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Adaptive control of ultrasonic motors using the maximum power point tracking method

The properties of ultrasonic piezoelectric motors depend on applied load and temperature. The output behaviour is characterized by the nonlinearity of the power transmission at the friction interface between stator and rotor. Because of these inherently nonlinear characteristics, a resonant frequency tracking control is imperative for some particular applications where direct speed or position sensing is not applicable and harsh environment conditions with large thermal gradients and important load variations subsist. We suggest a maximum power point tracking (MPPT) approach, which is well known from contactless energy transmission systems. The MPPT controller was compared to a phase-locked loop controller for ultrasonic motors known from literature. The resonant frequency can be tracked over a far wider working range compared to the phase locking range. Also, the MPPT control circuit is very easy to tune and no sophisticated loop gain design is needed. The analog version of the MPPT controller was built and tested with linear and rotary ultrasonic motors.