Table of Figures
1. CONCEPT AND HISTORY OF THE FG5.................................................. 1-1
1.1. The FG5 Absolute Gravimeter................................................................................................... 1-1
1.2. HISTORY...................................................................................................................................... 1-1
1.3. FG5 Design Features .................................................................................................................... 1-2
2. DESIGN: COMPONENTS AND FUNCTION ............................................. 2-1
2.1. The Dropping Chamber .............................................................................................................. 2-2
2.1.1. CART/DRAG-FREE CHAMBER ........................................................................................... 2-4
2.1.2. TEST MASS ............................................................................................................................ 2-5
2.1.3. DRIVE MECHANISM ............................................................................................................ 2-6
2.1.4. SERVICE RING ...................................................................................................................... 2-6
2.1.5. VIEWING PORT ..................................................................................................................... 2-7
2.1.6. THE DROP .............................................................................................................................. 2-7
2.2. The Interferometer Base.............................................................................................................. 2-9
2.2.1. LASER ..................................................................................................................................... 2-9
2.2.2. OPTICS AND BEAM PATH .................................................................................................. 2-9
2.3. The Superspring......................................................................................................................... 2-12
2.3.1. SUPERSPRING MASS ......................................................................................................... 2-12
2.3.2. SPHERE DETECTOR SYSTEM .......................................................................................... 2-12
2.4. The System Controller ............................................................................................................... 2-14
2.4.1. REQUIRED HARDWARE ................................................................................................... 2-15
2.4.2. OPTIONAL HARDWARE.................................................................................................... 2-15
2.4.3. SOFTWARE .......................................................................................................................... 2-15
2.5. Electronics................................................................................................................................... 2-15
2.5.1. TIMING SYSTEM ................................................................................................................ 2-15
2.5.2. DROPPER CONTROLLER .................................................................................................. 2-17
2.5.3. SUPERSPRING CONTROLLER.......................................................................................... 2-18
2.5.4. LASER CONTROLLER........................................................................................................ 2-18
2.5.5. POWER SUPPLIES............................................................................................................... 2-19
2.5.6. OPTIONAL SYSTEMS......................................................................................................... 2-19
2.5.6.1. ENVIRONMENTAL SENSORS....................................................................................... 2-19
2.5.6.2. ROTATION MONITOR.................................................................................................... 2-20
3. HOW TO SET UP AND RUN THE FG5..................................................... 3-1
3.1. Setting Up the FG5....................................................................................................................... 3-1
3.1.1. ELECTRONICS CASE ........................................................................................................... 3-1
3.1.1.1. Model ML-1 Laser:.............................................................................................................. 3-2
3.1.1.1.1. Warm Up........................................................................................................................ 3-2
3.1.1.1.2. Operation .......................................................................................................................3-2
3.1.1.2. WEO Model 100 Laser ........................................................................................................ 3-3
3.1.2. SUPERSPRING ....................................................................................................................... 3-5
i
Table of Figures
INTERFEROMETER............................................................................................................... 3-6
3.1.3.
3.1.4. DROPPING CHAMBER TRIPOD ..........................................................................................3-6
3.1.5. ION PUMP ...............................................................................................................................3-7
3.1.6. LEVELING THE DROPPING CHAMBER ............................................................................3-8
3.1.7. CABLE CONNECTIONS ........................................................................................................3-9
3.1.8. THE SUPERSPRING ............................................................................................................. 3-11
3.1.8.1. TRAVEL LOCK.................................................................................................................3-12
3.1.8.2. SUPERSPRING ZERO-POSITIONING............................................................................3-12
3.1.9. BEAM VERTICALITY .........................................................................................................3-13
3.1.10. FRINGE OPTIMIZING.......................................................................................................... 3-14
3.2. Running the FG5......................................................................................................................... 3-17
3.2.1. SYSTEM CONTROLLER .....................................................................................................3-17
3.2.2. DROPPER CONTROLLER ................................................................................................... 3-17
3.2.3. PROGRAM SETUP ...............................................................................................................3-17
3.3. Shutting Down the FG5 .............................................................................................................. 3-18
3.3.1. COMPUTER ..........................................................................................................................3-18
3.3.2. SUPERSPRING...................................................................................................................... 3-18
3.3.3. INTERFEROMETER............................................................................................................. 3-19
3.3.3.1. OPTION 1: Model ML-1 Laser.........................................................................................3-19
3.3.3.2. OPTION 2: WEO Model 100 Laser ...................................................................................3-19
3.3.4. DROPPING CHAMBER........................................................................................................ 3-19
3.3.5. POWER ..................................................................................................................................3-19
3.4. Disassembling and Packing the FG5.........................................................................................3-20
3.4.1. ELECTRONICS .....................................................................................................................3-20
3.4.2. SYSTEM CONTROLLER .....................................................................................................3-20
3.4.3. ROTATION MONITOR (IF INCLUDED)........................................................................... 3-21
3.4.4. DROPPING CHAMBER........................................................................................................ 3-22
3.4.5. DROPPING CHAMBER TRIPOD ........................................................................................3-22
3.4.6. INTERFEROMETER BASE.................................................................................................. 3-22
3.4.7. SUPERSPRING...................................................................................................................... 3-23
3.4.8. TURBO PUMP (IF USED) ....................................................................................................3-23
4. ADJUSTMENT AND MAINTENANCE........................................................ 4-1
4.1. The Dropping Chamber...............................................................................................................4-1
4.1.1. REPLACEMENTS AND ADJUSTMENTS ............................................................................4-1
4.1.1.1. Removing The Dropping Chamber Cover ............................................................................4-1
4.1.1.2. Replacing the Dropping Chamber Cover..............................................................................4-2
4.1.1.3. Replacing the Drive Belt ......................................................................................................4-4
4.1.1.4. Adjusting the Drive Belt Tension ......................................................................................... 4-5
4.1.1.5. Replacing the Ferrofluidic Vacuum Feedthrough.................................................................4-5
4.1.1.6. Replacing the V-Plate ...........................................................................................................4-6
4.1.1.7. Replacing the Linear Bearings.............................................................................................. 4-7
4.1.1.8. Replacing the Shaft Bearings—Drive Pulley........................................................................4-8
4.1.1.9. Replacing the Shaft Bearings—Top Pulley .......................................................................... 4-9
4.1.1.10. Replacing the Rotary Shaft Encoder................................................................................... 4-9
4.1.1.11. Pumping Down the Dropping Chamber ........................................................................... 4-10
4.2. The Interferometer .....................................................................................................................4-10
ii
Table of Figures
ALIGNMENT........................................................................................................................ 4-10
4.2.1.
4.3. The Superspring......................................................................................................................... 4-13
4.3.1. REPLACEMENTS AND ADJUSTMENTS.......................................................................... 4-13
4.3.1.1. Removing the Superspring Cover ...................................................................................... 4-13
4.3.1.2. Removing the Service Ring................................................................................................ 4-13
4.3.1.3. Replacing the Coil (Linear Actuator)................................................................................. 4-14
4.3.1.4. The Mass Mainspring/Hanger............................................................................................ 4-15
4.3.1.5. Replacing the Flexures....................................................................................................... 4-15
4.3.1.6. Assembling the Superspring............................................................................................... 4-18
4.3.1.7. Replacing the Focus Lever Motor...................................................................................... 4-19
4.3.1.8. Adjusting the Micro-Switches............................................................................................ 4-19
4.3.1.9. The Aneroid Wafer Assembly............................................................................................ 4-19
4.3.1.10. The Delta Rods................................................................................................................. 4-20
4.4. Timing System and Data Acquisition....................................................................................... 4-21
4.4.1. TIMING ................................................................................................................................. 4-21
4.4.1.1. Avalanche Photo Diode Printed Circuit Board .................................................................. 4-23
4.4.1.2. APD Board and Photo Diode Supply Module.................................................................... 4-23
4.4.1.3. Rubidium Oscillator........................................................................................................... 4-24
4.4.2. DATA ACQUISITION.......................................................................................................... 4-25
4.4.2.1. Computer Interface Cards .................................................................................................. 4-25
4.5. DROPPER CONTROLLER ..................................................................................................... 4-25
4.5.1. DROPPER CONTROL MODES ........................................................................................... 4-27
4.5.1.1. STANDBY.........................................................................................................................4-27
4.5.1.1.1. To Select ...................................................................................................................... 4-27
4.5.1.1.2. Function ....................................................................................................................... 4-27
4.5.1.1.3. To Deselect .................................................................................................................. 4-27
4.5.1.2. MANUAL ..........................................................................................................................4-27
4.5.1.2.1. To Select: ..................................................................................................................... 4-27
4.5.1.2.2. Function: ...................................................................................................................... 4-27
4.5.1.3. DROP................................................................................................................................. 4-27
4.5.1.3.1. To Select: ..................................................................................................................... 4-27
4.5.1.3.2. Function: ...................................................................................................................... 4-28
4.5.1.4. OSCILLATE...................................................................................................................... 4-28
4.5.1.4.1. To Select: ..................................................................................................................... 4-28
4.5.1.4.2. Function: ...................................................................................................................... 4-28
4.5.2. Analog Servo.......................................................................................................................... 4-28
4.5.2.1. Cart-Position ...................................................................................................................... 4-29
4.5.2.2. Cart Velocity...................................................................................................................... 4-29
4.5.2.3. Sphere Servo ...................................................................................................................... 4-30
4.5.2.4. Active Sphere Servo........................................................................................................... 4-30
4.5.2.5. Feedforward Sphere Servo................................................................................................. 4-31
4.6. Superspring Controller.............................................................................................................. 4-31
4.6.1.1. SUPERSPRING CONNECTIONS.................................................................................... 4-31
4.6.1.2. MOTOR DRIVE SELECTION ......................................................................................... 4-33
4.6.1.2.1. OFF .............................................................................................................................. 4-33
4.6.1.2.2. AUTO .......................................................................................................................... 4-33
4.6.1.2.3. WINDOW.................................................................................................................... 4-33
4.6.1.2.4. REMOTE ..................................................................................................................... 4-33
4.6.1.2.5. MANUAL.................................................................................................................... 4-33
iii
Table of Figures
Active Servo .......................................................................................................................4-33
4.6.1.3.
4.6.1.4. Coil Drive (Current Driver)................................................................................................ 4-34
4.6.2. Maintenance Schedule............................................................................................................4-34
4.6.2.1. Annually .............................................................................................................................4-34
4.6.2.2. Semi-annually.....................................................................................................................4-34
5. TROUBLESHOOTING ...............................................................................5-1
5.1. The FG5 System ............................................................................................................................ 5-1
5.2. System Problems ...........................................................................................................................5-2
5.2.1. DROPPING CHAMBER.......................................................................................................... 5-2
5.2.2. INTERFEROMETER AND LASER........................................................................................ 5-5
5.2.3. SUPERSPRING........................................................................................................................ 5-8
5.2.4. SYSTEM CONTROLLER .....................................................................................................5-10
5.2.5. ELECTRONICS .....................................................................................................................5-11
5.2.6. ENVIRONMENTAL SENSORS PACKAGE .......................................................................5-12
5.2.7. ROTATION MONITOR ........................................................................................................5-13
5.3. Gravity Site Selection..................................................................................................................5-14
5.3.1. GEOLOGIC STABILITY ......................................................................................................5-14
5.3.2. SITE STABILITY ..................................................................................................................5-14
5.3.3. ENVIRONMENTAL NOISE ................................................................................................. 5-14
5.3.4. TEMPERATURE STABILITY.............................................................................................. 5-14
5.4. AC POWER................................................................................................................................. 5-15
6. SWITCHING THE AC POWER .................................................................. 6-1
6.1. LASER ...........................................................................................................................................6-1
6.1.1. WEO Model 100 Laser .............................................................................................................6-1
6.1.2. ML-1 Laser ...............................................................................................................................6-2
6.2. Portable Ion Pump Power Supply ...............................................................................................6-2
6.3. Computer.......................................................................................................................................6-2
7. PUMPING/BAKING THE CHAMBER ........................................................ 7-1
7.1. Choosing the Pumping Method....................................................................................................7-1
7.1.1. Cold Pumping the Chamber...................................................................................................... 7-1
7.1.2. Baking Out the Chamber ..........................................................................................................7-1
7.2. Connecting the Turbo Pump to the Dropping Chamber........................................................... 7-2
7.3. Heating the Chamber....................................................................................................................7-3
7.4. Starting the Ion Pump ..................................................................................................................7-4
8. THE ROTATION MONITOR....................................................................... 8-1
iv
Table of Figures
8.1.
INSTALLATION ......................................................................................................................... 8-1
9. CHECKLISTS AND LOGS ........................................................................ 9-1
9.1. Switching the AC Power .............................................................................................................. 9-1
9.2. FG5 Setup...................................................................................................................................... 9-2
9.3. Dropping Chamber Pump Down and Bake-Out ....................................................................... 9-5
9.4. Dropping Chamber Maintenance ............................................................................................... 9-7
9.4.1. Removing Dropping Chamber Cover....................................................................................... 9-7
9.4.2. Replacing Dropping Chamber Cover....................................................................................... 9-7
9.4.3. Replacing Drive Belt................................................................................................................ 9-8
9.4.4. Adjusting Belt Tension ............................................................................................................ 9-9
9.4.5. Replacing Ferrofluidic Vacuum Feedthrough .......................................................................... 9-9
9.4.6. Replacing V-plate .................................................................................................................. 9-10
9.4.7. Replacing the Linear Bearings ............................................................................................... 9-11
9.4.8. Replacing Shaft Bearings (Drive Pulley) ............................................................................... 9-13
9.4.9. Replacing Shaft Bearings (Top Pulley).................................................................................. 9-14
9.4.10. Replacing Rotary Shaft Encoder ............................................................................................ 9-15
9.5. Interferometer Alignment.......................................................................................................... 9-16
9.6. Superspring Maintenance.......................................................................................................... 9-18
9.6.1. Removing Superspring Cover ................................................................................................ 9-18
9.6.2. Replacing Superspring Cover................................................................................................. 9-18
9.6.3. Removing Superspring Service Ring ..................................................................................... 9-19
9.6.4. Replacing Superspring Coil ................................................................................................... 9-19
9.6.5. Replacing Superspring Flexures............................................................................................. 9-20
9.6.6. Assembling Superspring ........................................................................................................ 9-22
9.6.7. Replacing Superspring Focus Lever Motor............................................................................ 9-25
9.6.8. Adjusting Superspring Micro-Switches ................................................................................. 9-26
9.7. Packing the FG5 ......................................................................................................................... 9-26
10. INDEX ........................................................................................................... 10-1
Figure 2-1 The FG5 System......................................................................................... 2-1
Figure 2-2 Front view of the dropping chamber ...................................................... 2-2
Figure 2-3 Side view of the dropping chamber ........................................................ 2-3
Figure 2-4 Front view of the cart/drag-free chamber..............................................2-4
Figure 2-5 Side view of the cart/drag-free chamber................................................2-5
Figure 2-6. Side view of the service ring................................................................... 2-6
Figure 2-7 Top view of the service ring. .................................................................... 2-7
Figure 2-8 Side view of interferometer optics and beam path .............................2-11
Figure 2-9 The Superspring. ...................................................................................... 2-13
Figure 2-10 The Superspring sphere detection system.......................................... 2-14
v
Table of Figures
Figure 2-11 Timing diagram...................................................................................... 2-16
Figure 2-12 Rotation Monitor....................................................................................2-21
Figure 3-1: Superspring / Interferometer Setup...................................................... 3-4
Figure 4-1 Drive Pulley Assembly............................................................................. 4-7
Figure 4-2: Interferometer Top Lid.......................................................................... 4-11
vi
Table of Figures
This Page is Intentionally Blank
vii
Concept and History of the FG5 1
1. Concept and History of the FG5
1.1. The FG5 Absolute Gravimeter
The FG5 absolute gravimeter is a high precision, high accuracy, transportable
instrument that measures the vertical acceleration of gravity (g). The operation
of the FG5 is simple in concept. A test mass is dropped vertically by a
mechanical device inside a vacuum chamber, and then allowed to fall a distance
of about 20cm. The FG5 uses a laser interferometer to accurately determine the
position of the free-falling test mass as it accelerates due to gravity. The
acceleration of the test mass is calculated directly from the measured trajectory.
The laser interferometer generates optical interference fringes as the test mass
falls. The fringes are counted and timed with an atomic clock to obtain precise
time and distance pairs. These data are fit to a parabolic trajectory to give a
measured value for g. This method of measuring gravity is absolute because the
determination is purely metrological and relies on standards of length and time.
The distance scale is given by a frequency stabilized helium neon (HeNe) laser
used in the interferometer. A rubidium atomic time-base provides the time scale
used for the accurate timing. The value of gravity obtained with the FG5 can be
used without the loop reductions and drift corrections normally required when
using relative instrumentation.
1.2. HISTORY
The FG5 is a new generation of absolute gravimeter based on technology
developed over the last thirty years by Dr. James Faller of the National Institute
of Standards and Technology (NIST), and his colleagues. Beginning with a
white-light-fringe interferometric system built in 1962, Faller and coworkers have
continuously improved the designs of the instruments. The most recent
predecessors of the FG5 was the series of six JILAg gravimeters, built in 1985 at
the Joint Institute of Laboratory Astrophysics (JILA), with support from NIST,
the Defense Mapping Agency (DMA), the National Oceanographic and
Atmospheric Administration (NOAA), the Canadian Geophysical Survey (GSC),
the University of Hanover Institute for Earth Measurement, Germany, the
Finnish Geodetic Institute, Finland, and the University of Vienna Institute for
Metrology and Geophysics, Austria.
1-1
Concept and History of the FG5 1
1.3. FG5 Design Features
The FG5 incorporates a number of significant advancements in design which
reduce or eliminate systematic errors identified in the earlier versions, and which
make the FG5 easier to use. These improvements are:
• An inline interferometer beam path which eliminates systematic errors
from tilt-induced path length changes.
• Complete redesign of the Superspring, a device for providing an inertial
mass that contains a retroreflective corner cube. The new Superspring has
improved performance, and at the same time greatly reduced size. The drift
problems of earlier designs have been reduced substantially.
• Completely new tripod design, which supports the test chamber, for extra
stability. The tripod is now built symmetrically with respect to the drop line.
• Improvements to the electronics reflect new technology and make the
instrument smaller and easier to use.
• This absolute gravimeter is designed to work with a new rugged iodinestabilized laser system (WEO model 100) traceable to the BIPM.
• The system controller has been updated to an Intel-based personal
computer with a standard language interface. The decision to use standard
PC technology has allowed the FG5 to offer more computing power while
reducing the size of the instrument.
• “g”, a user-friendly, Windows©-based, full-featured software program
takes interferometer data and environmental data. This software provides an
immediate value for the local gravity in real-time. The program is also a fullfeatured post-processing software program that allows complete ability to
vary data analysis procedures and to vary environmental corrections.
1-2
Design: Components and Function 2
2. Design: Components and Function
The FG5 System (Figure 2-1) consists of a: Dropping Chamber, Interferometer,
Superspring, System Controller, and Electronics. A test mass is allowed to free-
fall inside the evacuated Dropping Chamber. The Interferometer is used to
monitor the position of the freely-falling test mass. The Superspring is an active
long-period isolation device used to provide an inertial reference for the gravity
measurement. The System Controller (computer) allows a flexible user
interface, controls the system, acquires data, analyzes data, and stores the
results. The Electronics provides high accuracy timing necessary for the
measurement and provides system servo control.
Figure 2-1 The FG5 System
2-1
Design: Components and Function 2
2.1. The Dropping Chamber
The Dropping Chamber (Figure 2-2 and Figure 2-3) is an evacuated
chamber which contains the Cart/Drag-Free Chamber which houses the Test
Mass. A Drive Mechanism is used to drop, track, and catch the test mass inside
the drag-free chamber. Laser light (Figure 2-1) passes through a window in the
bottom of the Dropping Chamber to the corner cube (inside the test mass), then
is reflected back down through the window to the interferometer.
2-2
Figure 2-2 Front view of the dropping chamber
Design: Components and Function 2
Figure 2-3 Side view of the dropping chamber
2-3
Design: Components and Function 2
2.1.1.CART/DRAG-FREE CHAMBER
The cart /drag-free chamber (Figure 2-4 and Figure 2-5) houses the test mass.
The purpose of the drag-free chamber is to reduce the residual air drag inside the
evacuated dropping chamber. The chamber also reduces magnetic and
electrostatic forces on the test mass, and provides a convenient method for
dropping and catching the test mass, as well as returning it to the top of the
chamber for the next drop. A Light Emitting Diode (LED) , located on the cart,
directs light through an optical glass sphere attached to the test mass. The
sphere focuses the light onto a linear detector, also mounted on the cart. This
system senses the position of the cart with respect to the test mass. A servo-
motor/drive belt system (Figure 2-2) moves the cart inside the Dropping
Chamber, using active feedback from the position sensor to maintain the cart in a
constant position relative to the test mass during free-fall. Since there is
essentially no relative motion between the test mass and the drag-free chamber,
the effects of residual air drag are eliminated.
2-4
Figure 2-4 Front view of the cart/drag-free chamber
Design: Components and Function 2
Figure 2-5 Side view of the cart/drag-free chamber
2.1.2.TEST MASS
The Test Mass (Figure 2-4 and Figure 2-5) is a retroreflective corner cube
surrounded by a support structure and balanced at the optical center of the
corner cube. The corner cube is a three-surface mirror which has the special
optical property that the reflected beam is always parallel to the incident beam.
In addition, the phase shift of the reflected beam is virtually constant with
respect to any slight rotation or translation of the corner cube around its optical
center
1
1
.
Peck, Edson, J. Opt. Soc. Amer., 38, (1948)
2-5
Design: Components and Function 2
2.1.3.DRIVE MECHANISM
The drive mechanism (Figure 2-2) is a support structure inside the dropping
chamber on which the cart/drag-free chamber travels up and down, driven by a
DC servo motor.
2.1.4.SERVICE RING
The Service Ring (Figure 2-6 and Figure 2-7) is the base of the Dropping Chamber. It provides connection and mounting for the following:
• A bellows-type vacuum valve for the initial evacuation of the vacuum
system
• A Ferrofluidic rotary vacuum feedthrough which connects the motor
shaft to the cart drive mechanism
• A servo motor/rotary shaft encoder assembly which moves the cart and
senses its position
• An electrical vacuum feedthrough which allows connection of the test
mass tracking electronics to the controller
• An ion pump, mounted on a 2¾” Conflat flange, which maintains the
vacuum once the chamber has been evacuated by the roughing pump
• Spare 2¾” Conflat and Mini-Conflat flanges are blanked off, and can be
used for additional vacuum accessories
Figure 2-6. Side view of the service ring.
2-6
Design: Components and Function 2
Figure 2-7 Top view of the service ring.
2.1.5.VIEWING PORT
The viewing port (Figure 2-2 and Figure 2-3) is located in the top flange of the dropping chamber. It allows visual observation of the dropping chamber interior when the rotation monitor is not fitted to the system. The rotation monitor (when fitted to the system) is mounted to the top flange of the dropping chamber, directly above the viewing port. When the rotation monitor is not mounted, a cover for the port is used to exclude ambient light from the interior of the dropping chamber during measurements.
2.1.6.THE DROP
In drop mode, a signal from the computer to the dropper controller initiates the
drop sequence. The cart drag-free chamber is driven slowly from its bottom
position to the “hold” position at the top of the drop. A second pulse initiates
the drop, and the cart accelerates downward at more than 1 g, leaving the test
mass in free-fall.
2-7
Design: Components and Function 2
When the cart has traveled about 5 mm downward from the hold position (as
measured by the shaft encoder) a separation of about 3 mm between the cart and
test mass has been achieved. The dropper controller then uses feedback from the
linear detector to maintain this separation for the remainder of the drop.
The free-falling test mass generates an interference fringe for each halfwavelength (λ/2) of its movement. As the mass accelerates downward, the
fringes occur more and more closely in time. The resulting signal from the
avalanche photo diode (APD) is a “chirped” sine wave (Figure 2-11) whose
frequency is proportional to the free-falling test mass’s velocity.
Approximately a million fringes are generated during a single drop. A zerocrossing discriminator (comparator) transforms the sinusoidal fringe signals
from the APD into a series of square Transistor-Transistor Logic (TTL) pulses.
The pulses are scaled (i.e., divided) by a user-defined factor which is set in the
software (typically 1000). A Time Interval Analyzer (TIA) measures the time
interval between each scaled pulse. The g-program fits each time and distance
pair to a parabolic trajectory to determine the value of g.
When the cart and test mass have descended past the catch point, the controller
signals the cart to reduce acceleration and then come to a stop. The falling mass
catches up to the descending cart and is brought gently to rest.
The system resets for the next drop. The entire sequence takes about 2 seconds
and can repeated up to thirty times per minute.
2-8
Design: Components and Function 2
2.2. The Interferometer Base
The interferometer base is an aluminum housing which supports the optics for
splitting, directing, and recombining the laser beams.
2.2.1.LASER
The FG5 employs a stabilized helium-neon laser to provide an accurate and
stable wavelength used in the interferometric measurement system. There are
two lasers which are currently available for the FG5.
• The Winters Electro-Optics Model 100 iodine stabilized laser. This laser is a
primary standard for the definition of the meter at the Bureau International
des Poids et Measures (BIPM) in Sevres, France. It is a highly stabilized
distance standard having an absolute frequency accuracy of 1 part in 10
kHz).
10
(50
• The Micro-g Solutions Model ML-1 frequency/intensity stabilized HeNe laser
is characterized by a slow, linear drift. Unlike the WEO Model 100 Iodine
Laser, it must be periodically calibrated to achieve the best accuracy.
However, it is more rugged than the iodine laser.
2.2.2.OPTICS AND BEAM PATH
Refer to Figure 2-1 and Figure 2-8 for the following description of the beam path.
The optical fiber directs the laser beam from the laser head to the interferometer
base. At the input of the interferometer, a lens collimates the light from the
optical fiber. It is then directed to beamsplitter #1, where it is split into the test
beam and the reference beam. The reference beam is split again at beamsplitter
#2 and travels to the Avalanche Photo Diode (APD) and the fringe viewer. The
path length of the reference beam remains constant.
The test beam is reflected vertically at beamsplitter #1, and passes through a
compensator plate and a window in the bottom of the Dropping Chamber. It is
then reflected back down by the corner cube in the test mass. The test beam
returns through the window, the compensator plate, and passes down through
the interferometer base to the superspring. The test beam passes through the top
window of the superspring chamber to a corner cube in the superspring mass.
2-9
Design: Components and Function 2
The test beam is then reflected back through the window to the interferometer
base, where it reflects off mirror #1, passes through the translator plate
(twiddler), reflects off mirror #2, and is recombined with the reference beam at
beamsplitter #2.
This interferometer is a Mach-Zender interferometer with a fixed (reference) arm
and a variable (test) arm. During a drop, the motion of the test mass/corner
cube affects the path length of the test beam. The interference fringes which
result from the recombination of the test beam and the reference beam provide
an accurate measure of the motion of the test mass relative to the mass
suspended on the superspring.
Two separate complementary, recombined beams are produced at beamsplitter
#2. The vertical recombined beam is focused by a lens to strike the detector
(APD), and the interference fringes are converted to a Continuous Wave (CW)
signal. The CW signal is then converted to a Transistor Transistor Logic (TTL)
signal and transmitted to the time interval analyzer.
The other recombined beam travels horizontally until it reaches the attenuator
plate (rattler). This beam is split and reflects "rattles" between the beamsplitter
coating and the uncoated side of the attenuator plate. Three beams of decreasing
intensity emerge from the coated side. The first and brightest of these beams
travels horizontally into the fringe viewer. The second and third beams are
deflected vertically by a mirror. A flag in front of the mirror blocks the second
beam, allowing the third (dimmest) beam to exit the interferometer where it is
reflected off mirror #3 and enters the collimating telescope. The collimating
telescope is used to compare this weak reference beam with another beam
reflected off of an alcohol pool to allow alignment of the laser beam with the
local vertical.
2-10
Design: Components and Function 2
Figure 2-8 Side view of interferometer optics and beam path
2-11
Design: Components and Function 2
2.3. The Superspring
The superspring (Figure 2-9) is a long-period, active vertical isolator used to
compensate for small vertical motions of the first beam splitter. The superspring
has a short (20-cm) mainspring with a natural period of about 1 second. The
mainspring is contained in a support housing that is actively servo-controlled to
track the superspring mass at the end of the mainspring. The resulting system is
a long-period (30-60 second) spring-mass system which is suspended from the
interferometer base. The superspring isolated ground motions occurring at a
higher frequency than its own enhanced natural frequency.
2.3.1.SUPERSPRING MASS
The superspring mass contains a corner cube retroreflector and an optical glass
sphere.
2.3.2.SPHERE DETECTOR SYSTEM
The superspring sphere detector system (Figure 2-10) senses motions of the
superspring mass relative to the support housing. An infrared light emitting
diode (LED) located on the support housing directs light through an optical glass
sphere attached to the superspring mass. The sphere focuses the light onto a
split photodiode detector, also mounted on the support housing. The support
housing is itself servo-driven to cancel these motions using an electromagnetic
coil-type linear actuator (coil) is mounted between the support housing and the
superspring base. As vertical ground motion occurs the linear actuator moves
the support housing up or down as needed. The apparatus is constrained to
move only vertically by a linear way system constructed of five flexures (delta
rods) arranged in an upper V-shaped array, and a lower triangular array.
2-12
Design: Components and Function 2
Figure 2-9 The Superspring.
2-13
Design: Components and Function 2
Figure 2-10 The Superspring sphere detection system
A rough adjustment of the spring length is made with a DC motor-driven lever
system that supports the mainspring at the top of the mainspring housing.
Temperature-related length changes of the mainspring are compensated with an
aneroid wafer assembly (Figure 2-9).
2.4. The System Controller
The system controller is an IBM-compatible PC which is used to control the
gravimeter (initiate drops) as well as collect data (distance and time) for
computing the gravity value. It is also used to collect environmental and
rotation monitor data if the FG5 is equipped with these systems.
2-14
Design: Components and Function 2
2.4.1.REQUIRED HARDWARE
A Pentium II or better PC running Windows98©
• Memory: >32M RAM
• Hard Drive: 500Mb or larger recommended for high density data storage
• Combination A/D converter and Parallel Input-Output (PIO) board
• Time Interval Analyzer (TIA)
2.4.2.OPTIONAL HARDWARE
• Micro-g Solutions Environmental Sensors Package
• Micro-g Solutions Rotation Monitor
2.4.3.SOFTWARE
The FG5 software, “g”, is a Windows©-based application. The software
provides FG5 data acquisition, real-time processing, post processing, and
diagnostic testing. It also allows the user to customize the data acquisition for
each site including input/output files, printing options, and session control. The
software also allows user input of site-specific information such as site
name/code, geodetic coordinates and elevation, nominal air pressure, and
gravity gradient. In addition, the software allows the user to apply (or not
apply) all the gravity corrections independently. These options are available
both in the real-time and post-processing modes.
See the “g” Software Manual for additional information.
2.5. Electronics
2.5.1.TIMING SYSTEM
The timing system (Figure 2-11) consists of four main components:
• Avalanche Photo Diode (APD)
2-15
Design: Components and Function 2
• Rubidium Oscillator
• Time Interval Analyzer (TIA)
The Avalanche Photo Diode (APD) is located in the interferometer (Figure 2-8).
It detects the fringes created when the test and reference beams are recombined.
An ultrafast comparator chip located on the APD board detects the zerocrossings of the sinusoidal fringes and outputs a TTL (square wave) version of
the frequency-swept fringe signal. During a drop, the fringe signal sweeps from
DC to about 6 MHz.
The Rubidium Oscillator is an atomic resonance-controlled oscillator (or
equivalent) which outputs a stable sinusoidal signal of 10 MHz.
The Time Interval Analyzer (TIA) scales the fringes using a software-specified
scale factor and outputs the time of occurrence for each scaled fringe.
Figure 2-11 Timing diagram
2-16
Design: Components and Function 2
2.5.2.DROPPER CONTROLLER
The dropper controller uses three modes to operate the dropping chamber.
These modes are OSC, AUTO, and MANUAL. The operator also controls the
status of these modes and the dropper triggering with the RESET and INIT
switches and the trigger source (INT/EXT) switch. See Chapter 4 for a detailed
discussion of the modes and switches.
The dropper controller can direct the motor in the Dropping Chamber to lift the
cart and test mass to a specified height, to move the cart at a specified velocity,
and to track the test mass during free-fall.
The motor drives the cart/test mass assembly by turning a pulley and stainless
steel drive belt which is attached to the cart. The motor also turns an optical
shaft encoder that provides accurate information to the dropper controller on the
position and velocity of the pulley.
Information on the relative position of the test mass to the cart during free-fall is
provided by a sphere detector system. An LED and a linear detector are
mounted on opposite sides of the cart, and an optical glass sphere is mounted on
the test mass. The sphere focuses a beam of light from the LED onto the linear
detector, indicating the precise location of the center of the sphere relative to the
cart. The dropper controller uses this information to determine whether to
maintain, increase, or decrease current to the motor to achieve the appropriate
relative position of the cart and the test mass. This feedback system is a
conventional analog servo system.
2-17
Design: Components and Function 2
2.5.3.SUPERSPRING CONTROLLER
The purpose of the electronic and mechanical systems for the superspring is to
isolate the reference mass from any vertical motion of the instrument in order to
keep the path length of the test beam constant. Three systems provide coarse
and fine adjustment of the spring support structure: a motor attached to the top
of the mainspring, a linear actuator coil and magnet system, and an aneroid
wafer assembly. A controller circuit board drives the motor and the coil and
magnet system, while the aneroid wafer assembly responds automatically to
temperature changes.
A sphere detector system similar to the one used in the Dropping Chamber
provides information on the position of the reference mass relative to the
mainspring support system. An infrared LED and a photo detector are mounted
opposite each other inside the mainspring support housing. A sphere attached
to the bottom of the mass focuses the light from the LED onto the detector, which
transmits the resulting signal to a sphere signal preamplifier.
The zero-position of the sphere on the test mass can be adjusted by moving the
top of the main spring with a small DC motor with a very large gear ratio for fine
control.
The main servo electronics control, the coil-magnet forcer, moves the main
spring support in such a way to keep the main spring length constant. This active
servo effectively weakens the main spring, synthesizing a long period isolation
device. The active period of the superspring is nominally about 60 seconds.
2.5.4.LASER CONTROLLER
The laser controller supplies power and enables operator control for the WEO
Model 100 Iodine stabilized laser or the Micro-g Solutions Model ML-1
frequency/intensity stabilized laser. See Chapter 3 for setup and operation.
2-18
Design: Components and Function 2
2.5.5.POWER SUPPLIES
There are two primary power supply units which are required to operate the
FG5:
The power mains module is located in the rear of the electronics case. It is the
primary input for AC power, and contains all the DC power supplies which are
required to operate the FG5. Note that it accepts AC voltage from 100-240 V, 5060 Hz.
The Micro-g Solutions Model 125 Portable Ion Pump Power Supply is located in
the dropping chamber travel case. It supplies power to the ion pump for both
AC and DC operation. See the Model 125 manual for operating instructions.
Note that it accepts AC voltage from 100-240 V, 50-60 Hz.
2.5.6.OPTIONAL SYSTEMS
There are two optional systems for the FG5: the Environmental Sensors Package
and the Rotation Monitor. The environmental sensors package can be added to
the system by itself, but if the rotation monitor is purchased, the environmental
sensors package must be included in the system.
2.5.6.1.ENVIRONMENTAL SENSORS
The Environmental Sensors Package is used to record environmental data
(temperature and atmospheric pressure) as the system is operating. Atmospheric
pressure data is used to compute and apply the local barometric pressure
attraction correction while the system is operating. The primary components of
the environmental sensors package are:
• Temperature Probe: The patch panel (Channel 0) contains a built-in
temperature sensor that records ambient temperature. Note that
temperature is not actually used in the gravity determination calculation.
It is recorded purely for diagnostic reasons.
• Pressure Sensor: A digital barometer is used to sense the atmospheric
pressure. It is mounted on the inside of the power mains module at the
rear of the electronics case, and is read in to the computer via patch panel
Channel 4.
2-19
Design: Components and Function 2
2.5.6.2.ROTATION MONITOR
The rotation monitor (Figure 2-12) is used to monitor and record the rotation of
the test mass during each drop. The rotation monitor consists of a rigid anodized
aluminum housing mounted on the top flange of the dropping chamber, above
the viewing port. The rotation monitor employs a very sensitive optical lever
system to measure and record the rotation of the test mass which can be used as
a means to reject bad drops or determine when the mechanical system is not
functioning properly. A diode laser produces a visible beam which is directed
onto and reflects from a mirror attached to the top of the dropped object. The
reflected beam is sent through a lens and is focused onto a two axis position
sensitive photodetector. This system rejects translation and is only sensitive to
rotation. The diode laser beam reflects off mirror #1 and the beamsplitter (mirror
mount #2). The beam then passes down through the dropping chamber viewing
port, where it reflects off a flat mirror which is mounted to the top of the test
mass. The return beam from the test mass mirror passes through the
beamsplitter and reflects off mirror #3. The beam then passes through the 200
mm focusing lens, which is adjusted to eliminate cross coupling of translations
which would otherwise appear as rotations. The beam is then reflected off
mirror #4 and enters the detector box. The output from the quad detector is used
to provide rotation information to the computer/system controller. Each
rotation monitor is calibrated by Micro-g Solutions to determine the rotation and
translation sensitivity. These data are used to calculate rotation errors. The
rotation data can also be displayed on the computer screen, and is recorded in
the DDT output data file.
2-20
Design: Components and Function 2
Figure 2-12 Rotation Monitor
2-21
Design: Components and Function 2
This Page is Intentionally Blank
2-22
How to Set Up and Run the FG5 3
3. How to Set Up and Run the FG5
3.1. Setting Up the FG5
NOTE: These instructions are based on the assumption that all subsystems of
the FG5 are aligned correctly and operating properly. If adjustment or
alignment is necessary, consult chapter 4, “Adjustment and Maintenance” for
instructions, before proceeding with set up. When setting up the FG5, it is
helpful to use the FG5 Setup Checklist in Appendix D.
Locate and mark a reference point on the floor where gravity will be measured.
THE FLOOR SHOULD BE AS CLEAN, SMOOTH, AND LEVEL AS POSSIBLE.
IT IS BEST TO SET UP THE FG5 ON A CONCRETE OR HARD TILE FLOOR!
3.1.1.ELECTRONICS CASE
1. Place the electronics case in a convenient location about 1 meter from the
reference mark.
2. Check the input voltage settings and make sure they are set to the proper AC
line voltage. If the instrument is not set to the correct voltage see appendix A
for instructions on switching the input voltage settings.
3. Make sure the following switches are off:
• Main AC power (rear)
• Main DC power (rear)
• Laser power (main AC power and key switch)
• Superspring coil
4. If the ion pump has been maintaining the dropper vacuum on battery power,
open the dropping chamber case and apply AC power to the ion pump
power supply. See the Ion Pump Model 125 Manual for more details.
3-1
How to Set Up and Run the FG5 3
5. Open the Interferometer case, and place the laser on the floor about 1 m from
the reference mark. Take care not to stress the fiber optic.
6. Connect the main AC power cable from the mains power input (rear of
electronics case) to the AC power receptacle. If a GUPS (or similar
uninterruptible power supply) is used with the FG5, the GUPS should be
connected to the main AC power source and the FG5 AC power supply
should be connected to the GUPS output.
7. Turn on the main AC and DC power switches on (rear of electronics case).
8. Connect the applicable power cables to the laser and Turn on the laser power.
Consult the instructions below for the proper laser.
3.1.1.1.Model ML-1 Laser:
3.1.1.1.1.Warm Up
(Consult ML-1 operating manual for more details)
• Set the MODE switch to the WARMUP position.
• Turn on the main power switch and the key switch for the laser tube HV.
• Set the heater current to 0.3 V using the front panel monitor to view
current..
• Allow the laser to warm up for at least half an hour before locking.
3.1.1.1.2.Operation
• After the laser is warm, turn the REMOTE switch ON (the REMOTE LED
should light). This allows the computer to control the red/blue side-lock
choice.
3-2
How to Set Up and Run the FG5 3
3.1.1.2.WEO Model 100 Laser
• Turn on the power (main power and HV key switches).
• Select the proper iodine peak (usually “E”).
• Set the servo control to AUTO.
• Do not adjust any other controls. Nominal control settings are:
Signal Monitor Section:
Meter Select: 1F
Gain 1
Time Constant 1
Temperature Control Section:
Body Temp Mode TEMP
Temperature Control Section
Meter BIAS
Bias Voltage Set to 0V (meter)
• Allow the laser to warm up for at least two hours (or until the
temperature of the laser has stabilized) before beginning observations.
3-3
How to Set Up and Run the FG5 3
3-4
Figure 3-1: Superspring / Interferometer Setup
How to Set Up and Run the FG5 3
3.1.2.SUPERSPRING
Figure 3-1 illustrates the location of the superspring and interferometer base for
setup.
9. Remove the superspring tripod from the interferometer case and place on the
floor over the reference mark. Orient the tripod so the bull’s eye level
(mounted near one of the leveling feet) is facing the electronics case, if
possible. The bull’s eye level should be facing south to minimize the effect of
the Coriolis effect on the dropped object. The tripod can be centered over the
mark by viewing the mark through the hole in the center of the tripod.
10. Rough level the superspring tripod using the bull’s eye level.
11. Measure the lower reference height using the depth gauge provided. The
lower reference height is the distance between the superspring tray ring and
the reference mark (approximately 5-15 cm). Place the depth gauge parallel
surface on the machined inside ring of the superspring tripod. Pass the gauge
rod through the hole in the center of the superspring tripod and extend it
until it hits the reference mark on the floor. Tighten the locking screw and
measure gauge length using the scale fixture which is used to measure the
upper reference height (see step 37). Record this value in the system check
log.
12. Place the superspring on the tripod. Orient the superspring so the travel lock
(brass knurled knob on the service ring) is pointing toward the bull’s eye
level.
13. Clamp the superspring to the tripod by turning the three 5-lobe knobs fully
clockwise. This rotates the clamps in place over the base of the superspring.
14. Level the tripod using the two precision level vials on the base of the
superspring. Be sure to adjust the cross level first, then the long level. The
cross level is opposite the superspring travel lock (knurled brass knob). If the
long level is adjusted first, it will change when the cross level is adjusted.
When the cross level is adjusted first, it does not change when the long level
is adjusted. Only turn two of the feet while leveling; this insures that the
lower reference height does not change.
3-5
How to Set Up and Run the FG5 3
Note: While leveling the superspring and dropping chamber tripods, note that
turning the tripod feet clockwise lowers the dropping chamber tripod and raises
the superspring tripod.
3.1.3.INTERFEROMETER
15. Remove the interferometer base from its shipping case taking care not to
stress the fiber optic. Remove the dust cap from the top superspring window
and place the interferometer base on the superspring. Orient the
interferometer base so the fiber optic input is located directly above the
superspring travel lock. The alignment pins on the top of the superspring
assure that the interferometer base is oriented correctly.
16. Lock the interferometer base in place by tightening the four 5-lobe knobs.
3.1.4.DROPPING CHAMBER TRIPOD
17. Remove the tripod tray from the superspring case and place it carefully
upside down on the floor.
18. Remove the three tripod legs from the dropping chamber case and attach
them to the tray. Tighten the legs by using the ~30 cm “cheater” bar.
19. At this point the interferometer will be used to support the dropping chamber
tripod. First, remove the dust cap from the top of the interferometer base.
20. Carefully place the dropping chamber tripod on the interferometer. Orient
the tripod so that the small hole is
21. Carefully remove the dropping chamber from its case by the handles and
gently place it into the pocket in the top of the tripod tray, allowing the two
vertical alignment pins in the tray to engage the sockets in the dropping
chamber base. Orient the dropping chamber so the ion pump is directly
above the beam blocker controls on the interferometer.
22. Lock the dropping chamber in place with the three clamps by turning the 5lobe knobs fully clockwise. This rotates the dropping chamber clamps in
place over the base of the chamber.
23. Release the cart travel lock by turning the motor shaft slightly
counterclockwise with a 4 mm hex wrench or ball driver to release the
3-6
How to Set Up and Run the FG5 3
pressure on the travel lock mechanism. While holding this position, pull out
the brass knob, rotate it 90° in either direction, and gently release it so the pin
in the shaft rests in the lock. Gently release the wrench or ball driver from the
motor shaft.
3.1.5.ION PUMP
(If the ion pump is already on and maintaining the chamber vacuum, skip to step
to “Leveling the Dropping Chamber” below)
24. Recheck the AC, BAT, and HV switches on the ion pump power supply, and
make sure they are off.
25. Connect the ion pump HV cable to the ceramic connector on the pump.
Connect the small green safety ground umbilical of the HV cable to one of the
banana jacks located on the base of the dropping chamber near one of the
handles. Connect the safety HV ground to the other banana jack on the base
of the dropper. Tie both cables to a tripod leg with a velcro strap.
26. If the vacuum in the dropping chamber has remained intact since the
previous use, the ion pump alone may be sufficient to pump down the
chamber. The ion pump will probably start if it has been off for two hours or
less. Appendix B discusses the use of the turbo pump.
27. Before turning on the ion pump, set the front panel meter select knob to
PUMP VOLTAGE (kV). Turn on the pump and check the meter. The
voltage should be at least 2 kV within 5 minutes after turning on the ion
pump. An increasing voltage usually indicates that the ion pump is starting.
Nominal voltages (at operating pressure) are:
4 kV on AC
3 kV on battery power
28. If the ion pump does not start within 5 minutes, shut off the power at the ion
pump power supply, and prepare to pump the dropping chamber with the
turbo pump. See appendix B for instructions.
3-7
How to Set Up and Run the FG5 3
3.1.6.LEVELING THE DROPPING
CHAMBER
Note: While leveling the superspring and dropping chamber tripods, note that
turning the tripod feet clockwise lowers the dropping chamber tripod and raises
the superspring tripod.
29. Check the superspring levels and adjust, if necessary, by leveling the
superspring tripod.
30. When the superspring levels are centered, the dropping chamber tripod
levels should be within two divisions of the center position. If the levels do
not agree, this may indicate a problem. Consult the section on adjustment
and maintenance for instructions.
31. Remove the blue pads and brass tripod feet from the superspring case. Make
sure the pads and tripod feet are clean.
32. Place a tripod foot under each leg of the tripod. Raise each foot and slide a
blue pad under the foot.
33. Center the cone in each foot under the nylon ball on the end of each tripod
leg. Turn the leveling adjustment screws on the feet counterclockwise,
raising them until they just contact the balls. It is important that there is no
horizontal tension between the foot and the tripod leg because it will cause
the dropping chamber to shift sideways when it is lifted. It is helpful to
rotate, or wiggle, the foot slightly (while it is in contact with the nylon ball)
to release any horizontal tension.
34. After each foot is in contact with the tripod leg, rotate each tripod foot
leveling screw one revolution (counterclockwise), using the mark on the top
of the adjustment screw as a reference. Rotate each tripod foot leveling foot
one additional revolution as described previously. The two total turns raise
the tripod off of the interferometer so there is no contact between the two
components. Each counterclockwise revolution of the leveling screw raises
the tripod about 0.75 mm.
35. Level the tripod tray by adjusting the tripod feet (not the superspring tripod
legs). Be sure to adjust the cross level first, then the long level. The cross
level is parallel to the telescope and the long level is perpendicular to the
telescope. If the long level is adjusted first, it will change when the cross level
3-8
How to Set Up and Run the FG5 3
is adjusted. When the cross level is adjusted first, it does not change when
the long level is adjusted.
36. It is best to adjust the levels by raising the proper adjustment foot. This will
prevent the dropping chamber from contacting the interferometer.
! THERE MUST BE NO CONTACT BETWEEN THE TRIPOD/DROPPING
CHAMBER ASSEMBLY AND THE INTERFEROMETER DURING OPERATION.
THIS ALSO APPLIES TO CABLES. CABLES CONNECTED TO THE
INTERFEROMETER MUST NOT TOUCH THE DROPPING
CHAMBER/TRIPOD, AND CABLES CONNECTED TO THE DROPPING
CHAMBER/TRIPOD MUST NOT TOUCH THE
INTERFEROMETER/SUPERSPRING.
37. Check the superspring levels and adjust, if necessary, using the leveling feet
on the superspring tripod.
38. Measure the upper reference height using the scale fixture provided. The
upper reference height is the distance between the top of the interferometer
base and the bottom of the dropping chamber. Loosen the clamp on the scale
and pass the scale up through the access hole in the dropping chamber tripod
while pulling the scale slightly towards yourself (the hole is located directly
above fiber optic input on the interferometer base) until it contacts the top of
the interferometer base. The upper reference height is approximately 5 cm.
Record this value in the system check log. The sum of the upper and lower
reference heights (approximately 15 cm) will be entered as the reference
height term in “g” Process | Setup| Information. See the FG5 Software
Manual for instructions.
3.1.7.CABLE CONNECTIONS
39. If the computer is not mounted in the electronics rack, remove it from its
shipping case and place on the top of the electronics case. Make sure the
power switch is off.
40. Connect the computer power cable to the power supply panel.
3-9
How to Set Up and Run the FG5 3
41. Connect the gray ribbon cable from the “rear of the patch panel to the
connector on the computer. Note the location of Pin 1 (white arrow) in both
cases.
42. Connect the BNC cable from the interferometer base “TTL” connector to the
“FRINGES” connector on the computer time interval card, identified as
“Channel A” in the Time Interval Analyzer (TIA) operating manual.
43. Connect the BNC cable from the “10 MHz” connector on the power supply to
the “CLK” connector on the computer time interval card, identified as “EXT
CLK” in the TIA operating manual.
44. Connect the BNC cable from the TRIG OUT connector on the dropper
controller to the “TRIG” connector on the computer time interval card,
identified as “EXT ARM” in the TIA operating manual.
45. Connect the barometer cable from the power supply to Channel 4 on the
patch panel.g
46. For the WEO Model 100 laser:
Connect the BNC cable from the OUTPUT connector on the front panel of the
laser controller to Channel 3 of the patch panel . Make sure the meter select
switch on the laser controller is set to the “1F” position.
47. For the Model ML-1 Laser: Connect the LASER LOCK on the patch panel to
the REMOTE BNC on the back of the ML-1 controller (this allows the laser
mode to be switched from “red” to “blue”). Connect the DIG C5 output on
the patch panel to REMOTE2 BNC connector on the back of the ML-1
controller (this enables the warmup/lock mode) .
48. Connect the BNC cable from the SPHERE OUT connector on the superspring
controller to the Channel 1 of the patch panel.
49. Connect the BNC cable from the METER MONITOR connector on the ion
pump power supply to Channel 2 of the patch panel. Set the meter select
switch to the 10
-4
µA scale.
50. Connect the BNC cable from the TRIG OUT connector on the patch panel to
the TRIG EXT IN connector on the dropping chamber controller.
3-10
How to Set Up and Run the FG5 3
51. Attach the rest of the system cables as described below. Both ends of all
cables are labeled with the proper location for each connector.
• MAKE SURE THE DROPPING CHAMBER IS IN STANDBY (PRESS THE
RESET BUTTON TO FORCE THE DROPPING CHAMBER
CONTROLLER INTO STANDBY). Connect the dropping chamber signal
cable (white Lemo connector) from the power supply to the electrical
feedthrough on the service ring.
• Connect the rotary shaft encoder cable (blue Lemo connector) from the
power supply panel to the blue Lemo connector on the motor drive
assembly.
• Connect the DC motor power cable (orange Lemo connector) from the
power supply panel to the orange Lemo connector on the motor drive
assembly.
• Connect the APD power cable (green Lemo connector) from the power
supply panel to the interferometer base “power” connector.
3.1.8.THE SUPERSPRING
52. Make sure the COIL switch on the superspring controller is OFF. Connect
the superspring control cable (yellow Lemo connector) from the power
supply panel to the connector on the base of the superspring.
THE COIL SWITCH IS ON.
53. Adjust the leveling screws on the superspring tripod, if necessary, to level the
THEMSELVES. They are preset to provide the correct internal vertical reference
for the superspring.
! DO NOT ATTACH SUPERSPRING CONTROL CABLE WHEN
superspring with the two precision level vials on the base.
! CAUTION: DO NOT ADJUST THE LEVELING VIALS
3-11
How to Set Up and Run the FG5 3
3.1.8.1.TRAVEL LOCK
54. Release the superspring travel lock by pulling out the brass travel lock knob
until it engages the shaft and slowly rotating it counterclockwise until it
reaches the stop (180°). Slowly release the lock knob. The arrow on the lock
knob points down when it is locked (up when it is unlocked).
55. If necessary, one can use the travel lock knob to damp excess spring motion.
Carefully pull the knob out, and slowly turn it clockwise until the travel lock
just touches the spring support structure. Then slowly return the knob to the
unlocked position.
3.1.8.2.SUPERSPRING ZERO-POSITIONING
56. Allow the spring to settle for at least two minutes before setting the zeroposition of the mainspring. Use the DC motor to move the top of the
mainspring relative to the inner support structure. The zero-position is
monitored on the front panel BNC of the superspring controller marked
SPHERE OUT. The position can be adjusted manually or automatically to be
within about ±20 mV of 0 V.
The rotary knob on the front panel can be set to MANUAL, WINDOW,
AUTO, or REMOTE. The REMOTE setting should not be used. The
MANUAL setting allows the user to apply a voltage to the DC. motor that
can be varied with a front panel trimpot labeled MOTOR. The middle of the
trimpot range applies no voltage. Turning the knob towards UP causes the
motor to lift the mass (increasing positive voltage on the sphere BNC), while
turning the knob towards DOWN lowers the mass (decreasing the voltage on
the sphere BNC).
AUTO causes the motor to seek the zero-position automatically.
! NOTE: IN AUTO, THE DC MOTOR IS ALWAYS ACTIVE. THE
SUPERSPRING SHOULD NOT BE LEFT IN AUTO DURING NORMAL
OPERATION (WHEN THE COIL SERVO IS ACTIVE)
3-12
How to Set Up and Run the FG5 3
The WINDOW setting turns on the motor only when the spring position is
out of range (indicated by an LED on the front panel). In this case, the AUTO
mode is activated until the position is moved to zero, and then the motor is
deactivated. The superspring can be left in this condition, but it is still
advisable to turn the rotary knob to OFF before closing the superspring coil
loop (switch set to CLOSED on the front panel). WINDOW mode is
currently not supported by the FG5 software.
When setting the zero position, it is very important to make sure that the
mass is hanging freely, and is not out of range. A substantial variation in
SPHERE OUT voltage (many 100s of mV) when the servo loop is open
indicates that the mass is hanging freely. It is possible for the mass to be out
of range of the detector. In this case, a small positive or negative voltage
indicates that the mass is above or below the detector, respectively. This can
happen if there is a large change in gravity (usually as a result of a large
latitude and/or elevation change) from one site to the next. The zero
position can be set by using the AUTO mode, which moves the mass to the
zero position. One can also move the test mass position using the MANUAL
mode. Before switching to MANUAL mode, first set the trim-pot to move the
motor in the correct direction. The trim-pot should be set towards UP if the
SPHERE voltage is negative or towards DOWN if the SPHERE voltage is
positive.
57. Once the desired zero-position is reached (SPHERE output within 20 mV),
deactivate the motor by turning the front panel knob to OFF. The spring
should again be allowed to settle down for at least two minutes. Set the coil
switch to ON. This switch activates the superspring main servo loop for
normal operation. At this point, there may still be a rotation in the test mass
(three second period) which is not damped by the servo. This rotation mode
will eventually damp out, and the gravity data will be become quieter over
the first hour.
3.1.9.BEAM VERTICALITY
58. Loosen the lock on the side access door of the interferometer base (located
directly below the beam blocker controls) and slide the door open.
3-13
How to Set Up and Run the FG5 3
59. Remove the top cap of the alcohol container and place the container inside the
interferometer base. By eye, center the alcohol pool left/right, and slide it all
the way back until it contacts the rear wall. This insures the laser spot is
centered in the alcohol pool.
60. Focus the telescope crosshairs by placing a white card in front of the telescope
objective. Place the card at an angle to allow light to strike the card and
illuminate the crosshairs.
COLLIMATING TELESCOPE (adjust only the eyepiece). THE OPERATOR
WHO FOCUSES THE CROSSHAIRS SHOULD ALSO PERFORM THE
FOLLOWING ALIGNMENT STEP.
61. Pull out both beam blockers and align the test and reference beams by
! DO NOT ADJUST THE INFINITY FOCUS OF THE
making the beams coincident in the telescope. Align the beams by adjusting
the two screws on the fiber optic mounting plate (mirror mount) on the
interferometer base only! Do not adjust the twiddler or the mirror below the
telescope (mirror #2) when the alcohol pool is in the interferometer; only
adjust the fiber optic mounting plate. Note that both beams move with
respect to the telescope crosshairs as you adjust the mirror mount screws.
3.1.10.FRINGE OPTIMIZING
62. To optimize the fringe signal, the test and reference beams must be made
perfectly coincident and parallel. The two interfering beams should be
perfectly overlapped and also have no angular deviation for the greatest
signal. The translation of the test beam relative to the reference beam is done
by adjusting the translator plate (sometimes called twiddler). The angular
deviation is minimized by adjusting mirror mount # 2, located below the
telescope (see figure 2-8).
63. For convenience, it is possible to move the test and reference beams to the
center of the telescope viewfinder by adjusting mirror # 3, located in front of
the telescope objective. Note this does not affect the interferometer
alignment; it is only for the user’s convenience.
64. Look in the fringe viewer and adjust the twiddler until the test and reference
beams are coincident (overlapped).
3-14
How to Set Up and Run the FG5 3
65. Look in the telescope and adjust mirror # 2 so the test and reference spots are
overlapped. Use the two knobs that are diagonally opposite of each other.
The two beams are now coincident and parallel.
3-15
How to Set Up and Run the FG5 3
66. Connect the ANALOG output on the interferometer to an oscilloscope, with
the following settings:
Scale = 50 mV/div
Sweep = 2 µsec/div
AC coupled input
67. Make sure the laser is locked. Set the dropper to OSC mode and press
RESET/INIT. This moves the cart slowly up and down at a constant velocity
and produces a constant frequency fringe signal which is useful for adjusting
mirror # 2.
68. Maximize the fringe signal on the oscilloscope by adjusting mirror # 2.
69. Maximize the fringe signal on the oscilloscope by adjusting the twiddler.
Nominal fringe signal (peak-to-peak) is 280-360 mV. Record this value in the
system check log.
70. Terminate OSC mode by pressing TRIG on the dropping chamber controller.
This brings the cart safely down the bottom and places the system in RESET
mode. Note that you can press the TRIG button at any point in the cart’s
motion.
3-16
How to Set Up and Run the FG5 3
3.2. Running the FG5
The FG5 begins observations when the user selects Process | Go in the “g”
program.. Consult the FG5 Software Manual for instructions on operating the
program. The software manual also includes information about software
features, gravity corrections, output displays, and input/output file descriptions,
as well as data analysis and trouble shooting.
3.2.1.SYSTEM CONTROLLER
1. Make sure the time and date are correctly set to UT. Do this by doubleclicking on the clock in the Windows© Task Bar.
3. Enter the site information into Process | Setup | Information window in “g”.
4. If automatic laser peak detection is desired, check the 1f signals with a volt
meter and enter the information in to Process | Setup | System | Laser
5. Enter the desired acquisition parameters into Process | Setup | Acquisition
6. Enter the desired environmental/system corrections into Process | Setup |
Control.
NOTE: Make sure the laser is warm and the meter select switch of the WEO
Model 100 laser controller is set to 1f (see WEO manual). After reading the 1f
signals, be sure to reconnect the BNC cable between the “output” connector on
the laser controller and the Channel 3 connector on the patch panel.
3.2.2.DROPPER CONTROLLER
7. Make sure the cart is resting at its bottom position, and the green
RESET/INIT light is on. Set the mode switch to DROP. Press RESET/INIT
twice. The red light next to DROP should be on. The dropper is now waiting
for an external signal from the computer to initiate a drop.
3.2.3.PROGRAM SETUP
3-17
How to Set Up and Run the FG5 3
8. Begin observations by executing Process | Go with the “g” program. Consult
the FG5 Software Manual for more information.
3.3. Shutting Down the FG5
3.3.1.COMPUTER
1. Exit “g”.
2. Backup the data if desired.
3. Shutdown the computer.
3.3.2.SUPERSPRING
3. Turn the COIL switch off..
4. Engage the superspring travel lock. The arrow on the lock know points
down when it is locked and up when it is unlocked.
5. Disconnect the cable from superspring electronics.
3-18
How to Set Up and Run the FG5 3
3.3.3.INTERFEROMETER
3.3.3.1.OPTION 1: Model ML-1 Laser.
6.
a) Set the switch on the front panel of the laser controller to WARMUP.
b) Turn the REMOTE switch off.
c) Turn the key switch off. The HV indicator light should turn off.
d) Turn power switch off.
3.3.3.2.OPTION 2: WEO Model 100 Laser
6.
a) Set servo control to OFF (recommended but not required).
b) Turn the key switch off. The green indicator light should turn off.
c) Turn power switch off.
3.3.4.DROPPING CHAMBER
7. Press RESET on dropper controller.
8. Disconnect the shaft encoder, motor power, and cart control cables from the
dropping chamber.
9. Engage the dropping chamber travel lock.
3.3.5.POWER
10. Turn off any devices that are still on.
11. Turn off the AC and DC power (power supply panel).
3-19
How to Set Up and Run the FG5 3
3.4. Disassembling and Packing the FG5
NOTE: Please follow these instructions carefully. Care in packing the
components properly will result in easier and faster set-up in the field, and will
help protect the instrument from damage.
3.4.1.ELECTRONICS
1. Disconnect the following cables from the electronics rack and components.
• APD power cable (green)
• TTL fringe BNC cable
• Laser signal (umbilical) cable
• Laser HV BNC cable
• AC Mains power cord
• AC computer power cord
2. If the storage/movement of the FG5 is less than about 8 hours, it is
possible to travel the dropping chamber with the ion pump supply’s
internal battery backup. Make sure the DC and HV switches are on, and
pack the ion pump controller inside the dropping chamber travel case.
3. If the storage is longer term, pack the ion pump supply inside of the
dropping chamber travel case, as above, but turn off all the power
switches.
4. Put the remaining cables in the zippered pouch which is attached to the
inside of the rear electronics case lid.
5. Secure the lids to the electronics case.
3.4.2.SYSTEM CONTROLLER
6. Unplug power cords and printer cable from the computer and place them
in the system controller case.
3-20
How to Set Up and Run the FG5 3
7. Close computer lid and place computer in the system controller case.
8. Close the system controller case and secure all latches.
3.4.3.ROTATION MONITOR (IF
INCLUDED)
9. Unplug all BNC and power cables from the rotation monitor, detector box,
and electronics rack and store in the rotation monitor case.
10. Remove the rotation monitor from the dropping chamber (two M6x35
screws) and place in the rotation monitor case.
11. Close the rotation monitor case and secure all latches.
12. Replace the two M6x35 screws in the dropping chamber top flange, and
replace the viewing port cover.
13. Secure the lids to the electronics case.
3-21
How to Set Up and Run the FG5 3
3.4.4.DROPPING CHAMBER
14. Lock the cart by turning the locking hub (motor shaft) counterclockwise
using a 4 mm Allen wrench or ball driver until the cart stops moving.
15. Pull and rotate travel lock knob 90°, allowing the pin to drop onto the hub,
then rotate the locking hub clockwise until the pin engages the hub.
16. Open the three dropping chamber clamps by turning the 5-lobe knobs
fully counterclockwise so the clamps are outside the bottom flange of the
dropping chamber.
17. Gently lift the chamber off the tripod and set it in its case. If the HV and
safety ground cables are still connected, take care not to stress them.
3.4.5.DROPPING CHAMBER TRIPOD
18. Carefully remove the tripod from the interferometer base. Remove the
legs from the tripod. Place the legs in the dropping chamber case along
with the tripod tray and feet.
19. Close dropping chamber case and secure all latches.
3.4.6.INTERFEROMETER BASE
20. Insert the dust plug into the top of the interferometer base.
21. Loosen the four 5-lobe knobs which attach the interferometer base to the
superspring.
22. Remove the interferometer base from the superspring and gently place it
in the shipping case along with the laser. Take care not to stress the fiber
optic.
23. Close the interferometer case and secure all latches.
3-22
How to Set Up and Run the FG5 3
3.4.7.SUPERSPRING
24. Pull out the travel lock brass knob until it engages the locking mechanism,
and rotate the lock 180° clockwise to lock it in place. The arrow on the
lock knob points down when it is locked and up when it is unlocked.
25. Insert the dust plug in the top of the superspring.
26. Loosen the three 5-lobe knobs which attach the superspring to the
superspring tripod.
27. Remove the superspring from the tripod and place in its shipping case.
Note that the bullseye level points toward the air relief valve in the case.
28. Close the superspring shipping case and secure all latches.
3.4.8.TURBO PUMP (IF USED)
29. Make sure the blank flange is in place on the turbo pump intake flange.
30. Make sure all covers are in place on the flexible tube and turbo pump
exhaust flange.
31. Unplug the power cord from the turbo pump.
32. Store the power cord and flexible tube in the base of the turbo pump case.
33. Place the turbo pump in the turbo pump case.
34. Close the turbo pump case and secure all latches.
3-23
Adjustment and Maintenance 4
4. Adjustment and Maintenance
NOTE:
wherever possible. However, some “off-the-shelf” purchased components use
English screws and dimensions.
The FG5 has been engineered to use metric screws and dimensions
4.1. The Dropping Chamber
4.1.1.REPLACEMENTS AND
ADJUSTMENTS
4.1.1.1.Removing The Dropping Chamber Cover
When opening the dropping chamber, take great care not to contaminate the
inside surface of the chamber cover or any of the interior parts. Always wear
clean-room gloves when handling internal parts. If any of the parts are
contaminated, clean the part using accepted vacuum system cleaning procedures
before reassembly. When performing repairs in the field, it is sufficient to wipe
or flush the contaminated parts with alcohol. Whenever possible, vent the
chamber with dry nitrogen rather than air. This will reduce the pump down
time after the chamber is reassembled.
To vent the dropping chamber, remove the clamp and blank flange from the
vacuum valve on the service ring. Loosen the vacuum valve lock ring and
slowly open the valve by rotating the control knob, allowing the chamber to
return to atmospheric pressure. It is best to vent using dry nitrogen, but it can be
directly vented to air. In any case, try to ensure that the gas entering the
chamber is free of particulate matter.
Remove the six screws holding the top flange to the top of the dropping chamber
cover, and remove the flange. Loosen the four snubber lock nuts and back out
the screws which position the top ring of the dropping mechanism within the
4-1
Adjustment and Maintenance 4
chamber cover. Remove the six screws holding the chamber cover and handles
to the service ring, and carefully lift the cover up over the dropping mechanism.
Be sure to protect the O-ring surface on the exposed bottom flange of the
chamber cover, and keep the flange clean.
Note that whenever the chamber is vented to atmosphere, it is a good
opportunity to clean the bottom window of the chamber. It is normal, as the test
mass balls and vees wear, for the window to collect a fine dust of tungsten.
With the chamber vented, but still fully assembled (i.e. cover in place, etc.), travel
lock the cart, and gently tip the chamber on its side. Remove the six Allen screws
that hold the bottom window in place, and remove the window. Clean the
window using pure alcohol and lens paper, inspect the o-ring, and reassemble
the window. Be sure to tighten the screws in a star pattern to equally distribute
the load on the o-ring.
4.1.1.2.Replacing the Dropping Chamber Cover
Inspect the chamber O-ring and sealing surfaces. Coat the O-rings with a very
light film of Apiezon L grease (just enough to make it “shiny”), if necessary, and
re-install the chamber cover and lifting handles. Tighten the mounting screws in
a star pattern.
With the chamber cover mounted, rotate the four snubber screws on the top rod
ring out until they come in contact with the inside of the chamber cover walls.
Tighten the screws equally an additional 1/8 turn and lock in position with the
locking nuts.
Inspect the top flange O-ring and sealing surfaces. Coat the O-rings with a very
light film of Apiezon L grease, if necessary, and mount the top flange to the
chamber cover.
NOTE: Whenever the chamber cover is removed or the support snubbers of the
top rod ring are adjusted, the vertical alignment of the dropping chamber must
be checked and the level bubbles on the tripod tray must be reset. See the section
on “Leveling the Dropper”.
4-2
Adjustment and Maintenance 4
4-3
Adjustment and Maintenance 4
4.1.1.3.Replacing the Drive Belt
Follow the procedures described previously for removing the dropping chamber
cover.
Loosen the belt tension with the tension adjustment set screw located above the
top pulley assembly on the top rod ring. The belt is clamped to the back of the
cart with two socket head screws. Remove the clamp and slide the ends of the
belt off the dowel pin. Remove the belt.
Before installing a new belt, clean both of the pulleys with acetone or alcohol on a
cotton swab. Wipe the new belt with acetone or alcohol to remove any traces of
oil or fingerprints.
Thread new belt around upper and lower pulleys, and place ends over the dowel
pin. A wire with a small hook works well to assist in threading the belt around
the lower drive pulley.
Replace belt clamp, but do not fully tighten. Tension belt while manually
moving the cart up and down to allow the belt to locate its natural position on
the pulleys, then tighten the belt clamp screws.
NOTE: The drive belt may not run exactly in the center of the pulley. This is
normal, but there should be a minimum clearance of 1 mm between the belt and
the side walls of the pulley housing (yoke).
4-4
Adjustment and Maintenance 4
4.1.1.4.Adjusting the Drive Belt Tension
Adjust belt tension using the set screw on the top pulley assembly, located on the
top rod ring. Tighten the belt adjustment screw until the slack has been taken
out and the belt is straight. Then tension the belt by tightening the screw
approximately three turns. If you are uncertain of the proper tension, the screw
can be tightened until the tension spring is just short of coil bind. The belt can
also be tensioned using the torque required to slip the pulley on the belt to
determine belt tension. Use a torque wrench to manually drive the motor shaft
(drive assembly). Set the torque wrench in the 6-7 inch-lb. range and tighten the
set screw until the belt slips on the pulley when rotated into the lower stop.
4.1.1.5.Replacing the Ferrofluidic Vacuum Feedthrough
The chamber must be vented and opened for this procedure. If possible, use dry
nitrogen to vent the chamber.
Loosen the English 4-40 clamp screws on the Helical coupling between the motor
and the Ferrofluidic vacuum feedthrough by reaching through the access hole in
the motor mount. Remove the three ¼-28 English screws which attach the motor
mount to the Conflat vacuum flange on the service ring. Remove the motor
mount assembly (including motor, Helical coupling, travel lock plate, and
encoder) from the Conflat vacuum flange, leaving the Conflat flange and
Ferrofluidic vacuum feedthrough attached to the service ring.
Inside the service ring, loosen the socket head clamp screw on the Helical shaft
coupler where it attaches to the lower drive pulley shaft. Remove the three
remaining Conflat mounting screws. Remove the Conflat with Ferrofluidic
feedthrough attached.
Remove the Helical coupling from the feedthrough, and unscrew the
feedthrough from the Conflat flange.
Lubricate the O-ring on the new Ferrofluidic vacuum feedthrough with a light
coat of Apiezon L grease. Use pliers with padded jaws (e.g. blue pad) to gently
tighten the feedthrough to the Conflat flange.
4-5
Adjustment and Maintenance 4
Reverse the procedure for reassembly.
4.1.1.6.Replacing the V-Plate
The V-plate contains three tungsten V's which support the test mass. Since
removal and insertion of the tungsten V's in the V-plate requires special tools,
this cannot be done in the field. However, an entire new V-plate assembly can be
installed. The dropping chamber must be vented and opened for this procedure
(see “Removing the Dropping Chamber Cover” procedures above).
Remove the bottom drag-free cover from the cart by removing the three M3
screws. Detach the LED bracket from the side of the cart and pull it out of the
way, being careful not to detach or damage the wires connected to it. Remove
the two M3 screws and the threaded post attaching the top drag-free cover to the
cart, observing the position of the post. Gently lift the cover off, being careful not
to damage the wires connected to it.
To remove the V-plate, the test mass must be partially disassembled. First, note
the orientation of the top part of the test mass and the V-plate. It is very
important to replace these parts in the same orientation. Remove the three
beryllium copper M3 screws that secure the top part of the test mass to the three
posts which pass through the V-plate. Now the lower portion of the mass can be
lowered through the V-plate. Remove the six M2 screws holding the V-plate to
the cart.
Reverse the procedure to replace the plate.
The tungsten balls from which the test mass is held are part of the top hat
assembly. If these balls need replacement, send the entire mass assembly to
Micro-g Solutions for installation and rebalancing. Reassemble the test mass
using the three beryllium copper M3 screws, and pack it carefully before
shipping. All the pieces must be included for the balancing to be done correctly.
4-6
Adjustment and Maintenance 4
4.1.1.7.Replacing the Linear Bearings
The chamber must be vented and opened for this procedure. If possible, use dry
nitrogen to vent the chamber.
Remove the drive belt as described previously. Loosen the three M6 screws in
the split clamps on the top rod ring and remove the top rod ring.
Remove the upper bumper stop assembly from the rod by removing both
retaining rings from the rod. Remove the ribbon cable wires connected to the
cart. Remove the ribbon cable clamp on the cart, and gently lift the cart off the
guide rods.
Two retaining rings secure each bearing to the cart. To remove the rings, they
must be wound off the end of each bearing. Slide the linear bearings out of the
cart and slide the new ones in. Replace the retaining rings.
NOTE: Venting holes have been added to the linear bearings by Micro-g
Solutions. In addition, the normal bearing lubricant has been replaced by a
special low vapor pressure oil (Krytox 143AC).
Reverse the above procedure to replace the cart.
4-7
Adjustment and Maintenance 4
4.1.1.8.Replacing the Shaft Bearings—Drive Pulley
Refer to Figure 4-1, Drive pulley assembly. Remove the drive belt as described
previously. Disconnect the Helical coupling between the pulley shaft and the
ferrofluidic vacuum feedthrough. Remove the five screws holding the bottom
rod ring to the bottom flange. Rotate the guide rod structure so that the shaft
clears the service ring and lift the structure. Remove the two screws which fasten
the pulley yoke to the bottom rod ring and remove the yoke. Remove the bowed
retaining ring from the short end of the pulley shaft, noting the orientation of the
bow.
Figure 4-1 Drive Pulley Assembly
!CAUTION: DO NOT DEFORM THE BOWED RETAINING RING.
Remove the retaining ring from the other end of the shaft. Slide out the pulley
shaft, taking care not to lose the Woodruff key, and remove the pulley from the
bearing mounting yoke. Push the bearings out of the yoke.
Reassemble in reverse order. When reassembling the pulley shaft, be sure that
the bowed snap ring is seated fully in its groove.
4-8
Adjustment and Maintenance 4
NOTE: Pulley bearings are specially lubricated with Krytox LVP vacuum grease.
4.1.1.9.Replacing the Shaft Bearings—Top Pulley
The procedure for the top pulley is similar to the drive pulley, except that there is
no Helical coupling and no Woodruff key. To access the top pulley assembly,
remove snap rings on upper bump stop and slide down shaft. Remove two
small retaining rings on top of upper pulley yoke and slide pulley down to
remove.
Reverse procedure for replacement of upper pulley assembly.
When reassembling the pulley shaft, be sure that the bowed snap ring is seated
fully in its groove.
4.1.1.10.Replacing the Rotary Shaft Encoder
Insert a 1/32" Allen wrench into the access hole at the top right (1 o'clock
position) of the encoder. Rotate the shaft until the set screw that holds the disk
to the shaft is aligned with the wrench, and loosen the screw. Pry the main
encoder housing off the encoder back plate with a flat blade screwdriver. The
back plate must remain attached to the travel lock plate with three screws to
maintain proper alignment.
To reassemble, snap the new encoder over the encoder back plate, which was left
attached to the travel lock plate. Tighten the set screw and remove the Allen
wrench from its hole. Rotate shaft to check that the encoder disk is rotating
freely inside the encoder. Loosen and retighten set screw, if necessary, until the
disk rotates freely.
4-9
Adjustment and Maintenance 4
4.1.1.11.Pumping Down the Dropping Chamber
See Appendix B for instructions on pumping down the dropping chamber.
4.2. The Interferometer
4.2.1.ALIGNMENT
1. Set up the superspring and attach the interferometer base as described in the
setup instructions.
2. Center the precision bubble vials mounted on the base of the superspring by
adjusting the leveling feet on the superspring tripod.
3. Turn on the laser.
4. Roughly collimate the laser beam on the ceiling or on a target above the
interferometer base. The collimating assembly is on the output end of the
laser fiber. Use a 2.5 mm Allen wrench to remove the collimating assembly
from the interferometer. Adjust the collimation of the beam by loosening the
locking screw (2.5 mm Allen screw) on the slide of the fiber input and moving
the slide. A properly collimated beam should have a diameter of 6 mm.
5. Slide the loosened assembly back into the interferometer (note the orientation
key) and precisely collimate the laser beam by use of the slide until the beam
size in the collimating telescope is minimized.
6. Tighten the locking screw on the slide. Then tighten the assembly into the
interferometer using the second Allen screw
7. Remove the dust plug on the top of the interferometer base and place the
dropping chamber tripod and dropping chamber on top of the interferometer
(see setup instructions).
8. If possible (both spots visible), verticalize the beam as described in the setup
instructions.
9. Slightly loosen the two screws directly above the fiber optic input on the
interferometer base, just sufficient to allow movement of the fiber optic input
stage.
4-10
Adjustment and Maintenance 4
10. Block the test beam (push in test beam blocker) and pull out the reference
beam blocker.
11. Translate the fiber optic input until the beam is centered in the fringe viewer
and tighten the screws.
12. Focus the telescope crosshairs and adjust mirror # 3 until the reference beam
is centered in the telescope viewfinder.
13. Re-verticalize the beam.
14. Check to see if the reference beam is still centered in the fringe viewer. If the
beam is not centered, repeat steps 8-14.
15. Place the superspring alignment fixture (pigsnout) over the top superspring
window. Orient the alignment fixture so the two alignment holes are parallel
to the telescope axis.
16. Verify that the beam is traveling properly through the superspring by making
sure the beam enters and exits the superspring through the two holes in the
alignment fixture.
17. If the beam is not traveling properly through the superspring, verify that the
reference beam is centered in the fringe viewer, and verticality is correct.
18. Using mirror # 2, make the test and reference beams coincident in the
collimating telescope.
19. Use the translator plate (twiddler) to align the test and reference beams in the
fringe viewer.
20. Remove the dropping chamber and dropping chamber tripod from the
interferometer base.
21. Block the test beam by pushing the beam blocker in.
22. Remove screws that attach the small plate to the back of the interferometer
base. See Figure 4-2, APD alignment adjustment access.
4-11
Adjustment and Maintenance 4
Figure 4-2 APD alignment adjustment access
23. Adjust the X-Y stage of the APD to maximize the voltage at the “analog” BNC
on the interferometer base. This voltage is negative. Adjust the stage to
obtain smallest absolute value. A digital voltmeter or oscilloscope is
recommended for this procedure. In both the X and Y directions, there
should be a “plateau” of stable, less negative voltage. Adjust both the “X”screw (accessed through a hole on the rear of the I.B.) and “Y”-screw
(accessed by “grabbing” the knurled knob of the XY stage) so that the APD is
in the center of both “plateaus”.
24. Replace the rear plate on the interferometer base and tighten all screws.
4-12
Adjustment and Maintenance 4
4.3. The Superspring
4.3.1.REPLACEMENTS AND
ADJUSTMENTS
4.3.1.1.Removing the Superspring Cover
If it is necessary to remove the top flange for any reason, be sure to replace the
flange in the same orientation. Align the large ear of the top flange directly
above the travel lock knob on the service ring.
! DO NOT OPEN THE COVER BY REMOVING THE BOTTOM
FLANGE, DUE TO INTERNAL ELECTRICAL CONNECTIONSTHROUGH THE
SERVICE RING.
The top flange and Superspring cover may be removed as a unit by removing the
six screws holding the cover to the top of the service ring and lifting the cover
assembly straight up.
Most routine maintenance can be accomplished at this point, except for replacing
the coil (linear actuator), the mainspring itself, and its upper hanger. To
accomplish these tasks, the service ring and bottom flange must be removed.
4.3.1.2.Removing the Service Ring
Disconnect the wires between the bulkhead fitting and the circuit board, emitter,
detector, and motor by unplugging the in-line connector closest to the bulkhead
4-13
Adjustment and Maintenance 4
(electrical feedthrough connector), then remove the electrical feedthrough from
the service ring.
! MAKE SURE TO NOTE THE COLOR CODING ON THE
CONNECTORS FOR LATER RECONNECTION.
Remove the travel lock knob by removing the flat-head screw from the center of
the knob and sliding the knob off the shaft. The knob is comprised of three
pieces: the brass knob, the control spring retainer, and the internal spring. With
the knob removed, the two screws holding the travel lock knob assembly are
exposed. Remove these and slide the assembly out of the service ring (the travel
lock to service ring interface is sealed with Teflon sealer).
The service ring and bottom flange can be taken off together by removing the
three screws securing the base plate of the Superspring structure to the bottom
flange.
4.3.1.3.Replacing the Coil (Linear Actuator)
The permanent magnet assembly of the coil is attached to the base plate, and the
voice coil pusher is attached to the emitter/detector block of the support
structure.
Remove the service ring as described previously.
Remove the base plate by loosening the three 3mm barrel clamp screws that
secure the rods to the plate. Carefully lift rod assembly off bottom plate.
Remove two shoulder screws that hold travel lock fork on (from underneath)
and slide travel lock fork assembly off main rod. Unhook three support springs
from O-rings and remove lower triangular spring plate. This plate is held on by
four 3-mm screws. Be careful not to cut or score the O-rings. Note orientation of
emitter-detector block and remove block (six 3mm screws in deep counter bore
holes). Remove the permanent magnet assembly from the base plate.
4-14
Adjustment and Maintenance 4
Remove the voice coil by removing its three attachment screws inside the
emitter-detector block.
Reverse the procedure for reassembly.
4.3.1.4.The Mass Mainspring/Hanger
which is soldered into the Superspring hanger and the coarse adjustment screw.
Mishandling of the mainspring may cause the wire to break, allowing the
hanger, spring, and test mass to drop.
THIS ASSEMBLY.
! The upper spring hanger assembly has a flexible thin wire member
! BE VERY CAREFUL NOT TO BEND THE FILAMENT WIRE IN
It is highly recommended that this procedure be done at Micro-g Solutions.
4.3.1.5.Replacing the Flexures
Necessary tools and fixtures:
• A small table with leveling screws that can hold the Superspring (in the
field, one could use the entire tripod or just the leveling feet).
• A pulling tool (music wire 16” long with m 1.6 threads on one end) to pull
the main spring through the main tube.
• Metric hex wrenches.
• Metric open-end wrenches(m 5.5 and m 6).
• Plastic Gloves: It is best to handle the test mass with plastic gloves.
• Measurement tools (calipers and/or ruler).
4-15
Adjustment and Maintenance 4
Remove the Superspring cover as previously described. Put a dust cover over
top of the main tube to keep dust from falling on the test mass and upwardfacing corner cube (a piece of paper or foil over the tube will suffice).
Remove the service ring as previously described.
Place a shim of hard foam or rubber between top rod ring and top triangular
plate approximately 6mm thick and use a cable tie to fasten the two plates
together. (Limiting the travel of the center tube assembly will help to avoid
damaging the delta-rod flexures during disassembly).
Loosen the three 3mm barrel clamp screws that hold the three rods to the
bottom plate. Disconnect wires from pre amp circuit board and carefully lift rod
assembly off bottom plate. Note : it is easier to do the next steps if the assembly
is blocked up 6’’- 8’’ so one can work underneath the assembly. Remove two
shoulder screws that hold travel lock fork on (from underneath) and slide travel
lock fork assembly off main rod. Unhook three support springs from O-rings
and remove lower triangular spring plate. This plate is held on by four 3-mm
screws. Be careful not to cut or score the O-rings. Note orientation of emitterdetector block and remove block ( six 3mm screws in deep counter bore holes).
Note: The test mass, spring, and flexure will come out with the emitter-detector
block. Wear plastic gloves when handling copper test mass to prevent finger
prints and rubbing off black coating. Remove spring from copper test mass.
Note how far upper anchor is screwed into the main spring (count how many
spring turns are on the screw). Put the lower lock nut onto the new flexure
assembly. Remove broken flexure-anchor and carefully screw new flexureanchor into spring to the original position.
Thread the pulling tool through one nut, top lever and down center tube.
Carefully thread tool into top of flexure (coarse adjustment screw) of main spring
assembly. Pull tool up until spring hangs and then thread copper test mass onto
lower anchor of main spring assembly (tighten lower anchor). Pull wire tool,
lifting test mass ,and guide coarse adjustment screw through top lever. Screw on
top nut to hold in position then remove the tool. (Set approximately in center of
4-16
Adjustment and Maintenance 4
coarse adjustment screw travel). Rotate test mass so beam holes in top lever and
copper test mass roughly match.
4-17
Adjustment and Maintenance 4
4.3.1.6.Assembling the Superspring
Replace emitter-detector block in the original orientation. The emitter (two
terminal device with red/white wires) should be oriented below lever pivots.
Replace lower triangular spring plate and attach support springs. Slide travel
lock fork assembly onto shaft and re-install the two shoulder screws. Replace
spring assembly into lower plate being careful of voice coil . Note: do this on a
flat surface so the rods seat flush with the bottom of the lower plate , then tighten
barrel clamps. Check movement of center tube by blocking up travel lock fork
assembly (so it doesn’t rub), removing wire tie and foam back, then gently
bouncing center tube and checking to see if it moves freely. NOTE: Test mass
must be centered in cage (hanging plumb), and wires must not drag. Replace
spring assembly into service ring. Insert 6mm screws but don’t fully tighten.
Replace the travel lock assembly. Rotate the main spring assembly in the service
ring so that the travel lock fork has equal clearance around the two shoulder
screws, then tighten 6mm screws.
Install electrical feedthrough. Re-connect all electrical connections. Level the
Superspring by viewing on each side of copper mass and centering in cage (place
entire Superspring on an adjustable table to change where the mass hangs). Note
that the mass should hang down approximately 1.5mm from the point where it
makes contact with the top of the cage assembly as the system is travel -locked.
Use the Superspring controller to drive the zero-positioning motor (ZPM) until
the sphere voltage is zero. Measure the gap between the two levers near the
ZPM. Adjust the coarse adjustment screw so that when zeroing the sphere
voltage, the motor will end up in its center of travel position (about 6mm gap
between lever arms ±0.5mm). Note that when you are far off of the detector the
sphere voltage starts at zero and then goes to a maximum and then over a short
range goes through zero (it looks like an S curve). Make sure sphere detector
voltage goes +/- to assure true zero position.
Viewing from top, carefully rotate the corner cube and visually align the beam
holes in top lever with the ones in the copper Test mass, adjust by rotating the
coarse adjustment screw and lock in position by tightening both top and bottom
nuts.
4-18
Adjustment and Maintenance 4
Note 1: This will take time as the Test mass must settle down after each
adjustment.
Note 2: Be careful not to let dust or debris fall onto corner cube. Re-check the
level of the test mass and set level bubbles by turning brass screws. Lock position
with center lock screw. Replace the Superspring cover as previously described.
4.3.1.7.Replacing the Focus Lever Motor
Loosen and remove the nut on the focus adjustment screw. Lower the focus
lever motor assembly off the fulcrum lever.
Loosen the set screw holding the hex bushing to the motor shaft and raise the
bushing out of the way. The screws holding the motor to the motor mount are
now accessible, and can be removed. Reverse the procedure to install the new
motor.
4.3.1.8.Adjusting the Micro-Switches
Adjust the trip positions of the micro-switches that control the limits of travel of
the focus adjustment motor by changing the position of the set screws in the
actuator arms. The limit switches should shut off the zero-positioning motor
(ZPM) when the gap between the focus lever and the fulcrum plate is 4-8mm.
4.3.1.9.The Aneroid Wafer Assembly
To compensate for the thermal expansion and contraction of the Superspring, an
aneroid wafer assembly adjusts the position of the top hanger. The position of
the aneroid assembly is set by the manufacturer, and should not need to be
adjusted.
4-19
Adjustment and Maintenance 4
4.3.1.10.The Delta Rods
Five delta rods (arranged in an upper V-shaped array, and a lower triangular
array) provide a linear way system for the internal support structure of the
Superspring. If a delta rod needs replacement, contact Micro-g Solutions for
parts and procedures.
! THE POSITION OF THE DELTA RODS DETERMINES THE
CLEARANCE BETWEEN THE VOICE COIL (PUSHER) AND THE MAGNET
ASSEMBLY. IT IS IMPERATIVE THAT THE ASSEMBLY NOT RUB OR DRAG!
To field check the alignment, measure the gap between the support
structure pins and the center tube assembly with a feeler gauge. This gap
should be equally spaced (approximately .003-.005”) all around. If the support
pins touch the center tube assembly, a bent delta rod is indicated.
4-20
Adjustment and Maintenance 4
4.4. Timing System and Data Acquisition
4.4.1.TIMING
The timing system consists of the avalanche photo diode (APD) board, the
rubidium oscillator, and the Time Interval Analyzer (TIA). Data are taken by
two different interface cards inside an IBM-compatible Pentium II (or better)
personal computer. The computer processes the data, compiles statistics, and
computes a gravity value, including certain corrections (e.g., for the tides and the
gradient).
Optical fringes are produced in the interferometer by combining the portion of
the laser beam hitting the freely falling and reference retroreflectors (the corner
cubes) with the portion traveling directly through both beam splitters. A fringe
is produced every time the falling object traverses a distance equal to the
wavelength of the laser, lambda, over two (
object changes, the frequency (f) of the fringe signal is swept according to f =
2gt/λ, where g = gravity and t = time. The optical fringes are detected by an
APD mounted in the interferometer base.
λ/2). As the velocity of the falling
The zero-crossings of the fringes provide very good fiducial marks which can be
used for timing. The zero-crossing points of the a.c.-coupled fringe signal are
determined using an ultrafast comparator. The comparator outputs a squarewave version of the frequency-swept fringe signal. The comparator is located on
the APD circuit board.
The TIA scales the fringes using a software-specified scale factor (usually 1000),
then times each scaled fringe. The TIA times events from t=0 at trigger.
The time of occurrence of each scaled fringe and the distance derived from the
number of fringes that have passed can be expressed as a time and distance pair.
The data are then fit to a parabola by the computer to determine a best value for
the acceleration—a gravity value.
4-21
Adjustment and Maintenance 4
4-22
Adjustment and Maintenance 4
4.4.1.1.Avalanche Photo Diode Printed Circuit Board
This circuit detects the optical fringes produced in the interferometer. The FG5
uses a 50-MHz APD which is powered by a high-voltage module mounted inside
the interferometer base.
A high-speed comparator and 50-Ω driver on the APD board minimize noise
problems on the long cables between the interferometer base and the TIA. The
analog and digital versions of the fringe signal are both available on BNC
connectors mounted on the interferometer.
4.4.1.2.APD Board and Photo Diode Supply Module
The APD board is mounted inside the interferometer base, along with the high
voltage power supply module. The APD is mounted directly on the board. The
APD's high-oltage bias is zener-limited to 600 volts (on-board). A
potentiometer is used to set the voltage supplied to the APD.
The analog output is buffered by an OP AMP directly from the APD. The zerocrossing of the fringe signal is determined by the high speed comparator, which
in turn drives a 50-Ω line driver chip. A 40-mV hysteresis is implemented on the
discriminator to avoid multiple triggering. Positive zero-crossings of the fringe
signal are detected and begin the leading edge of the TTL fringe signal. This TTL
fringe signal leaves the board through an SMA connector, and is available
outside the interferometer on a BNC connector labeled TTL. The TTL signal is
used for timing by the gravimeter electronics.
All on-board voltages are derived from the ± 15V supply with linear regulators:
± 6 volts for the APD , and ± 5 volts for the discriminator and 50Ω TTL driver.
4-23
Adjustment and Maintenance 4
APD SIGNALS
CONNECTOR TYPE DESCRIPTION
Digital (SMA) output
TTL fringe signal (50-Ω)
Analog (SMA) output buffered fringe output
HV 6 Pin Amp HV Module
Power 3 Pin Amp power from Lemo connector
*NOTE: The Power Technologies module is supplied from the +15V supply,
and is connected to J3, J5, and J6 of the APD board.
Table 4-1
4.4.1.3.Rubidium Oscillator
The FG5 uses a rubidium oscillator as a frequency standard (atomic clock). The
oscillator generates a 10-MHz sine wave with amplitude of .5Vrms into 50 Ω. It
is used by the TIA card in the personal computer to provide accurate time
information. The clock is located in the power supply mains and is connected to
the TIA card via a BNC cable.
4-24
Adjustment and Maintenance 4
4.4.2.DATA ACQUISITION
4.4.2.1.Computer Interface Cards
The computer running the system is a Pentium II machine (or better). The
combination A/D converter and Parallel Input Output (PIO) board is used for
interfacing the computer with the environmental sensors. The other computer
card is the Time Interval Analyzer (TIA). Inputs for the TIA are listed in the table
below.
TIME INTERVAL ANALYZER (TIA) INPUT SIGNALS
Name Type Destination Description
Channel A BNC
Interferometer
Base “TTL”
FRINGES
Channel B BNC NOT USED
Trigger Arm
(EXT ARM)
External Clock
(EXT CLK)
BNC
BNC
Patch Panel
“trigger out”
Power Supply
“10 MHz”
External Trigger
External Clock
4.5. DROPPER CONTROLLER
The dropper controller is a flexible control circuit (programmable servo
controller) that can direct the motor to servo the cart (and test mass) to a
specified height in the dropping chamber using a rotary shaft encoder, or to a
specific velocity, again using the shaft encoder. The controller can also direct the
motor to track the test mass during free-fall using the sphere detector system.
The dropper controller board uses an EPROM to allow control over the motor
drive signal sources (the shaft encoder and the sphere detector), as well as
programmable offsets (command voltages) for each servo mode. This EPROM
4-25
Adjustment and Maintenance 4
also controls the state-machine clock source, clearing the state-machine counter,
and clearing the shaft encoder.
A second EPROM holds a programmable comparator level used for the setting of
trigger and hold points within the dropping chamber. One bit of the second
EPROM is also used to control the time-out circuitry (a safe-guard that protects
the motor and the test apparatus).
Eight bits address the two EPROM’s, giving a total of 256 programmable states.
One bit is reserved for a fail/standby state, reducing the system to 128 nonstandby states. These remaining states are subdivided into modes defined using
three bits of latched data (a total possibility of eight modes). Each mode can
have an associated four-bit state-machine cycle (sixteen possible states). A
counter which can be clocked by an external signal (the computer), by a window
comparator level, or by a programmable reference level in the dropping chamber
controls the latter four bits. These clock choices are stored in the first EPROM.
This architecture allows flexibility to program many different modes of
operation. Each mode can be associated with a programmable cyclic statemachine. The circuitry also allows programmable digital set points for critical
positions such as launch points or hold points. The set points are well-controlled
against environmental variables.
The controller circuit board currently supports four different modes of
operation: STANDBY, MANUAL, OSCILLATE and DROP.
4-26
Adjustment and Maintenance 4
4.5.1.DROPPER CONTROL MODES
4.5.1.1.STANDBY
4.5.1.1.1.To Select
This mode is chosen upon power-up, when the front panel RESET button is
pressed, or when a time-out has occurred (usually indicating failure).
4.5.1.1.2.Function
The controller is in standby. The motor is turned off.
4.5.1.1.3.To Deselect
Press the front panel INIT button.
(Note: The initialize function can also be executed remotely through the INIT
BNC connector.)
! WARNING: IT IS DANGEROUS TO ALLOW THE COMPUTER
REPEATED INITIALIZE CAPABILITY. THE SYSTEM CAN ENTER THIS
STATE THROUGH TIME-OUT, WHICH MAY INDICATE SYSTEM FAILURE.
4.5.1.2.MANUAL
4.5.1.2.1.To Select:
THIS MODE SHOULD NOT NORMALLY BE NEEDED BY THE USER AND
SHOULD NOT BE USED WITHOUT FULL UNDERSTANDING OF THE
CONTROLLER!
This mode is selected by turning the front panel selector switch to MANUAL and
pressing the INIT button.
4.5.1.2.2.Function:
The front panel potentiometer controls a servo position for the cart. The cart
servo will not initiate until the trimpot is within a predefined window of the
actual cart position. This will time out after 20-30 seconds if the time out is
enabled (default).
IF THE TIME OUT IS DISABLED, BE CAREFUL TO NOT BURN OUT THE
MOTOR BY DRIVING IT INTO THE STOPS AT THE TOP OR BOTTOM.
4.5.1.3.DROP
4.5.1.3.1.To Select:
Set the selector to AUTO. Press the INIT button to initiate the drop mode.
4-27
Adjustment and Maintenance 4
4.5.1.3.2.Function:
The DROP mode has six states:
1. Standby: The motor is off and the system waits for a trigger signal from
the computer or from the internal timer (selected on front panel).
2. Lift: The cart servos to a constant lift velocity until the cart comes within
the window of the hold position.
3. Hold: The cart servos to a constant hold position.
4. Track: The cart servos to track the test mass with a separation of about 3
mm. This is adjustable.
5. Soft-catch: The cart tracks the test mass with a very slight separation.
6. Catch: The cart servos to a decreasing ramp velocity servo until about 5
mm. At this point the state-machine returns to the standby mode (1).
4.5.1.4.OSCILLATE
4.5.1.4.1.To Select:
Press INIT while the selector switch is set to OSC.
4.5.1.4.2.Function:
This mode causes the cart to move up and down smoothly (with a
constant velocity) in the dropping chamber. It is very useful for generating
fringes. DO NOT LEAVE IN OSC MODE UNATTENDED! Cumulative belt slip
can cause the cart to drive into the bottom or top of the dropping chamber and
burn out the motor.
4.5.2.Analog Servo
The analog servo has three different sections. They are: Cart-position, Cartvelocity, and Sphere-position. The cart servos use the rotary shaft encoder as a
position/velocity sensor. The sphere servo uses the optical sensor mounted on
the cart.
4-28
Adjustment and Maintenance 4
4.5.2.1.Cart-Position
The cart position is given by an optical shaft encoder that is mounted on the
motor shaft. The base resolution is 500 counts per revolution. The outputs are
two quadrature signals which give information about the amount and direction
of shaft rotation.
The shaft encoder quadrature outputs are preconditioned by a custom
programmed gate array logic (GAL) chip called the AXQD2X. The outputs of the
AXQD2X are glitch-free clock pulses and an up/down bit. The resolution of the
shaft encoder is multiplied by two, giving 1000 counts per revolution. The
AXQD2X also has logic that helps clear the counters and keeps them from an
overflow or underflow condition.
The sixteen-bit counters feed a twelve-bit digital to analog chip (DAC) for use in
the analog servo. The top seven bits are also fed to a comparator that is used for
level settings that trigger different phases of the servo. The output of the DAC is
available on the front panel BNC called CART position.
The DAC output has a programmable offset corresponding to either (1) the
manual position controlled by the front panel knob, or (2) the hold position
trimpot on the controller board. The servo adds in some derivative or damping
that is set by the CART DAMP trimpot on the PC-board. The overall gain is set
by the HOLD GAIN trimpot on the PC-board.
4.5.2.2.Cart Velocity
The servo takes the cart position derivative. A velocity lead trimpot on the PCboard adds phase margin which tends to speed up or damp the servo in the
velocity mode. Servos that measure position and control velocity tend to have a
slow exponential response without this precaution.
A programmable reference voltage is added to the velocity signal. The servo
makes the actual cart velocity track these reference voltages. The four references
are called: throw, lift, soft throw, and catch.
The throw and lift are constant offsets set by trim pots on the circuit board.
4-29
Adjustment and Maintenance 4
The soft throw and catch are both linear voltage ramps whose slopes are
controlled with trim pots labeled SOFT THROW RAMP and CATCH RAMP.
Each reference voltage also allows an offset to be added using the trim pots
labeled THROW OFFSET and CATCH OFFSET. These ramps are used to
accelerate the cart during the throw phase and decelerate the cart during the
catch phase.
The gain for the velocity servo is set by the VELOCITY GAIN trimpot on the PCboard.
4.5.2.3.Sphere Servo
The sensor for this servo is an LED focused spot-sensor. The operational
principle is a simple optical lever arrangement. An LED and linear detector are
mounted on opposite sides of the cart. A spherical lens mounted on the test
mass focuses the LED onto the linear detector, giving relative position between
the cart and test mass. The signal is preconditioned by a preamplifier mounted
on the cart. The position can be monitored on a front panel BNC labeled
SPHERE.
The servo consists of an active feedback servo and a passive feedforward servo.
The feedforward provides the approximate correct motor voltage during the
drop.. This reduces the demands on the feedback servo.
4.5.2.4.Active Sphere Servo
The sphere signal is added with an offset called height and another offset called
hover height. The height offset is always added to provide a tracking difference
between the cart and the test mass during free-fall. The hover height is switched
in by the EPROM and gives an overall reduced offset which makes the cart track
the test mass very close to the rest position. This is useful for catching the object
near the bottom of the drop.
The sphere signal (sphere and offsets) servo uses a proportional and a first
derivative term. The proportional term is controlled with a trimpot called
4-30
Adjustment and Maintenance 4
SPHERE GAIN. The derivative term is controlled with the trimpot called
SPHERE DAMPING.
4.5.2.5.Feedforward Sphere Servo
This servo is made with a ramp waveform. The slope of the ramp is given by the
trimpot called RAMP SLOPE. The zero point, which should be at the top of the
drop or throw, is set by the ramp offset
4.6. Superspring Controller
The Superspring electronics comprise a system for locating the Superspring mass
relative to the support housing, a motor-driven mechanical lever system for
raising and lowering the mainspring, and an electromagnetic coil that enhances
the natural frequency of the mainspring by 2000 to 3000 percent.
Inside the Superspring are an SE-3455 infrared LED, a photo detector, and a
sphere signal preamplifier. An infrared beam from the LED shines through an
optical glass sphere attached to the bottom of the Superspring mass, which is
suspended from the end of mainspring. The sphere focuses the beam on the
photo detector. The detector outputs a signal to the preamplifier that indicates
the position of the Superspring mass in relation to the mainspring support
system.
4.6.1.1.SUPERSPRING CONNECTIONS
Name TYPE Destination Name
Front Panel:
SPHERE OUT BNC BNC16 2H
COIL OUT BNC NC
Back Panel:
COMP OUT BNC NC
4-31
Adjustment and Maintenance 4
NOISE INJECT BNC NC
Table 4-2 Superspring Connections
The preamplifier relays the signal to the Superspring controller, which controls a
motor. The motor drives a lever system that raises and lowers the mass and
mainspring. Two micro-switches and diodes keep the motor from raising or
lowering the mass beyond a specified range.
A linear actuator coil and magnet system pushes and pulls the mainspring
support structure to track the Superspring mass.
The Superspring controller has six main parts:
1. LED driver
2. Sphere signal buffer
3. Window comparator
4. Motor driver
5. Active filter
6. Coil driver
The LED driver supplies a constant current to drive the LED in the Superspring
can. This current can be adjusted internally using the potentiometer (pot).
The sphere signal buffer buffers the signal from the Superspring can so it can be
routed to different parts of the board. The Sphere Detector Out BNC can be low
pass filtered at 1.26 Hz (FILTER ON).
The window comparator checks the LP-filtered sphere signal to see if it is within
the preset range. If the signal is within the window the Range Status will be
"High;" if the signal is out of the window the Range Status will be "Low."
4-32
Adjustment and Maintenance 4
4.6.1.2.MOTOR DRIVE SELECTION
The motor driver circuit can be selected by an external switch with five positions:
OFF, WINDOW, AUTO, REMOTE, and MANUAL. This controls a DC motor
which can position the top of the main spring relative to the main bracket. The
position may need to be adjusted as the main spring slowly stretches over time,
or due to temperature or gravity changes. The DC motor can lift (lower) the test
mass so that the sphere position becomes more positive (negative). This motor
should only be used to position the mass but should not be activated during the
Superspring operation as a long period isolation device.
4.6.1.2.1.OFF
This setting disables the DC motor that positions the top of the main spring. The
Superspring should be left in this mode when the active servo is activated.
4.6.1.2.2.AUTO
The Superspring controller will try drive the DC motor using a feedback loop so
that the sphere signal is zero.
4.6.1.2.3.WINDOW
The Superspring controller will activate the motor when the sphere voltage is out
of range (larger than the window setting). The motor drives the sphere voltage to
zero and then is turned off. In principal the Superspring can be left in this mode
but it could re-zero during a measurement causing excess noise.
4.6.1.2.4.REMOTE
The Superspring can be made to zero using an external TTL signal.
4.6.1.2.5.MANUAL
In manual mode, the spring motor is controlled by the potentiometer (pot) on the
front panel. A setting greater than 5 will move the spring up less than 5 will
lower the spring. Moving the spring up will case the BNC Sphere out to move in
a positive direction while down will move voltage output in a negative direction.
4.6.1.3.Active Servo
The filter is a band pass accomplished with a high pass (HP) and then a low pass
(LP) filter. The Q-value and frequency of each filter is adjustable via the onboard trim pots. The phase lead and lag and the servo gain are also adjustable.
2
2
Nelson P.G., An Active Vibration Isolation System for Inertial Reference and Precision Measurement,
Rev. Sci. Instrum., 1991, 62, 2069-2075.
4-33
Adjustment and Maintenance 4
4.6.1.4.Coil Drive (Current Driver)
The filter section feeds the coil driver which produces a current for the coil using
a high power op amp. The signal feeding the coil driver can be viewed at COMP
OUT. The signal returning from the coil can be monitored at COIL. The coil
driver can be engaged with the COIL switch. You can also insert a signal to the
coil at the Noise Inject BNC on the back panel.
4.6.2.Maintenance Schedule
To keep the instrument at its optimum performance level proper maintenance
procedures should be periodically performed.
4.6.2.1.Annually
Inspect, replace, and adjust the cart drive belt.
Inspect, replace, and lubricate the drive system bearings.
Inspect and replace the tungsten support contacts on the test mass and cart.
Reassemble the vacuum system and test for leaks. Pump down and bake out the
vacuum system.
Test and adjust the system electronics.
Clean and align the optics.
4.6.2.2.Semi-annually
Calibrate the ML-1 He-Ne laser to an iodine standard.
Calibrate the rubidium oscillator frequency.
4-34
Troubleshooting 5
5. Troubleshooting
This section is intended as a rough guide to identification of system problems
based on observed symptoms. There will be cases where the observed symptom
is not described here and other times where the correct cause is not listed in this
chapter because of the complexity of the mechanical, electrical, optical, and
software relationships. In these cases, please contact Micro-g for a better
diagnosis. Also any new symptoms/causes that are observed should be reported
so that future versions of this manual can be updated.
5.1. The FG5 System
The FG5 absolute gravimeter is an extremely sensitive instrument which must be
set up and operated with the utmost care to achieve optimum results. It is
especially important to maintain a system check log during set up and
observation. Conscientious recording of system information (e.g. laser power,
ambient temperature and atmospheric pressure, Superspring position, ion pump
current, fringe signal amplitude, dropping chamber and Superspring level
positions, etc.) can be extremely helpful in diagnosing problems. The Micro-g
Solutions Environmental Sensor Package is highly recommended for monitoring
system status, because it automatically records ambient temperature and local
atmospheric pressure during observations. It is also helpful to maintain a log of
operating hours for the system. This will assist the operators in performing
preventative maintenance (e.g. cleaning laser or interferometer optics, replacing
the ion pump, etc.)
5-1
Troubleshooting 5
5.2. System Problems
5.2.1.DROPPING CHAMBER
It is extremely important that all dropping chamber systems (dropper
servo, vacuum, etc.) operate properly to achieve the best results possible. Table 51 lists problems related to the dropping along with possible solutions.
Problem Solution
Dropping chamber will not drop Travel lock engaged
Controller not initiated
Current at portable ion pump power
increases after each drop.
Ferrofluidic rotary feedthrough
needs to be replaced.
Vacuum level (ion pump current)
consistently high.
OSC mode does not operate
continuously, and cart drives into
lower spring
Noisy catch. Catch offset and ramp need to be
This can be caused by a small leak
in the dropping chamber (usually
at an O-ring seal). Also, the ion
pump may need to be replaced.
Leak check the dropping chamber.
If no leaks are found, replace the
ion pump. The life of the Ion Pump
is rated as 20,000 hours at 1 x 10
torr (continuous operation).
However, ion pump life is usually
much less if the pump is shut off
and restarted often.
Velocity servo needs to be adjusted
adjusted
-6
5-2
Troubleshooting 5
Hum at the top of the drop. Hold servo gain needs to be
reduced.
Shaft encoder is dirty and needs to
be blown clean with high pressure
air.
Upward or downward trend in gravity
values (time series plot).
Ion pump is not turned on.
Time or date on PC controller
have been set incorrectly.
Tide correction flag has not been
set in the Process | Setup |
Control.
Latitude or longitude have been
entered incorrectly in Process |
Setup | Information.
Set up on non-rigid floor and
verticality of the beam is drifting.
Single clicking sound during lift and
drop.
Drive belt developing fracture and
needs to be replaced.
Stops after one drop and resets itself Drive belt could be loose and may
need to be tightened.
Multiple clicking sounds during lift
and drop.
Drive bearing needs to be
replaced.
Cart drives to the top very fast No shaft encoder signal. Check
cable connection. Check that the
cart voltage changes as you
manually lift the mass (using a hex
wrench).
Very bad sound during drop Sphere signal is not present
because of the cable is not
connected or a there is a bad
cable/connector. Look for a sphere
signal that changes as the mass is
travel locked.
5-3
Troubleshooting 5
Sharp spikes (faster than 1ms) on the
sphere signal during drop with a bad
sound during drop
Many bad drops occur with extremely
high residuals. [High number of dropouts]
Table 5-1 Dropping Chamber Problems and Solutions
Bad cable/connector
The cart travel is not aligned with
the vertical. The bubble levels on
the tripod need to be reset or the
dropping chamber is not sitting in
the tripod tray correctly.
Dropper guide rods are not
straight. Can be due to snubbers
tightened asymmetrically or
because of stress from bad
alignment of rods in the top ring.
5-4
Troubleshooting 5
5.2.2.INTERFEROMETER AND LASER
To achieve the best possible results, the interferometer optics must be clean and
correctly aligned. In some cases, it is possible to detect alignment problems
during setup. The operator should always be extremely observant while leveling
the interferometer, and adjusting beam coincidence and verticality so any
problems in misalignment can be detected and corrected before observations
begin. Table 5-2 gives a list of problems and possible solutions which are related
to the interferometer.
Problem Solution
WEO laser not producing beam Check cable connections. If the
front panel show no sign of power
check the fuse and power switches.
WEO laser power too low Laser optics need cleaning.
Alignment of back laser mirror
should be adjusted.
Iodine cell needs rotation
adjustment to maximize beam.
Laser tube needs to be replaced.
Fiber alignment needs to be
optimized
WEO laser will not lock Ambient temperature too high
(30C)
Laser not warmed up
Laser sweep turned on
Incorrect toroid voltage setting
Large systematic signal in the leastsquares residual plot.
Interferometer base and dropper
tripod are mechanically coupled.
The top interferometer plates are
not fastened.
5-5
Troubleshooting 5
Beam spot viewed in T2-telescope is
larger than normal
Test beam viewed in periscope has flat
side (not circular).
Cannot obtain >200mv amplitude on
analog fringe output with alignment
optimized.
T2-telescope infinity focus needs
calibration.
Beam expander is not set correctly
for collimation.
Optics in beam path need
cleaning.
Alcohol pool dirty. Shake pool or
replace alcohol.
Dropper not aligned properly
with interferometer.
Superspring not leveled or not
aligned properly with
interferometer.
Rotate λ/4plate lens in front of
faraday isolator to increase beam
intensity.
Analog fringe output clipped with
alignment optimized.
Intentionally mis-align the input
into the fiber optic. Set fringes at
about 340 mV
Larger than expected single drop
distributions or multi-node histogram
on computer display or simply too
large of a scatter at a quiet site
(100µGal)
This can be due to the software
incorrectly detecting a peak hop.
Rerunning the data without the
peak detect should solve the
problem in replay. The problem
could be due to the 1f signal is not
hooked up correctly or the time
constant is set to FLAT. The time
constant should be set to 1s.
Data drop-outs Laser not locked (possibly due to
mechanical vibrations, air flow on
instrument, or optical feedback)
5-6
Troubleshooting 5
Large residual amplitude Check parameter file, do not fit
past point 170
Bad cables, especially PIO ribbon
cable
Bad grounding of fringe signal
10nm residual frequency swept Wrong laser type selected
Incorrect laser modulation
frequency entered in the
fg5param.dat file
No fringes at all No power to APD.
One/both beam blockers are
pushed in
Very bad alignment of the
test/reference beams. Look into
the telescope and make sure that
the test and reference beams are
overlapped. Also make sure that
the beams are overlapped in the
fringe viewer with the twiddler
Fringe amplitude varies as the cart is
moved up or down.
Table 5-2 Interferometer Problems and Solutions
The iodine-stabilized laser is
running multi-mode. This can
indicate the laser servo gain being
set too high.
5-7
Troubleshooting 5
5.2.3.SUPERSPRING
A Superspring problem usually results in gravity data with a large drop-to-drop
scatter. A chart recorder is very helpful in monitoring the status of the
Superspring. A two channel recorder can monitor both sphere and coil output.
However, if a one channel recorder is used, it is best to monitor coil output. A
plot of the coil output can also be helpful in identifying seismic activity or
unusually large environmental noise, which produce a larger than average dropto-drop scatter. During setup, the operator should exercise care in leveling the
Superspring. Also, if the change in gravity from one site to the next is very large,
it is common for the test mass to be out of range (as indicated by an LED on the
front panel of the Superspring controller). If the change in gravity from the most
recent station to the present one is positive and the sphere is out of range, the
spring will be low and should be driven up into range. If the change in gravity is
negative, the spring should be driven down into range. It is always very
important to monitor the voltage at the front panel BNC of the Superspring
controller marked SPHERE OUT while the servo loop is still open. The sphere
position should be moving substantially (+/- voltage) when the loop is open and
the Superspring is unlocked. This indicates that the test mass is hanging freely.
See Chapter 3, paragraph 53-54 for more details. Table 5-3 lists possible
problems and solutions pertaining to the Superspring.
Problem Solution
Test mass can be driven in one
direction but cannot be adjusted in the
other direction.
Sphere shows no oscillations when
unlocked and cannot be driven into
range.
Spring sphere signal looks quiet but the
gravity data is still noisy as if the
Superspring is not isolating.
Limit switch is limiting the motion.
The solution is either to widen the
range for the limit switch or adjust
the coarse position of the main
spring using the nut on the top of
the flexure.
Upper flexure on main spring is
broken and needs replacement or a
delta rod bent.
The mass is not in range of the
detector.
The Superspring is still locked.
5-8
Troubleshooting 5
Spring damps suddenly after
unlocking.
Spring is off level, check bubble
levels for proper level.
Spring is out of range and is
hitting upper or lower physical
limit of the spring.
Horizontal flexure ( delta rod )has
been damaged and needs
replacement.
The lock on the controller is
locked.
Gravity is extremely noisy (mGal) It is possible that there is an
earthquake event occurring
somewhere on earth. The
Superspring is very sensitive to
earthquakes no matter where they
occur. The gravity will continue to
be noisy for hours or even days
when the earthquake event is close
to the gravimeter location.
Table 5-3 Superspring Problems and Solutions
5-9
Troubleshooting 5
5.2.4.SYSTEM CONTROLLER
Since the system controller performs real time gravity
computations/corrections, records data, and initiates the dropping sequence, a
failure of this component results in a catastrophic system failure. Table 5-4 gives
a list of problems and possible solutions for the system controller. The primary
source of all problems encountered with the controller units have been with poor
cable connections or poor internal connections of the cards to the computer.
Problem Solution
“g” responds with “Time Out: Check
Trigger, Fringe Amplitude, TTL
Cable” message.
General message indicating no
fringe data arrived at the
computer. Check for alcohol pool
in the I.B., beam blocker pushed in,
TTL cable to computer, TRIG cable
from dropper controller to
computer. Check that the fringe
amplitude is >250 mV. Check that
the dropper is functioning in the
DROP mode.
Gravity value is wrong Check all gravity corrections
especially vertical transfer to the
specified height. Check
latitude/longitude and date/time.
Check laser peak lock
hardware/software
Table 5-4 System Controller Problems and Solutions
5-10
Loading...