The piezoelectric ceramics have been considered not to suitable for detecting quasi-static mechanical pressures because their output signal is not directly proportional to the pressure but responses in a so-called differential operation. This is because the resultant electric charges are discharged through the finite internal resistance as well as externally connected circuit. In order to overcome the problem, we have newly devised an intelligent or smart actuator having with a built-in pressure sensor, consisting a multi-layered piezoelectric actuator and a piezoelectric ceramic plate uni-axially joined together. The variation of the electrostatic capacitance of the sensing ceramics upon pressure application has successfully detected by the frequency change of the RC oscillator circuit which makes it possible to realize digitally controlled feed back systems. A result of application of the sensing method for an air regulator has been also discussed.
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