Piezoelectric
Transducers
Piezoelectric
transducers produce an output voltage when a force is applied to them. They are
frequently used as ultrasonic receivers and also as displacement transducers,
particularly as part of devices measuring acceleration, force and pressure. In
ultra- sonic receivers, the sinusoidal amplitude variations in the ultrasound
wave received are translated into sinusoidal changes in the amplitude of the
force applied to the piezoelectric transducer. In a similar way, the
translational movement in a displacement transducer is caused by mechanical
means to apply a force to the piezoelectric transducer. Piezoelectric
transducers are made from piezoelectric materials. These have an asymmetrical
lattice of molecules that distorts when a mechanical force is applied to it.
This distortion causes a reorientation of electric charges within the material,
resulting in a relative displacement of positive and negative charges. The
charge displacement induces surface charges on the material of opposite
polarity between the two sides. By implanting electrodes into the surface of
the material, these surface charges can be measured as an output voltage. For a
rectangular block of material, the induced voltage is given by:
V = kFd/A
Where F
is the applied force in g, A is the area of the material in mm, d is the
thickness of the material and k is the piezoelectric constant. The polarity of
the induced voltage depends on whether the material is compressed or stretched.
Where F
is the applied force in g, A is the area of the material in mm, d is the
thickness of the material and k is the piezoelectric constant. The polarity of
the induced voltage depends on whether the material is compressed or stretched.
Materials
exhibiting piezoelectric behaviour include natural ones such as quartz,
synthetic ones such as lithium sulphate and ferroelectric ceramics such as
barium titanate. The piezoelectric constant varies widely between different
materials. Typical values of k are 2.3 for quartz and 140 for barium titanate.
Applying equation (13.1) for a force of 1 g applied to a crystal of area 100 mm2
and thickness 1 mm gives an output of 23 µV for quartz and 1.4 mV for
barium titanate.
The
piezoelectric principle is invertible, and therefore distortion in a
piezoelectric material can be caused by applying a voltage to it. This is
commonly used in ultrasonic transmitters, where the application of a sinusoidal
voltage at a frequency in the ultra- sound range causes a sinusoidal variation
in the thickness of the material and results in a sound wave being emitted at
the chosen frequency. This is considered further in the section below on ultrasonic
transducers.
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