Guralp Systems CMG-5T User Manual
CMG-5T Compact
Compact Triaxial Accelerometer
Ope rator's guide
Part No. MAN-050-0004
Designed and manufactured by
Güralp Systems Limited
3 Midas House, Calleva Park
Aldermaston RG7 8EA
England
Proprietary Notice: The information in this manual is
proprietary to Güralp Systems Limited and may not be
copied or distributed outside the approved recipient's
organisation without the approval of Güralp Systems
Limited. Güralp Systems Limited shall not be liable for
technical or editorial errors or omissions made herein,
nor for incidental or consequential damages resulting
from the furnishing, performance, or usage of this
material.
Issue E 2011-05-05
CMG-5T Compact
Table of Contents
1 Introduction............................................................................................................... 3
2 Installation................................................................................................................. 5
2.1 Unpacking and packing...................................................................................... 5
2.2 Initial testing.......................................................................................................6
2.3 Installing the sensor........................................................................................... 7
2.3.1 Temporary installations..............................................................................8
2.3.2 Installation in Hazardous environments....................................................8
2.4 Removing offsets from the 5T Compact ............................................................9
2.4.1 Manual offset removal.................................................................................9
2.4.2 Removing offsets using a terminal connection........................................10
2.4.3 Removing offsets using a third party digitisers........................................11
2.5 The hand-held control unit..............................................................................11
2.5.1 Signal meter...............................................................................................11
2.5.2 Calibration................................................................................................. 12
2.5.3 Open-loop response...................................................................................12
2.5.4 Banana sockets.......................................................................................... 12
3 Calibration............................................................................................................... 13
3.1 Absolute calibration.........................................................................................13
3.2 Relative calibration........................................................................................... 13
3.3 Calibrating accelerometers............................................................................... 14
3.3.1 Open-loop calibration...............................................................................17
3.4 The calibration pack.........................................................................................17
3.4.1 The calibration sheet.................................................................................17
3.4.2 Frequency response...................................................................................18
3.4.3 Obtaining copies of the calibration pack..................................................19
4 Inside the 5T Compact............................................................................................ 20
4.1 The force transducer......................................................................................... 21
4.1.1 The sensor transfer function.....................................................................21
4.2 Electrical connections......................................................................................22
5 Connector pinout..................................................................................................... 24
6 Specifications.......................................................................................................... 26
7 Revision history....................................................................................................... 27
2 Issue E
Operator's Guide
1 Introduction
The CMG-5T Compact is a three-axis, strong-motion, force-feedback
accelerometer in a sealed case. The accelerometer is self-contained
except for its 10 – 36V power supply, which can be provided through the
same cable that carries the analogue data. An internal DC–DC converter
ensures that the sensor is completely electrically isolated. Optionally,
this converter can be omitted, in which case an external ±12 Volt DC
three-way or 'dual rail' power supply is required. Sensors with this
option have substantially lower power requirements than those fitted
with the DC–DC converter.
The 5T Compact system combines low-noise components with high
feedback loop gain to provide a linear, precision transducer with a very
large dynamic range. In order to exploit the entire dynamic range, two
separate outputs - high and low gain - are provided. Nominally, the high
gain outputs are set to output a signal 10 times stronger than the low
gain outputs.
The 5T Compact sensor outputs are all differential with an output
impedance of 47 Ω. A single signal ground line is provided as a return
line for all the sensor outputs.
Full-scale low-gain sensitivity is available from 4.0g down to 0.1g. The
most common configuration is for the 5T Compact unit to output 5 V
May 2011 3
CMG-5T Compact
single-ended output for 1 g (≈ 9.81 m/s2) input acceleration. The
standard frequency pass band is flat to acceleration from DC to 100 Hz
(although other low pass corners from 50 Hz to 100 Hz can be ordered.)
A high frequency option provides flat response from DC to 200 Hz.
Each accelerometer is delivered
with a detailed calibration sheet
showing its serial number,
measured frequency response,
sensor DC calibration levels and
the transfer function in
poles/zeroes notation.
Installation is simple, using a
single fixing bolt to attach the
sensor to a hard surface. If
required, you can also level the
sensor using its two adjustable
levelling feet.
Optionally, you can use a Güralp Hand-held Control Unit (HCU) and
breakout box to distribute power and calibration signals to the sensor
and to receive the signals it produces. It is available in standard, rackmounted and water-resistant portable formats.
The accelerometer housing
itself is completely
waterproof, with a hard
anodised aluminium body
and “O”-ring seals
throughout. CMG-5T
Compact instruments have
been tested for long periods
of total immersion in water.
4 Issue E
Operator's Guide
2 Installation
2.1 Unpacking and packing
The CMG-5T Compact accelerometer is delivered in a single cardboard
box with foam rubber lining.
The packaging is specifically designed for the 5T Compact and should be
re-used whenever you need to transport the sensor. Please note any
damage to the packaging when you receive the equipment and unpack
on a clean surface. The package should contain:
• the accelerometer,
• a signal connection cable (if ordered),
• a 26-pin connector (unless fitted to the supplied cable).
Place the accelerometer on a clean surface and identify:
• the signal cable connector on the top of the unit,
• the N/S orientation line, engraved on the lid,
• the bubble level,
• the screw-on cover for the output offset adjuster (see Section 2.4
on page 9),
• the central hole for the main fixing bolt and
• the serial number.
If you need to request the sensor production history, you will need to
quote either the serial number of the sensor or the works order number,
May 2011 5
CMG-5T Compact
which is also provided on the calibration sheet.
2.2 Initial testing
To test the 5T Compact before installation, you will need a power source
which can deliver >100 mA at 10 to 36 V to cover initial transients and a
digital voltmeter (DVM) with 1 and 10 V ranges. Also ensure that the
supplied cable is connected with the correct polarity.
To make it easier to measure the output from the sensor, you can use a
Hand-held Control Unit or an improvised interface box, which can be
manufactured from a screw clamp connector block. This will simplify
the connections to the appropriate connector pin outputs.
1. Place the 5T Compact sensor on a flat, horizontal surface.
2. Connect the power supply, observing the correct polarity for the
cable supplied and switch on.
3. Connect the voltmeter to pins A and B of the output connector
(corresponding to the low gain vertical component). Measure the
output of the low gain vertical component. The steady output
voltage should be about zero (± 10 mV).
4. Repeat the measurement for the N/S and E/W low-gain component
outputs (pins C/D and E/F respectively).
5. Now turn the sensor on its side, propping it carefully to stop it
rolling.
6. The low gain vertical component should now read about –5 V,
corresponding to –1g.0
7. Roll the instrument until the N/S line is vertical, with N at the top.
8. The low gain N/S component should now read +5 V,
corresponding to +1g.
9. Roll the instrument until the N/S line is horizontal.
10.The low gain E/W component should now read +5 V.
If the performance so far has been as expected, the instrument may be
assumed to be in working order and you may proceed to install the unit
for trial recording tests. In many cases, there will be a slight offset to the
readings. This can be compensated for after installation, by adjusting
potentiometers (see Section 2.4 on page 9),
6 Issue E
Operator's Guide
2.3 Installing the sensor
You will need a hard, clean surface such as a concrete floor, to install the
5T Compact.
If you are in any doubt about how to install the sensor, you should
contact Güralp Systems.
1. Prepare the surface by scribing a N/S orientation line and
installing a grouted-in fixing bolt on the line, near the middle. A
6mm (0.25 inch) threaded stud is suitable, as is an expanding-nut
rock bolt or anchor terminating in a threaded stud. The bolt
should be about 120mm (5 inches) long.
2. Place the accelerometer over the fixing bolt and rotate to bring the
orientation line and studs accurately into registration with the
scribed base-line.
3. Level the sensor, using its adjustable feet, until the bubble lies
entirely within the inner circle of the level indicator.
The feet are mounted on screw threads. To adjust the height of a
foot, turn the brass locking nut anticlockwise to loosen it and
rotate the entire foot so that it screws either in or out. When you
are happy with the height, tighten the brass locking nut clockwise
to secure the foot. The locking nut must be tightened so that it
moves to the bottom of its travel: although the foot will also be
locked if the nut is at the top of its travel, performance will be
reduced.
The instrument can use internal simulated level adjustment to
compensate for tilt, as long as it is fixed within 1 ° of the
horizontal.
4. Secure the accelerometer in place using a fixing nut with spring
washer. Do not screw down the instrument too tightly, as the
casing may be deformed.
5. If required, make a screening box for the sensor to shield it from
draughts and sharp changes of temperature. A suitable box can be
constructed from expanded polystyrene slabs (e.g. 5 cm building
May 2011 7
CMG-5T Compact
insulation slabs) with the
joints between them sealed.
A hole should be drilled for
the connector. You can then
use high-grade glass fibre
sealing tape to fix the leads in
position and to fasten the box
securely to the mounting
surface. Commercially
available duct sealing tape is
ideal for this purpose.
6. Connect the sensor to your digitizing equipment or Hand-held
Control Unit to start receiving signals.
2.3.1 Temporary installations
5T Compact sensors are ideal for monitoring vibrations at field sites,
owing to their ruggedness, high sensitivity and ease of deployment.
Temporary installations will usually be in hand-dug pits or machineaugered holes. Once a level base is made in the floor, the
accelerometer can be sited there and covered with a box or bucket.
One way to produce a level base is to use a hard-setting liquid:
1. Prepare a quick-setting cement/sand mixture and pour it into the
hole.
2. “Puddle” the cement by vibrating it until it is fully liquefied,
allowing its surface to level out.
3. Depending on the temperature and type of cement used, the
mixture will set over the next 2 to 12 hours.
4. Install the sensor as above, then cover and back-fill the
emplacement with soil, sand, or polystyrene beads.
5. Cover the hole with a turf-capped board to exclude wind noise
and to provide a stable thermal environment.
If you prefer, you can use quicker-setting plaster or polyester mixtures
to provide a mounting surface. However, you must take care to
prevent the liquid leaking away by “proofing” the hole beforehand.
Dental plaster, or similar mixtures, may need reinforcing with sacking
or muslin.
2.3.2 Installation in Hazardous environments
The fully enclosed, aluminium case design of the 5T Compact
8 Issue E
Operator's Guide
instrument makes it suitable for use in hazardous environments where
electrical discharges due to the build up of static charge could lead to the
ignition of flammable gasses. To ensure safe operation in these
conditions, the metal case of the instrument must be electrically bonded
('earthed') to the structure on which it is mounted, forming a path to
safely discharge static charge.
Where electrical bonding ('earthing') is required during the installation of
a 5T Compact instrument, the central mounting hole that extends
through the instrument should be used as the connection point. This is
electrically connected to all other parts of the sensor case. Connection
can be made by either a cable from a local earthing point terminated in a
8mm ring tag or by the mounting bolt itself.
2.4 Removing offsets from the 5T Compact
Once installed the sensor needs to be as level as possible to ensure the
output offset is minimised. Levelling is carried out by adjusting the feet
until the bubble level on the top of the sensor has the bubble completely
within the scribed ring.
When power is applied to the CMG-5T Compact, offset adjustments are
carried out automatically.
For more information on offsets go to:
http://www.guralp.com/articles/20060315-howto-sensor-zeroing
Should there be a need to reset the offsets, the simplest method is to
power cycle the sensor. If this is not possible the following methods can
be used.
2.4.1 Manual offset removal
Early versions of the CMG-5T Compact had a port in the
sensor lid to allow for manual offset adjustment. If the
sensor has a knurled screw cover then there is a manual
offset adjustment option.
1. Remove the screwed cover protecting the level adjusters. The
cavity contains three adjustment screws.
2. Power up the sensor and connect a digital multimeter to its lowgain vertical outputs (pins A and B). Alternatively, use a
Hand-held Control Unit (see Section 2.5, page 11) to monitor the
outputs more easily.
May 2011 9
Loading...