Page 395 - Mechatronics with Experiments

JWST499-Cetinkunt
October 9, 2014 8:1
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SENSORS 381 254mm×178mm
Using the operational amplifier,
V = V ⋅ (C ∕C) (6.170)
out s s
V ⋅ C s
s
= ⋅ x (6.171)
K ⋅ A
2
V ⋅ C ⋅ K 1
s
s
= ⋅ ΔP (6.172)
K ⋅ A
2
The signal flow relationship for the sensor operation is as follows,
ΔP → x → C → V (6.173)
out
where the pressure differential results in a change in the distance between two plates of the
capacitive sensor which in turn changes the capacitance of the sensor.
6.9 TEMPERATURE SENSORS

Three classes of temperature sensors are discussed below:
1. sensors which change physical dimension as a function of temperature,
2. sensors which change resistance as a function of temperature (RTD and thermistors),
and
3. sensors which work based on thermoelectric phenomena (thermocouples).
Pictures of various RTD and thermocouple type temperature sensors are shown in Fig-
ure 6.51.

6.9.1 Temperature Sensors Based on
Dimensional Change

Temperature is an indicator of the molecular motion of matter. Most metals and liquids
change their dimension as a function of temperature. In particular, mercury is used in
glass thermometers to measure temperature since its volume increases proportionally with
the temperature. Then the glass tube can be scaled to indicate the measured temperature
◦
(Figure 6.52). It has a typical accuracy of about ±0.5 C. Similarly, bimetallic solid materials
change their dimension as a function of temperature. As a result, they can be used as a
temperature sensor by converting the change in the dimension of the bimetallic component
into a voltage. However, mercury is rarely used as a temperature sensor element due to its
highly toxic nature and environmental concerns if it is spilled.

FIGURE 6.51: Pictures of various temperature sensors: thermocouples and RTDs.