Tektronix Keithley AIM9 LVDT/RVDT Module User's Manual Rev. B User manual

AIM9

LVDT/RVDT Module

This documentation describes the features, installation, and operation of the AIM9
Lm/RVlYI Module in the Series 500 or System 570. This manual also contains impor­tant progr amming information and several example programs.

The AIM9 is a dual-channel module for making measurements with AC-driven
transducers such as LvDT’s, RVDT’s, and variable-reluctance transducers. Typically,
linear variable differential transformers (LvDTs) measure linear displacement, while
rotary variable differential transformers (RVDT’s) measure angular displacement.
Throughout this manual, the term “transducer” implies an LVM; RVDT, or other AC­driven, displacement-type sensor.

The AIM93 important features include the following:

Dual channels, each with differential input.
A quick-disconnect terminal block for each channel. Connections include excitation

output, three shield/common terminals, signal A input, and signal B input.
Selectable excitation frequencies of lkHz, 2kHz, 5kHz, lOkHz, or 2OkI-k
Provision for driving the excitation circuitry of up to nine AIM9 modules from the

master oscillator of one AIM9.
Selectable low-pass filter with Wz, 2OHz, and 2OOHz bandwidths. The filter is

software-selectable through the IONAME FILT% parameter.
Continuously variable on-board gain of Xl (*WV full-scale input) to X20 (*0.5V full-

scale input).
Offset adjustments for zeroing the output of the AIM9 gain amplifiers.
Phase adjustment potentiometers to align the excitation and returning signals for

each channel.
Compatibility with Series 500 or System 570. The System 570 accepts one AlM9.

The Series 500 can accept up to nine AIM9 modules.

The AIM9 is intended for transducers which require AC excitation and produce AC out­put signals. However, it is also compatible with some types of general-purpose DC­driven transducers which normally produce DC output levels. Examples are strain gages
and potentiometric transducers

-

Figure 1 identifies the adjustment potentiometers, jumpers, switches, and important test
points on the AIM9 module.

Document Number: 501-902-01 Rev. B

AIM9-1

MASTER

lo(0_lO_I

MASTER/SLAVE

OSCILLATOR JUMPER

SLAVE

Wl

TEST

POINT

DETl

TEST

POINT POINT

DETO

TEST

GND

EXCITATION OFFSET

FREQUENCY

ADJUST

C46

.n.

PHASE

ADJUST

GAIN 1 1 j T;;E.;/;L ;;;fIN

ADJUST

I I I

rm Jl?

CHANNEL (

Requirements for Using the AIM9

The AIM9 is hardware-compatiile with Keithky’s IBM-Version Series 500 and System
570 products. When used in the Series 500, the AIM9 requires a master analog input
module AMMl or AIM1 in slot 1. The AIM1 requires either an ADMl or ADM2 A/D

module in slot 2. The System 570 already contains the master analog input and AID

functions, and accepts one AIM9.

The AIM9 is programmed with Soft500 Version 4.0 or later. Soft500 runs under IBM PC
Advanced BASIC (BASICA) included in IBM PC-DOS. IBM PC-DOS versions 3.1 and
later are recommended for use with IBM PC, m, and AT computers.

Compaq computers must run Soft500 under Compaq DOS 3.0 or later, with the mat­ching BASICA version. Earlier versions of Compaq DOS and BASICA are not compati-

ble with Soft500 V4.0 or later.

Soft500 is also compatible with many 100% IBM-compatible computers which run GW-

BASIC under MS-DOS (Version 3.0 or later). Regardless of the brand or rev level of the

DOS, you must use the GWBASIC version which accompanies or is recommended for
the DOS version. Mixing DOS and BASIC versions can cause problems.

The AIM9 module can also be programmed directly using BASIC& PEER and POKE

functions, or the corresponding memory read and write functions of other programm­ing languages. This capability permits the AIM9 to be programmed outside the Soft500
environment.

Installation

Install the AIM9 in any of the slots 2-10 of the Series 500 (slots 3-10 if the AIM1 is us­ed). For maximum immunity to noise, install the AIM9 and any other analog input
modules in the lowest-numbered available slots. The System 570 can accept one AIM9
module in its option slot. For either system, update the configuration table to show the
location of the AIM9.

User-Configured Features

The AIM9 module requires a number of settings and adjustments for best performance
with a given transducer. The user-definable parameters and adjustments include fre­quency selection, phase correction, zero offset adjustment, and gain adjustment. A
bank of DIP switches sets the excitation frequency. Potentiometers control the phase,
offset, and gain adjustments.

To get the maximum utility from the AIM9 and transduceq the two must be calibrated

as a unit, and the calibration factor entered into the configuration table as part of an

IONAME. Even if you do not elect to enter IONAME’s in the configuration table, you

must select an excitation frequency, adjust the phase potentiometers, and adjust offset.

AlM9-3

This manual provides programs and other information to help you derive a calibration
factor for a chosen transducer. The calibration factor applies only to the transducer and
AIM9 as a pair. You must repeat calibration if you change the AIM9 gain, phase adjust­ment, or excitation frequency. Since the transducer and AIM9 are calibrated as a pair,
you must also recalibrate if you replace either the transducer or the AIM9.

Generally, an AIM9 set up involves the eight steps listed below. The list and the accom­panying detailed instructions assume that the transducer is connected to channel 0. The
instructions refer to various test points and adjustments. Be sure to select those test and
adjustment points for channel 0. The corresponding controls for channel 1 are physical­ly near those for channel 0 (see Figure 1).

1. Select a transducer which is suited to the application.

2. Perform the mechanical installation of the transducer on the test/calibration fixture.

3. Connect the transducer to the AIM9 channel 0 terminal block.

4. Select an excitation frequency.

5. Install the AIM9 in the data acquisition system and turn on the system. Turn the

Channel 0 GAIN potentiometer fully CW (maximum gain position).

6. Monitor the detector test point DETU with an oscilloscope. Adjust the Phase 0 and/or

Phase Z potentiometers for the proper waveform.

7. Run a short Soft500 program to read the voltage output of the gain amplifier. Adjust

the AIM9’s offset potentiometer OS0 for an output of zero.

8. Adjust for a suitable gain with the channel 0 GAIN potentiometer.

The following paragraphs discuss these steps in greater detail.

Selecting and Connecting the llansducer to the AIM9
You must select a suitable transducer for the experiment or measurement system. For

more information on this topic, refer to manufacturers’ catalogs and literature covering
various types of transducers and applications.

An Lm or RVDT generally has two secondary signal windings (A and B), a primary

winding for excitation, and a movable core. The primary and secondary windings com-

prise a transformer. The amplitudes of voltages induced into windings A and B vary in-

versely with each other as the core is moved.

You can connect a transducer’s signal windings to the AIM9 in a number of configura-

tions. The series-opposing connection (Figure 2) allows linear measurement of the core

position to either side of the center (null) position. At one extreme of the core’s move-

ment, the AIM9 output will be negative. At the other extreme, the AIM9 output will be

positive. The terminals for “winding A Ground” and ‘Winding B Ground” are not us-

ed for the series-opposing configuration.

AIM9-4

TYPICAL LINEAR VARIABLE DIFFERENTIAL
TRANSFORMER TRANSDUCER

A

EXCITATION

IRON CORE DEMODULATOR

OISPLACEMENT-e

OUTPUT

VOLTAGE

OUT-PUT

VOLTAGE

SECONDARY

AC SIGNAL TO

-+DISPLACEMENT

>

B

Figure 2A. LVDT Series-Opposing Connection to AIM9, with Corresponding

Signal Variation

AIM93

TYPICAL VARIABLE INDUCTANCE TRANSDUCER

PRESSURE PORTS

Pl

P2

1

METAL--+

DIAPHRAGM

tz4

< COMMON

J

Figure 2B. LVDT Series-Opposing Connection to AIM9, with Corresponding

Signal Variation

AIM9-6

The AIM9 card has two quick-disconnect terminals, each with six screw terminals (see
Figure 1). The screws make connections to each channel’s oscillator (excitation) output,
ground, and channel A and B inputs.

Pin Terminal Name

Oscillator Out

:

Ground
3 Signal A Input
4 Ground
5 Signal B Input
6 Ground

To connect wires to a terminal block, first loosen the screws several turns. Strip %” of
insulation from a wire lead, insert the lead in the receptacle beneath the screw, and
tighten the screw.

To make the task of connecting the leads easier, you can remove a terminal block by
pulling it off the board in a perpendicular direction with a firm, even pressure. Do not
pry the terminal blocks off with a screwdriver or other sharp tools or you may damage
the circuit board. After you have connected the wires to the terminal block, reinstall the
block on the AlM9.

Selecting the Excitation Frequency

The AIM9 uses a Wein-bridge oscillator for AC excitation of both channels. The single

oscillator feeds an adjustable phase shift network and buffer amplifier for each channel.

The oscillator also feeds a third adjustable phase shift network which produces the

“Phase Z” signal.

Transducer Connect
Primary

Primary Ground
Secondary Winding A
Wmcbng A Ground

Secondary Wmding B

Winding B Ground

The Phase 0 and Phase 1 potentiometers adjust for any phase difference between the

oscillator and signal returning from the transducer. The Phase Z potentiometer adjusts

for proper phase relationship between the oscillator and detector.

The module design produces a non-adjustable excitation amplitude of 5V T&IS nominal.

Oscillator drive buffers boost the excitation drive current to 1OOmA.

Individual DIP switches on the AIM9 select oscillator frequencies of &Hz, 2kHz, 5kH2,

lOkHz, or 2OkHz. Turn on only one frequency select switch at a time. Turning on more
than one frequency select switch will produce a non-standard frequency. See Figure 1
for the location of the frequency switches.

The choice of excitation frequency depends on the application, type of transducer, and

other factors. The transducer manufacturer may recommend a particular excitation fre-

quency to

phase controls for channel 0, 1, and Phase Z can compensate for constant phase shifts

in the transducer.

minimize phase shift or other undesirable effects in the transducer. The

AlM9-7

Where the transducer measures simple displacements of an otherwise static element,
there is no implicit advantage to using any given frequency. Unless the transducer
manufacturer recommends a specific frequency, set the AIM9 for 5kHz.

Measuring the displacement of a vibrating element requires a careful evaluation of the
vibration frequency, type of vibration, filter, and sampling rate. Generally, the excitation
frequency should be at least ten times the vibration frequency of the element being
monitored with the transducer.

Driving Multiple AIMS’s From One Master Oscillator

If you operate more than one AIM9 in a Series 500, you may need to drive all AIM9 ex-

citation circuits from the oscillator of one AIM9. This will assure that the excitation
delivered to all transducers is at precisely the same frequency.

The AIM9 has a single jumper for setting the oscillator in either master or slave mode

(see Figure 1). With the jumper in the master position, the AIM9’s oscillator drives its
own excitation circuitry and a common daisy-chain line in the Series 500 bus. Placing
the jumper in the slave position disconnects the ATM9’s oscillator from the phase shift
and buffer circuits, and connects this excitation circuitry to the daisy-chain line in the
baseboard.

To operate several AIM9 modules from one oscillator, first refer to Figure 1. For the the
master AIM9, install the jumper block on the center and left-most pins of Wl. To make
an AIM9 a slave, install the jumper block on the center and right-most pins of Wl. The

daisy-chain line in the-&r&s 5QO baseboard bus automatically makes the proper con­nections between the master and slave AIM% whenthes<‘modules a.r~plugged-iriIo
the Series 500.

Adjusting the Phase 0, Phase 1, and Phase Z Potentiometers
The phase adjustments permit the relative phases of the channel 0 and channel 1

return signals to be aligned with each other and Phase Z. The Phase 0 and Phase 1
potentiometers control the phase of the excitation for channel 0 and channel 1 relative

to the oscillator. The Phase Z potentiometer controls the phase shift of a third oscillator

signal used by the AIM9 demodulator (detector). The phase potentiometers give an ad­justment range of approximately 170’ at the lkH2 excitation frequency.

The need for phase adjustments becomes clearer if one considers a simple LVJYI’

measurement. In practice, the AIM9 excites the transducer which returns AC waveforms

to the AlM9 “Ai’ and “5” inputs. The amplitudes of these waveforms vary inversely

with each other depending on the displacement of the Lvllvr core.

As is often the case, the return waveforms may experience some phase shift relative to
the oscillator, and to Phase Z which represents the oscillator waveform. For proper
decoding of the signals returning from the transducer, the phase-sensitive ATM9 detec­tor circuit requires that the transducer signals be properly aligned relative to Phase Z.
The Phase 0 and Phase Z adjustments provide for this alignment.

AIM9-8

When two transducers are used, there may be a need to align the phase angles of the

channel 0, channel 1, and Phase Z waveforms. The separate phase controls for each
channel and Phase Z gives a broad range of adjustment between the channel 0 and
channel 1 input signals and Phase Z.

You must use an oscilloscope to adjust the Phase 0, Phase 1, and Phase Z poten­tiometers. Wth the transducer connected to the AlM9, monitor the test point DElU
with the oscilloscope. The signal level at DE’IU will be on the order of 5OmV45OmV
depending on the degree that the LVDT core is off-center electrically. If the output of a
transducer is zero, the waveform will be a straight line or nearly so. If this is the case,
move the transducer off electrical center to provide a workable waveform at DETO.

For channel 0, adjust the Phase Z and/or Phase 0 potentiometers to produce a
waveform at DETO resembling Figure 3. There will be an adjustment range for both
potentiometers wherein either will correct an out-of-phase condition. UltimateIy you
may turn one or the other potentiometer beyond the point where it has any effect, and
it will no longer be possrble to achieve alignment. Therefore, the adjustment may re­quire some trial-and-error to find the best positions for each potentiometer. You may
have to turn each potentiometer lock-to-lock to find its active range.

Once set, the phase potentiometers should require no further adjustment unless you
change the excitation frequency or the orientation of the transducer.

For two-channel operation, first adjust Phase 0 and/or Phase Z for the proper waveform
at test point DETO. Next, monitor test point DETl and adjust Phase 1 for the proper
waveform at test point DETl. You may have to readjust Phase 0 or Phase Z to find the
potentiometer positions which mutually align the waveforms at DETO and DETl.

AlM9-9