Actuators that have characteristics such as light weight, high thrust to weight ratio, and force controllability are desirable for applications such as finger robots and power assist suits. Recently, an indirect force control method using an ultrasonic motor (USM) and a spring has been attracting considerable attention because this method has the potential to satisfy all of the desirable characteristics, namely light weight, high thrust to weight ratio, and force controllability. The USM controls the length of the spring, and the tensional force of the spring is indirectly controlled by the length. As the resonance frequencies of indirect force control systems are low due to the springs, it is difficult to select a high feedback gain. This research therefore proposes a novel resonance ratio controller for indirect force control systems. The validity of the controller is verified by simulation and experiment. The overshoot ratio of the conventional controller is more than 10%, while that of the proposed controller is around 1%, when the rise time is 0.30 s.
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