Page 394 - Mechatronics with Experiments

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6.8.3 Piezoelectric Based Pressure Sensor
Piezoelectric based pressure sensors are the most versatile pressure sensor types. The
pressure of the diaphragm is converted to a force acting on the piezoelectric element
(Figure 6.41). The piezoelectric element will generate a voltage proportional to the force
acting on it which is proportional to the pressure. The piezo based pressure sensors can
have bandwidth in the order of kHz range.
The charge produced by the piezoelectric effect as a result of pressure is

q = K qp ⋅ p (6.164)
and the output voltage is
q
V out = (6.165)
C
Then,
K qp
V out = ⋅ p (6.166)
C
= K ⋅ p (6.167)
1

6.8.4 Capacitance Based Pressure Sensor
The diaphragm pressure sensing concept can also be used to change the capacitance between
two charged plates inside the sensor. The displacement of the diaphragm results in a
proportional change in capacitance. Using an operational amplifier, reference voltage, and
reference capacitor, the change in the capacitance of the sensor can be converted to a voltage
output signal proportionally (Figure 6.50).

x = K ⋅ ΔP (6.168)
1
K ⋅ A
2
C = (6.169)
x

Vout = (Cs/C) Vs
_
p +
1
Fixed
metal plate
High gain
amplifier
- +
Capacitor, C x

Thin metal
diaphragm C s

Vs
p
2
FIGURE 6.50: Capacitive pressure sensor and its operating principle.