Abstract
An ultrasonic motor (USM) can produce a “free state” where the frictional force between the rotor and the stator diminishes when a standing wave is excited in the USM instead of a traveling wave. In this paper we propose a three state PWM drive method in which the duty ratios of a traveling wave, a standing wave and a switch-off period are changed in order to control the motion of the USM. It was confirmed that a hysteresis of torque curve for the duty ratio of the traveling wave was reduced by introducing a standing wave to the stator. If the duty ratio of the standing wave is small, the large torque due to a self-locking characteristic is obtained even at the small duty ratio of a traveling wave. When the standing wave ratio is increased, the relationship between the duty ratio of a traveling wave and the output torque of an USM becomes approximately linear. This linearity is optimum at the moderate duty ratio of a standing wave with a short switch-off period.
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