A system for controlling an ultrasonic transducer utilizes an indication of the relative phase of the voltage on the transducer and the current passing through the transducer to control the relative phase to a value that has been preselected to correspond to resonance of an electromechanical system that includes the ultrasonic transducer. In one example application, the system includes a waveform generator that produces an oscillating signal, and the frequency of the oscillating signal is adjustable under control of the control processor. The control processor adjusts the frequency of the oscillating signal according to a proportional control law at least some of the time, and according to a proportional-integral control law at least some of the time. The control processor may also control the amplitude of the oscillation of the ultrasonic transducer.