It explains the resistive displacement transducer

1. Resistive Displacement
Measurement
R K Shukla,
Asst. Professor, IC dept
Govt Engg College, Rajkot, Gujarat
R K Shukla, IC Dept

2. Introduction
• Length -> most measured physical parameter.
• Alternative names-displacement, movement,
motion.
• Often the intermediate stage of systems used
to measure other parameters.
• Older methods were largely mechanical
• Electrical techniques -> electronic outputs is
now much widely used
R K Shukla, IC Dept

3. Electronic length measurement
• Any physical principle that relates a length to a
physical variable has the potential to be used for
converting length to another equivalent signal.
• Output signal -> electronic form.
• Transduction -> converts length into an electrical
entity directly or indirectly.
• Suitable for smaller ranges using electromechanical
structures
• Electrical resistance, inductance, or capacitance
R K Shukla, IC Dept

4. Electrical resistance
• In essence some mechanical arrangement is made in
which the electrical resistance between two ends of
an interval is made to vary as the interval changes
length.
• Methods divide into two groups
– Change in length as the changing position of a contact point.
– The bulk properties of the structure
R K Shukla, IC Dept

5. Electrical resistance
• Sliding contact
length sensors.
• Voltage Vs is
applied ->
resistance unit.
• The output voltage
Vout -> the length l
• Vout = (Vs)/(L) * l
R K Shukla, IC Dept

6. Electrical resistance
• Vout gives a direct measure of length l. (Vs and
L are constant)
• Vs can be either d.c. or a.c. voltage.
• Sources of errors in the transduction process
– Non-uniform heating effects (Changes R and l)
– Loading effect (readout circuit must not load the
resistance) which results in output-to-length
relationship does not hold in a linear manner
R K Shukla, IC Dept

7. Electrical resistance
• Sliding-contact sensors
– Inexpensive
• Suffer from
– Granularity as the contact moves from wire to wires
– Noise caused by the mechanical contact of the wiper
– Wear to ensure adequate contact
• These practical reasons often rule them out as serious
contenders for an application.
• Their use is, however, very simple to understand and
apply.
R K Shukla, IC Dept

8. Electrical resistance
• Discrimination depends upon the granularity of the wire
diameter in the wound types.
• Sliding a contact along (rather than across) the wound wire as
a continuous motion.
• Range is from mm to a meter with discrimination of the order
of up to 1/1000 of the length.
• The frequency response of such units depends more upon the
mechanical mass to be moved during dynamic changes.
• Signal-to-noise performance can be quite reasonable
• But not as good as can be obtained with alternative inductive
and capacitive methods.
R K Shukla, IC Dept