A resonant frequency tracking circuit for a low-coupling-coefficient piezoelectric transducer

This paper presents a circuit which can track the resonant frequency of a piezoelectric transducer (PT) with a low coupling coefficient. Conventionally, the input reactance of a PT approximates zero at its resonant frequency. This characteristic can be used to track the resonant frequency of the PT. However, if the coupling coefficient of a PT is low, the frequency with zero reactance might not exist and could not be directly regarded as the resonant frequency. By actuating the PT with a square wave instead of a sine wave voltage, the phase of the mechanical current, which has zero phase difference with the actuating voltage at the resonant frequency, can be known. Thus, the resonant frequency of the PT can be tracked with techniques related to phase-locked loops. A prototype inverter circuit has been built to verify the proposed control method. The experimental results show that the PT with a low coupling coefficient can be automatically operated at the frequency close to the ideal resonant frequency.