7. DISPLACEMENT SENSORS
Explore the physical principles and functionality of following displacement sensors:
o resistance based sensor
o optical incremental sensor
o linear variable differential transformer – LVDT
1. Resistance based and optical incremental sensors of position
Task
Measure the transfer characteristics of a resistance based displacement sensor Novotechnik
TR100 (output voltage versus displacement); use potentiometer circuit. Estimate the influence
of the load applied to the sensor output. Use an optical incremental sensor SL101LB
connected to ADP1 indicator as a reference. Estimate the resistance sensor’s linearity in given
range.
Fig. 1: Tool for measurements with resistance and optical based sensors
Principle and used sensors
The sensing of position with a resistance sensor is one the most simple principles. The base is
a variable resistor, either linear or rotational. The measured object is mechanically attached to
the slider. The resistor is made by winding a resistance wire on a suitable support frame or by
vapor plating on a suitable substrate.
Fig. 2: Resistance based displacement sensor
Practically it is more convenient to use “potentiometer” circuit (Fig. 2) rather than “rheostat”
– “adjustable resistor” circuit using only two terminals – the slider (terminal 3 in Fig.2) and
one of the two remaining terminals). The potentiometer circuit eliminates the resistance
changes caused e.g. by temperature variations (see Fig. 3).
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Fig. 3: “Potentiometer” circuit for measurements with resistive sensors
The resistance based sensor usually exhibit high linearity of a transfer characteristics
(linearity error < 0.1 % or better). The effect of the loading resistor RZ must be minimized in
order to reach the maximum linearity (RZ ideally → ∞). Op-amp based buffers (voltage
followers) are used to provide desired low impedance output. It is also beneficial to use
current source to power up the sensor – it eliminates the effect of connecting wires resistance.
Tab. 1: Parameters of the resistance based sensor
Novotechnik TR100
Parameter
Value
Range of measurement
100
Nominal resistance
5
Linearity error
± 0.075
Suggested current through
≤1
slider
Maximum current through
10
slider during failure
Max. voltage U1
42
Unit
mm
kΩ
%
µA
mA
V
There is a 560 Ω resistor connected in series with the slider terminal – it limits the current
during short circuit conditions (during failure). This resistance is constant for the whole
measurement range.
Fig. 4: Resistive sensor of linear position Novotechnik TR100
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Optoelectronic incremental sensor SL101LB is designed to provide precise position
information in a range of 0 to 150 mm (different types up to 3140 mm). SL 101LB transfers
the information about the linear displacement into electrical impulses. The number of
impulses corresponds to the change of a position; their frequency is proportional to the speed
of position change. SL101LB is composed of two parts; one is fixed (contains glass scale with
optical marks), the second is movable (contains light source and photo-detector). The inner
parts are protected by flexible rubber flaps. The scale contains 50 marks per mm and
reference marks each 50mm. The outputs are TTL signals as shown on Fig.5. There is a signal
evaluation unit ADP1 with numerical display connected to the sensor; it directly presents the
measured distance.
Tab. 2: Parameters of the optoelectronic incremental sensor
SL101LB
Parameter
Value
Unit
Range of measurement
150
mm
Resolution
1
µm
Accuracy of the
±5
µm/m
measurement/per distance
Output signal
TTL
Fig. 5: Output signals (TTL levels) of the optoelectronic sensor SL 101LB (output 3 –
reference mark)
Fig. 6: Optoelectronic sensor SL 101LB and ADP1 display unit
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Measurement instructions
1. Rotate the control wheel of the linear displacement system in order to reach the most
left position of the resistance sensor.
2. Power up the display unit of the optoelectronic sensor and set the zero position by
pressing REF and then NUL buttons
Let the teacher check your circuit connection before you apply the power to the
circuit!
3. Set-up a current limit of 5mA on the power supply (protection against over-current
going through the slider during failure state). Apply 5VDC from the power supply to
the resistance sensor.
4. Measure the transfer characteristic of the resistance based sensor in a range of 0 –
100 mm with a step of 5 mm. Plot it as a function f(d)=U2, where the reference
displacement is measured by the optical incremental sensor. The U2 voltage is
measured by voltmeter at the slider terminal. See the circuit on Fig.3. Measure for
three different values of the loading resistors RL (use decade-resistance box as RL):
a) RL = 99 kΩ
b) RL = 9 kΩ
c) RL = ∞
5. Plot the measured values to graph, add linear approximation and specify the
coefficients of the approximation (sensitivity-slope & offset). Specify the linearity
error for the measured range.
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2. LVDT - Linear Variable Differential Transformer
Task
Measure the transfer characteristics of the LVDT sensor. Pay special attention to both outer
positions. The sensor’s signal conditioning is based on AD598 IC from Analog Devices.
Connect the oscilloscope directly to the outputs of the secondary coils of the LVDT sensor (be
careful while connecting the grounds – do not short circuit the coils via oscilloscope common
ground).
The position reference for the transfer characteristic is given by the slide caliper.
Fig. 7: Tool for measurements with the LVDT sensor
Sensor principle
The basic principle of LVDT sensor operation is a change of mutual inductance between the
primary and secondary windings by changing position of a ferromagnetic core. The primary
winding is powered by AC power supply (as any other transformer). The output voltage of the
secondary coils is proportional to the mutual inductances M1 and M2 which depend on the
position of a movable ferromagnetic core inside the sensor (see Fig. 8).
Fig. 8: Principle of the LVDT sensor position measurement
The induced voltage (U0) in the secondary coils increases the more the higher is the mutual
coupling between the primary coil and the appropriate secondary section (more % of length of
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the ferromagnetic core is in the appropriate section. The result of difference of U2a and U2b is
directly proportional to the core displacement |±∆x|. There are two ways of an output signal
conditioning:
a) In order to distinguish the direction of a position change the phase of the signals is
evaluated (phase with respect to primary voltage). The phase is reversed when the core
passes its center position. Synchronous detection is often used to do this functionality.
b) The output voltages are evaluated by so called “ratio detector”. Numerical calculation
(A - B)/(A + B) is realized either analogically or digitally. The phase information is no
more needed in this case and this principle also suppresses the error caused by primary
voltage variations.
The integrated circuit AD598 (by Analog Devices) produces sine wave for excitation of the
primary winding and also contains above mentioned ratio-metric output signal evaluation
(uses duty cycle signal processing). You can find more information about the circuit and its
functionality in the datasheet (see Fig. 9 and 10)
Fig. 9: AD598 IC principle
Fig. 10: The ratio-metric signal conditioning method implemented in AD598 IC
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Instructions
1. Go with the slider to the most right position.
2. Connect power supply to “POWER SUPPLY” terminals (±15 V a GND).
3. Connect a DMM (DC voltage measurement) to the “SIGNAL OTPUT” terminals and
connect two channels of an oscilloscope to “SECONDARY OUTPUTS” terminals
(LVDT secondary coils).
4. Shift the ferromagnetic core with the use of slide caliper in a range 0 – 100 mm with a
step 5 mm.
a) Measure the static transfer characteristic as a function f(d)=UO, where d is a
slide caliper reading and UO is the output voltage of the AD598 IC.
b) Measure the static transfer characteristic with the use of an oscilloscope based
mathematical signal processing. Set-up a mathematical channel (A - B) and
measure its effective voltage (RMS). Watch the phase of the signal in order to
correctly interpret the sign.
5. Find the center position (UO = 0) and measure the transfer characteristic more in detail
in a range of ±5 mm with a 1 mm step.
6. Construct a graph from the measured values, add a linear approximation of the
dependency and get the coefficients (sensitivity, offset). Determine the linearity error
for the whole measurement range.
7. Determine such a measurement range within which the linearity error does not exceed:
a) 5 %
b) 1 %
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