Patent ID: RE40709

Claim:
A drive circuit for a piezoactuator, comprising: at least one piezoelectric element having a first oscillation mode and a second oscillation mode, the at least one piezoelectric element being adapted to oscillate when an AC signal is applied to it, the second oscillation mode having a different oscillation direction than that of the first oscillation mode; a driver for applying an AC drive voltage signal to the at least one piezoelectric element; and a frequency control unit for detecting a first electrical signal from the at least one piezoelectric element indicative of oscillation in the first oscillation mode, for detecting a second electrical signal from the at least one piezoelectric element indicative of oscillation in the second oscillation mode, and for controlling the frequency of the AC drive voltage signal to optimize the phase difference between the first and second electrical signals for a particular operating condition of the at least one piezoelectric element , such that the phase difference is substantially maximized, the frequency control unit further comprising: a phase difference detection circuit for detecting the phase difference between the first and second electrical signals; a time differentiating circuit for determining a time differential of the phase difference detected by the phase difference detection circuit; and a frequency adjusting circuit for increasing the frequency of the AC drive voltage signal when the time differential is positive, and decreasing the frequency of the AC drive voltage signal when the time differential is negative .