A piezoelectric sensor - or 'piezo sensor' for short - exploits the direct piezoelectric effect, in that a mechanical stress upon a piece of piezoelectric material yields an electric charge at the surface of the material. For a piezo sensor, rather than in the conversion of energy, interest is in the conversion of information. A piezo sensor converts information from the mechanical domain into an electrical signal that can then be further processed.
An example of a piezo sensor is a piezoelectric 'knock sensor' that monitors the behavior of an engine in order to ensure optimal operation. A knock is a sudden increase in cylinder pressure caused by preignition of the air fuel mixture. A piezo sensor that monitors the sequence of knocks allows the engine to run with the ignition timing as far advanced as possible. The computer will continue to advance the timing until the piezo sensor detects pinging. At that point the computer retards the ignition timing just enough for the pinging to stop.
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