Page 353 - Mechatronics with Experiments

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JWST499-Cetinkunt
JWST499-c06
SENSORS 339 254mm×178mm
and the more accurate tempareture measurement is
T = 40 ⋅ 1.0002∕0.8 (6.43)
= 50 ⋅ 1.0002 (6.44)
◦
= 50.01 C (6.45)
Note that if the input resistance of the measurement device is small compared to the
nominal resistance in the Wheatstone bridge, the measurement error will be much larger.
For instance, if R = 1000 Ω, the resulting temperature measurement would be
m
◦
T = 50 ⋅ 1.2 = 60 C (6.46)
which has a 20% error in measurement due to the low input impedance of the measurement
device relative to the impedance of the circuit.

6.4 POSITION SENSORS

There are two kinds of length measurements of interest: (i) absolute position (the distance
between two points), (ii) incremental position (the change in the position). If a sensor
can measure the position of an object on power-up relative to a reference (the distance
of the object from a reference point on power-up), we call it an absolute position sensor.
If the sensor cannot tell the distance of the object from a reference on power-up, but can
keep track of the change in position from that point on, we call it an incremental position
sensor. Examples of absolute position sensors include a calibrated potentiometer, absolute
optical encoder, linear variable differential transformer, resolver, and capacitive gap sensor.
Examples of incremental position sensors include incremental optical encoder and laser
interferometer. Most of the position sensors have rotary and translational (linear) position
sensor versions.

6.4.1 Potentiometer
A potentiometer relates the absolute position (linear or rotary) into the resistance (Fig-
ures 6.9 and 6.10). The resistance change is converted to a proportional voltage change in
the electrical circuit portion of the sensor. Hence, the relationship between the measured

Resistive Brush
Resistive X element
element
Brush
V r

V out = kV x V r V out = kV r
r

(a) (b)
FIGURE 6.9: Linear and a rotary potentiometer for position measurement.