Positioning Control System of a 3-Dimensional Ultrasonic Motor with Spherical Rotor

This project is concerned with the design of the control system for the accurate positioning of the spherical rotor of an ultrasonic motor equipped with piezoelectric actuators. The motor is intended to be used for trajectory following of flying objects as well as to support human articulations (shoulder, knee) of handicapped. The ultrasonic motor has 3 stators with piezoelectric actuators in a horizontal arrangement to support and transmit motion to the spherical rotor. The rotor has a bar attached to it which moves in a spherical fashion. To determine the position of the bar, two optical encoders are used and their signals are input to a computer which calculates the 3 phases to be applied to each stator. A control algorithm is proposed which integrates a complex structure of geometrical relations with a PID controller to compute the phases which should be in the range between –90o and 90o. Experimental results show that the rotor bar achieves the desired spatial positions and follows desired trajectories with fast response and good accuracy even in the presence of external disturbances. These results demonstrate the feasibility of ultrasonic motors to perform positioning tasks with high degree of precision, robustness and autonomy. .