Symmetrical crystalline materials such like Rochelle salt , Quartz and Barium Titanate generated an emf when they are placed under stress. This property is utilized in Piezo-electric transducers, where a crystal is placed among a solid base and the force-summing member such that an externally applied force, entering the transducer from its pressure port, applies pressure onto the top of a crystal. This generates an emf across the crystal proportional to the magnitude of applied pressure.
As the transducer has a very good HF response, its principal use is in HF accelerometers. The device required no external power source and is, thus, self generating. The disadvantage is that it may not measure static conditions. The output voltage is also affected by temperature variation of the crystal. The basic expression for output voltage E is given by
E = Q /Cp
where Q = generated charge, and
Cp = shunt capacitance.
This transducer is inherently a dynamic responding sensor and does not measure readily static conditions (As it is a high impedance element, it needs careful shielding and compensation).
For a piezo-electric transducer under pressure, part of the energy is modified to an electric potential that seem on opposite faces of the element, analogous to a charge on the plates of a capacitor. The rest of the applied energy is converted to mechanical energy, analogous to a compressed spring. Whereas the pressure is eliminated, it returns to its original shape and loses its electric charge.
An alternating voltage applied to a crystal causes it to vibrate at its natural resonance frequency. As the frequency is a very stable quantity, piezo-electric crystals are principally used in HF accelerometers. The principal disadvantage is that voltage shall be generated as long as the pressure applied to the piezo-electric element changes.