Robust resonant frequency tracking control for ultrasonic-motor drive

Ultrasonic motors, used as compact servomotors for precision positioning, have advantages over electromagnetic motors. However, they are difficult to operate, especially since they must be driven by a variable frequency power source, and require using the resonant frequency tracking control for precision tracking of the motor's resonant frequency. In conventional systems, the frequency tracking control is based on the closed-loop control technology which is represented using a phase-locked loop (PLL) technique. A problem with this approach is that the resonance is easily affected by various factors such as waveform distortions, frequency hysteresis, and external noises, a situation unfavorable both to the response characteristic during the intermittent pulse drive and to the torque. In this paper, a frequency tracking method was proposed which is resistant to these disturbances. It includes a novel frequency scan method for searching the maximum value of the motor current which is proportional to the torque using the open-loop sampling control. This method was tested and found to be effective in terms of better resistance to disturbances, by comparison tests with the conventional PLL method. The obtained results demonstrated that the response characteristic can be improved up to 10 times that of the conventional PLL method. The output torque showed a 25% improvement, and the torque pulsations were reduced 10-fold. A one-chip LSI implementation of the proposed circuit was also attempted and these results are reported. The research findings demonstrated that it is possible to achieve high-quality economical control for ultrasonic motors, allowing their successful practical applications. (C) 1997 Scripta Technica, Inc.