Page 1167 - The Mechatronics Handbook

OUTPUT
(CAPACITOR CONNECTIONS)

x
INPUT
INPUT
SHAFT
CAPACITOR
PLATES
(END VIEW)

∆x
(a)
+

OUTPUT
R + ∆R

−

∆x
(b) INPUT x
OUTPUT
CRYSTAL

OUTPUT
INPUT INPUT
x x

∆x ∆x
ISOLATION
(c) MASS

FIGURE 45.3 Sensitive displacement transducers: (a) differential capacitor, (b) strain gauge resistor, (c) piezoelectric
crystals.

usually incorporated into one of the several bridge circuits, and the output of the bridge is taken as an
indication of strain.
The piezoelectric effect is used in several techniques for sensitive displacement measurements
[Fig. 45.3(c)]. In one technique, the input motion deforms the crystal by acting directly on one electrode.
In another technique, the crystal is fabricated as part of a larger structure, which is oriented so that input
motion bends the structure and deforms the crystal. Deformation of the crystal produces a small output
voltage and also alters the resonant frequency of the crystal. In a few situations, the output voltage is
taken directly as an indication of motion, but more frequently the crystal is used to control an oscillator,
and the oscillator frequency is taken as the indication of strain.

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