Pre-loading Mechanism and Angular Positioning Control Algorithm for Multi-DOF Ultrasonic Motor with a Weight Load

A multi-degree of freedom (multi-DOF) ultrasonic motor is anticipated as an actuator for the next generation because it can drive three-DOF motion with high torque and low noise. Before a multi-DOF ultrasonic motor can be used as a servomotor in actual robots, we have to develop a proper pre-loading mechanism and control algorithm for quick multi-DOF motion. This paper proposes a pre-loading mechanism and control algorithm for the multi-DOF ultrasonic motor, considering application to the auditory tele-existence robot “Telehead”. TeleHead is an elaborate dummy head system that transfers real sound fields to remote users. The dummy head must track three-DOF human head movement quietly in real time. The pre-loading mechanism is designed to meet the physical and geometrical requirements of the neck mechanism of TeleHead. The pre-loading method, by using springs and a counter weight, can generate enough pre-loading force and reduce resistance torque when the dummy head as a weight load is in the inclined state. The control algorithm accomplishes smooth multi-DOF motion of the motor. In order to prevent the motor from serious jagging, the control algorithm uses a self-tuning of feedback gain based on reinforcement learning method. Experimental results show that a motor with a 1-kg weight load can be properly driven with sufficient speed by using the proposed methods.