PWM driving characteristics of robot hand with fingers using vibration-type ultrasonic motors

A Langevin-type ultrasonic vibrator works as an ultrasonic motor when it is pressed slant wise against the surface of an object. This paper describes a robot hand with three fingers that each works as an ultrasonic motor. This robot hand can simultaneously grasp a cylindrical body and rotate the body about its axis. The authors propose a PWM (pulse width modulation) method for controlling the speed of ultrasonic motors by changing the duty ratio of the voltage applied to the motors. The vibration of the ultrasonic motor and the driving characteristics of a robot hand were examined experimentally. It was confirmed that the vibration speed of the tip of the ultrasonic motor wets approximately proportional to the duty ratio of the applied voltage. The velocity of the motor increases with an increase in duty ratio, except for the offset region. The torque characteristic curves approach the point of origin, moving parallel as the duty ratio decreases.