Pi LVDT and LVDT Amplifier
APPLICATION NOTE
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Product Safety Data Sheets relating to these materials are available on request.
2
LVDT Application Note
Contents
Introduction .............................................................................................. 4
LVDT Construction and Theory of Operation ....................................... 6
LVDT Types .................................................................................... 7
Pi LVDT Amplifier ........................................................................... 9
Connecting the LVDT and Amplifier ............................................. 10
Installation Notes ................................................................................... 12
LVDTs ........................................................................................... 12
LVDT Amplifier ............................................................................. 12
Calibration and Testing ......................................................................... 13
Pi LVDT and Amplifier .................................................................. 13
Error States ................................................................................... 13
Interchanging Pi LVDTs and LVDT amplifiers ............................. 14
Pi LVDTs ....................................................................................... 14
Specifications ......................................................................................... 15
LVDT Dimensions ......................................................................... 17
Parts Numbering .................................................................................... 19
Frequently Asked Questions ................................................................ 20
Glossary .................................................................................................. 22
Appendix A: Fault Finding .................................................................... 23
␣
Contact information ............................................................................... 24
3
Introduction
LVDT is an acronym of Linear Variable Differential Transformer (LVDT), which as the name
implies is a transformer with variable secondary coupling dependent upon the core
position.
The Pi LVDT range and LVDT Amplifier are designed to make using an LVDT as simple
as using a linear potentiometer, with all the advantages associated with using LVDTs of
extreme ruggedness, low friction, infinite resolution and high temperature operation.
LVDTs are used to directly measure linear or rotary (RVDT) displacement or indirectly to
measure parameters such as force or pressure. LVDTs are available in wide variety of
ranges from a few millimetres to a few metres in stroke length.
The LVDT like any transformer must be driven with an AC voltage which produces an AC
voltage at the secondaries, the relationship of this can be used to determine the core
position displacement. The complex drive (modulation) and decode (demodulation)
circuitry required by the LVDT is all provided by the Pi LVDT amplifier.
Configuring the modulator/demodulator circuitry for different LVDTs can be complex and
specialised. The Pi LVDT and Pi LVDT Amplifier together simplify this problem by
producing the same output voltage range, independent of stroke length. This makes using
the Pi LVDT and Amplifier as simple as using a linear potentiometer; with the amplifier
producing a DC voltage proportional to position. The added advantage is that unlike a
potentiometer, a well regulated supply is not required.
4
LVDT Application Note
In particular the Pi LVDT and LVDT Amplifier combination produce an output voltage
range of 0.5V to 4.5V (where the centre position is 2.5V) irrespective of stroke length. This
simplifies changing Pi sensors and amplifiers as only a change in the software calibration
is required for a change in LVDT stroke length.
=
OUTPUT
VOLTAGE
=
4,500
2,500
0,500
RETRACT
EXTEND
TOTAL ELECTRICAL STROKE
␣
LVDT Output Voltage Range
5
LVDT Construction and Theory of Operation
An LVDT consists of a primary winding, two secondary windings and a core. The primary
and secondary windings are housed in a metal barrel, with a moveable magnetic core rod
which goes inside the barrel. The Pi LVDT has an outer sleeve which provides mechanical
protection and low friction bearing surfaces.
Primary
Barrel
Moveable magnetic core
Secondaries
Cross Sectional Diagram of an LVDT
When the primary is driven with an AC voltage a corresponding AC voltage is induced in
the two secondary windings, the magnitude of which is dependent upon the core position
and hence displacement.
SEC+
VsecA
PR1+
SEC CENTRE
Vpr
PR1–
VsecB
SEC–
AC Voltage in the Two Secondary Windings
6
LVDT Application Note
The cores are normally wound in series opposition so the two secondary windings are
180° out of phase with respect to the centre tap. The example above shows the electrical
zero position where both voltages are equal magnitude but 180° out of phase with respect
to the centre tap.
The electrical and mechanical centre of travel is the same for many LVDTs. The electrical
range of the LVDT is the range over which the electrical specifications hold true. The
mechanical range of the LVDT is usually longer than the electrical range and it is not
necessarily true that it will remain within its electrical specification, as the accuracy or
linearity may suffer.
The Pi range of LVDTs have three grooves etched in the barrel to denote the nominal
retract, centre and extend positions.
LVDT Types
LVDTs are available with either four wires or five wires, where the extra wire is the centre
tap. The Pi LVDT amplifier only functions with five wire LVDTs as it is a phase insensitive
design and requires the centre tap as a reference voltage.
The relationship of the output voltage to the mechanical position is specified by LVDT
manufacturers in two ways:
␣
1.
As a sum and difference ratio at full scale with respect to the centre tap
position i.e. R = (VsecA–VsecB)/(VsecA+VsecB). Common ratios are 0.3, 0.4
and 0.5.
7
=
R = 0.5
=
0.5
RETRACT
EXTEND
-0.25
-0.5
Total Electrical Stroke
LVDT Types
2.
As a sensitivity ratio dependent upon the primary excitation and core position.
This is usually specified as Output Volts per Volt Primary Drive per mm Stroke
Length, where the units are in the form mV/V/mm. For example, if a 25mm
LVDT with sensitivity of 16mV per Volt In per mm is driven by 2V RMS at
+10mm the output would be 2 x 0.016x10=0.320V RMS measured across the
windings.
The equation of Mode 1 has the advantage of being independent of primary excitation
level, thus not affected by loading and temperature drift problems. Many LVDTs, although
specified in Mode 2 terms, can actually be used successfully in Mode 1.
The LVDT’s electrical performance is specified at a spot frequency with 2.5 kHz and 7.5
kHz being common. The lower frequency usually gives better linearity but requires more
drive current. Unfortunately, the sensitivity of the LVDT varies with frequency; if an LVDT
is being used at a frequency which is far removed from the specified sensitivity, the new
sensitivity should be measured.
8
LVDT Application Note
Pi LVDT Amplifier
In order to derive the position from an LVDT, a modulator is required to generate the
primary AC Voltage and a demodulator is required to translate the AC secondary voltage
to a DC voltage proportional to position. All this circuitry is contained within the Pi LVDT
amplifier.
Modulator
Core
Vin+
Demodulator
Vin–
LVDT
LVDT Modulator/Demodulator Circuitry
The modulator has to be capable of generating different frequencies dependent upon the
LVDT requirements. The demodulator requires different gains and operating mode
dependent upon the LVDT type and scaling. The Pi LVDT amplifier supports both LVDTs
specified as Mode 1 (sum and difference ratio) and Mode 2 (sensitivity mV/V/mm).
Mode 1 is the preferred operating mode for the Pi LVDT amplifier as this gives the highest
accuracy with the additional benefit of indicating some errors such as broken LVDT
primary wiring.
␣
Many demodulator circuits are phase sensitive. Phase errors from primary to secondary
may also introduce errors. The Pi LVDT amplifier circuit is phase insensitive so does not
suffer from this problem, but it will not function with 4-wire LVDTs as it requires the centre
tap as a reference.
9
The standard operating mode for the LVDT amplifier is in Mode 1, with 0.5 sum and
difference ratio and output range of 0.5V to 4.5V, with electrical zero corresponding to
2.5V. This allows errors such as broken LVDT drive and system looms to be detected by
the logging system. If the LVDT amplifier detects the loss of primary excitation in Mode 1
it will flag this by going to between 7V and 8V.
Connecting the LVDT
and Amplifier
A Pi LVDT and Amplifier require a nominal 12V supply and consume a maximum of 30mA.
It generates an output voltage between 0.5V and 4.5V which is proportional to distance,
of which the amplifier typically requires 20mA. Other LVDTs may need more current than
the Pi LVDT and thus may require connection to a high power (100mA) channel.
When operating in Mode 1, many LVDT wiring errors such as a broken LVDT loom are
indicated by the analogue output going between 7V and 8V. If the amplifier loses power
or the output voltage is lost, the logging system shows the analogue input at 0V.
A negative output sense wire is provided to eliminate the effect of voltage drop along the
0V wire supplying power to the LVDT amplifier. This connection will give the highest
performance but is optional.
Connection of the LVDT amplifier to a standard 4-pin Lemo connector used on Pi junction
boxes is shown below:
FGG.OB.304
1
AS606-05-PB-HE
1
2
2
3
4
4
Connection of the LVDT Amplifier to a 4-pin Lemo Connector
10
LVDT Application Note
Grounding
The LVDT amplifier body and connectors are joined to the system ground connection back
to the logger and provide an effective RF shield. This means that care must be taken to
avoid ground loops.
The LVDT screen is connected at the LVDT Amplifier connector but not at the LVDT.
To avoid a ground loop the LVDT amplifier must be mounted on industrial Velcro or AntiVibration (AV) mounts. When using other manufacturers LVDTs take care that the LVDT
screen and LVDT amplifier LVDT connector screen are not connected otherwise an earth
loop is formed.
The looms to the LVDT Amplifier from the system and LVDT to the amplifier should each
not exceed 5m.
Indicates Electrical Connection
Logging System
LVDT Amp
AV Mount
(Electrically Isolated)
No Connection
Chassis
LVDT
Optionally Electrically Connected
␣
LVDT Amplifier Body
11
Installation Notes
LVDTs
■
■
■
■
■
■
■
■
■
Select a position where the sensor will not be in contact with water, fuel or oil.
Make sure nothing is pushing the sensor barrel to one side.
The grooves on the LVDT sleeve indicate the middle and ends of the stroke,
use these as a first attempt at a mechanical datum and to check the
mechanical movement range.
Do not use the sensor as a physical end stop.
Make sure that the sensor can move freely over its range without interfering
with other parts of the vehicle.
Use the 3mm rod end bearings to mount the sensor.
Keep the temperature below 120°C.
Try not to place the sensor near sources of electrical interference such as
ignition coils, plug leads, alternators and telemetry antennas.
Ensure that the movement range being measured does not exceed the
mechanical stroke of the LVDT.
LVDT Amplifier
■
■
■
■
12
LVDT Application Note
Select a position where the amplifier will not be in constant contact with water,
fuel or oil.
Make sure that the amplifier will not be affected by heat soak.
Isolate the amplifier chassis from the vehicle chassis using AV mounts or
industrial Velcro. Check that none of the LVDT amplifier metal parts including
connectors will be in contact with the vehicle chassis.
Try not to place the amplifier near sources of electrical interference i.e. ignition
coils, plug leads, alternators and telemetry antennas.
Calibration and Testing
Pi LVDT and Amplifier
The LVDT amplifier is designed to have low zero offset and gain error to ease
interchangeability between amplifiers.
The standard output range for the LVDT and Amplifier combination is 0.5V to 4.5V with
the mechanical/electrical zero point being at 2.5V with the LVDT excitation frequency
being nominally 2.5 kHz.
Displacement
Output (10 bit ADC)
Output (12 bit ADC)
Full Scale In
Centre
Full Scale Out
102
512
922
408
2048
3688
Error States
For a Pi LVDT, if the LVDT position is in the stroke range and not as above within a few
counts, there is a fault either in the installation, the wiring, the LVDT or the Amplifier.
The 0.5V to 4.5V range is chosen so that a disconnected system loom or LVDT loom can
be detected. The LVDT amplifier output when used in Mode 1 will go to around 7.5V if
certain faults occur on the LVDT loom, such as a broken primary excitation.
Loss of power to the LVDT or broken output wiring will result in the analogue output
dropping to 0V.
␣
For further information on fault finding see Appendix A.
13
Interchanging Pi LVDTs
and LVDT amplifiers
When interchanging LVDT amplifiers it should not be necessary to change calibration.
When interchanging Pi LVDTs of the same stroke length it may be necessary to adjust the
offset due to slight variations in position in the mechanical zero. When interchanging
LVDTs of differing stroke length additionally the software calibration will require
modification for the change in stroke length.
Pi LVDTs
Pi LVDTs are five wire LVDTs specified as Mode 1 with a constant sum and difference ratio
at full scale, irrespective of stroke length. This means that changing between different Pi
LVDTs requires only a software calibration change. The mechanical and electrical zero are
the same throughout the Pi LVDT range, the excitation frequency being 2.5 kHz.
The Pi LVDT amplifier is configured as standard for operation with Pi LVDTs and produces
an output voltage of 0.5 to 4.5V across the stroke, the centre being at 2.5V. Using other
LVDTs means that the output range will differ in span but still be centred around 2.5V at
electrical zero.
When using LVDTs from other manufacturers, contact Pi Support to discuss your
requirements. This is particularly important as many LVDTs require a higher excitation
frequency than the standard 2.5 kHz, resulting in the amplifier not functioning correctly.
14
LVDT Application Note
Specifications
Dimensions
LVDT
27.00
1.06”
SYS
38.00
1.50”
M3 clear
(2 posns)
32.00
1.26”
27.00
1.06”
45.00
1.77”
␣
LVDT Amplifier
Description
Value
Input Voltage
Supply Current
Output Voltage
LVDT disconnected
Frequency Response
Zero Accuracy
Zero Drift
Gain Accuracy
Gain Drift
Vibration
Temperature Range
Environmental
Weight
8.5V–18V DC
20mA@12V Typical
0.5V–4.5 V
Output goes to 7.5V
0–200Hz
±5mV Typical
20uV/°C Typical
±0.5% of Full Scale Typical
85ppm/°C Typical
50g Max
0°C to 70°C
IP65
75 grams
15
LVDT Amplifier Connectors
1
1
System
LVDT
Description
Amplifier Connector
Mating Connector
System
LVDT
AS206-05SB-HE
AS206-05SN-HE
AS606-05PB-HE
AS606-05PN-HE
LVDT Amplifier Connector Details
16
Pin
System Connector
Pin
LVDT Connector
1
2
3
4
5
Scr
+12V
GND
GND Sense (Optional)
Analogue Output
Case
Connected
1
2
3
4
5
Scr
Primary Excitation +ve
Primary Excitation –ve
Secondary +ve
Secondary –ve
Secondary Centre
Isolated at LVDT
LVDT Application Note
12.0 typ
LVDT Dimensions
C ±0.5mm retracted
Electrical stroke
Ø13.0 max
Ø3.05
500.0
␣
Electrical Stroke mm 25 (±12.5)
50 (±25)
75 (±37.5) 100 (±50) 125 (±62.5) 150 (±75)
Dimension C
125
150
195
220
280
285
Weight grams
95
115
15 5
175
20 0
220
17
The following specification is common throughout the LVDT Range.
Description
Value
Sensitivity R
Excitation Voltage
Excitation Frequency
Input Impedance
Load Resistance
Frequency Response
Accuracy
Linearity
Gain Drift
Shock
Temperature Range
Environmental
Mechanical Stroke
0.5
1.0V–10V RMS
400Hz to 12.5 kHz Sinewave
>300 ohms @2.5 kHz
>50K ohms
0–200Hz
±1.0% of Full Scale
±0.5% of Full Scale
±10ppm/°C
20g
–20°C to +125°C
IP66
Electrical Stroke +1mm
LVDT Connectors
Description
Sensor Connector
Mating Connector
LVDT
AS606-05PN-HE
AS106-05-SN-HE
LVDT Connector Details
18
Pin
LVDT Connector
1
2
3
4
5
Scr
Primary Excitation +ve
Primary Excitation –ve
Secondary +ve
Secondary –ve
Secondary Centre
Isolated at LVDT
LVDT Application Note
␣
Parts Numbering
Product
Part Number
LVDT 25mm Total Stroke
LVDT 50mm Total Stroke
LVDT 75mm Total Stroke
LVDT 100mm Total Stroke
LVDT 125mm Total Stroke
LVDT 150mm Total Stroke
LVDT Amplifier
System Connector Loom
LVDT Extension Loom
01B-01549
01B-01550
01B-01551
01B-01552
01B-01553
01B-01554
01B-050330
03E-01837
03L-01813
19
Frequently Asked Questions
How can I check that my LVDT is correctly connected?
This can be achieved before connection by using a DVM to measure the resistance
between windings using an LVDT break-out loom.
Measure the resistance between the Primary +ve and the Primary –ve; this should be
<1K ohms.
Measure the resistance between the Sec +ve and SecCentre; this should be <1K ohms.
Measure the resistance between Sec –ve and SecCentre; this should be <1K ohms and
similar to the previous value.
Finally measure between Sec –ve and Sec +ve; this should be <4K ohms and
approximately twice the value previously measured.
An LVDT simulator which plugs directly into the LVDT loom connections can be obtained
from Pi for checking the LVDT loom and amplifier operation.
Why do you specify an output range of 0.5V to 4.5V?
This range is specified to give reasonable resolution in the 0V–5V range but also allow
errors such as a broken system loom or LVDT to be flagged to the logger. A broken system
loom is seen as 0V and a broken LVDT is seen as 5V by a Pi datalogger.
Why do I get different outputs from the amplifier with different LVDTs?
Different types of LVDTs have different sensitivities. When amplified through the LVDT
amplifier this results in a different output range. If different LVDTs of the same sensing
range produce varying output, this is usually a direct indication that the accuracy between
LVDTs is poor.
The Pi range of LVDTs simplifies this problem by having the same sensitivity throughout
the range.
20
LVDT Application Note
How long can the cables be?
The cables to the LVDT Amplifier from the system and from the Amplifier to the LVDT can
each be up to a maximum of 5m.
What is the advantage of the standalone amplifier over the built-in amplifiers in Pi
Loggers?
␣
The standalone amplifier is optimised for use with the new range of LVDTs. It also typically
offers better offset accuracy, gain accuracy and lower temperature drift compared to the
built in amplifiers. In addition, it can also signal broken LVDT connections in sum and
difference ratio mode.
21
Glossary
Electrical Range
Electrical Zero
Mechanical Range
Mechanical Zero
Sensitivity
Sum and Difference Ratio
22
LVDT Application Note
The electrical range of the LVDT is the range over
which the electrical specifications hold true.
The point where both voltages appearing on the
secondary are the same magnitude.
The mechanical range of the LVDT is usually longer than
the electrical range but it is not necessarily true that it will
remain in electrical specification.
This is the centre of the mechanical range. The
electrical and mechanical zero for many LVDTs including
the Pi LVDT is the same.
This is used by some manufacturers to specify the output
range and is in units mV Out per Volt In per mm. Where
Volt In is the primary excitation in V RMS and mm is the
stroke position.
Other LVDT manufacturers specify their LVDTs as a Sum
and Difference Ratio i.e. (VsecA–VsecB)/(VsecA+VsecB)
where Vsec A and Vsec B are the LVDT secondary
voltages.
Appendix A: Fault Finding
Below is a non-exhaustive fault finding diagram. If you experience any further problems
please contact Pi Research.
Move LVDT to Centre
Position
Faulty
< 0.2V
Check System
Loom
LVDT Amp
Reading
> 7V
Faulty
OK
OK
Move LVDT to Retract
Position
< 8.5 > 20V
Reversed
Check Power
Supply
OK
Check
LVDT Wiring
Replace LVDT Amplifier
or LVDT
GAIN FAIL
0.2 < 0.480 or 0.520 < 2.5V
Box A
LVDT Amp
Reading
> 2.5V
PASS
0.480 <> 0.520
Move LVDT to Extend
Position
GAIN FAIL
2.5 < 4.480 or 4.520 > 7
Functional
Incorrect Gain
Box B
LVDT Amp
Reading
< 2.5
PASS
4.480 < 4.520
LVDT & Amplifier
Functioning Correctly
LVDT Fault Finding Diagram
␣
The LVDT Simulator which plugs directly into the LVDT loom can be obtained from Pi to
help the fault finding process. Alternatively the LVDT connections resistance can be used
to check the wiring.
23
Contact information
For more information about Pi products and details of worldwide authorised agents,
please contact:
Pi Research
Brookfield Motorsports Centre
Twentypence Road
Cottenham
CAMBRIDGE
UK
Customer Support Tel +44 (0) 1954 253600
CB4 8PS
Fax +44 (0) 1954 253601
Pi Research, Inc.
8250 Haverstick
Suite #275
Indianapolis
IN 46240
USA
Tel +1 (317) 259-8900
Fax +1 (317) 259-0137
Research
Part Number: 29B-071185-3E
Issue 3.0 September 1999
Pi and the Pi logo are trademarks of Pi Group Limited
© Pi Research 1999
www.piresearch.com
24
LVDT Application Note