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FIGURE 19.36 Modes of operation for a simple plate as a piezoelectric device [4].

Crystal twister ''bimorph''

Ceramic bender ''bimorph''
or ''multimorph''

Crystal bender ''bimorph''

FIGURE 19.37 Curvature of “twister” and “bender” piezoelectric transducers when voltage applied [4].

caused by an external force, alters the oscillation frequency. Hence, a dynamic force can be measured by
the change in frequency of the oscillator.
Resistive Method
The resistive method employs the fact that when the multiple contact area between semiconducting
particles (usually carbon) and the distance between the particles are changed, the total resistance is altered.
The design of such transducers yields a very small displacement when a force is applied. A transducer
might consist of 2–60 thin carbon disks mounted between a fixed and a movable electrode. When a force
is applied to the movable electrode and the carbon disks move together by 5–250 µm per interface, the
transfer function of their resistance against the applied force is approximately hyperbolic, that is, highly
nonlinear. The device is also subject to large hysteresis and drift together with a high transverse sensitivity.
In order to reduce hysteresis and drift, rings are used instead of disks. The rings are mounted on an
insulated rigid core and prestressed. This almost completely eliminates any transverse sensitivity error.
The core’s resonant frequency is high and can occur at a frequency as high as 10 kHz. The possible
measuring range of such a transducer is from 0.1 to 10 kg. The accuracy and linear sensitivity of this
transducer is very poor.
Inductive Method
The inductive method utilizes the fact that a change in mechanical stress of a ferromagnetic material
causes its permeability to alter. The changes in magnetic flux are converted into induced voltages in the
pickup coils as the movement takes place. This phenomenon is known as the Villari effect or magneto-
striction. It is known to be particularly strong in nickel–iron alloys.
Transducers utilizing the Villari effect consist of a coil wound on a core of magnetostrictive material.
The force to be measured is applied on this core, stressing it and causing a change in its permeability
and inductance. This change can be monitored and used for determining the force.

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