Torque control of traveling-wave-type ultrasonic motors using the friction contact model

As an alternative to an electromagnetic motor, a traveling-wave-type ultrasonic motor (USM) has been attracted considerable attention. In this paper, torque control system design of the USM is discussed to implement the USM for a robot. The torque control method is proposed to apply the friction contact model as the control model of the USM and vibration amplitude control is built into the inner loop. Actually, torque control is conducted using the designed system. Firstly, the USM is modeled as the 3 models : the vibration model, the friction contact model and the rotor model. The friction contact model is the discrete spring model taking into account the slip. Vibration amplitude control is proposed and designed because the spring model assumed that the vibration amplitude is known. Secondly, the control apparatus using FPGA (Field Programming Gate Allay) is constructed and the control experiments are conducted. Relationship between torque and rotational speed is measured to verify the friction contact model. Finally, torque control experiment is conducted using the designed vibration amplitude control. Experimental results show that the designed torque control has an advantage over previous studies.