Flexible Ultrasonic Motor

Organisms that exploit soft structures produce the incredible capabilities for locomotion and manipulation in complex natural environments. While conventional robots with discrete links and rigid actuators are fast, strong, and easy to control, they struggle to operate in such surroundings. Soft robots with a deformable, continuum body and soft actuators can potentially approach this problem due to their elasticity, safety of interaction, and environmental adaptability. However, soft actuators have several problems including a limited stroke, difficulty of control, and slow response time, restricting their deployment. On the other hand, rigid actuators such as electromagnetic and piezoelectric motors widely adopted today solve these drawbacks, but do not have the softness. 

A combination of soft and classical technologies may address this challenge. The author proposes a flexible ultrasonic motor that consists of a rigid motor's stator and a flexible elongated shaft. It consists of a single metal cube stator with a hole and an elastic and long coil spring inserted into the hole. When voltages are applied to piezoelectric elements on the stator, a shaft inserted the hole moves back and forth. The coil spring has a slightly larger diameter than the stator hole and contacts the inner surface of the stator. Not only does the coil spring provide flexibility to the slider, but also acts as a pre-pressure mechanism to improve motor output. This motor can obtain both a flexibility and stroke (travelling distance) by designing the dimensions of the coil spring.The coil spring also works as a position sensor by regarding itself as a variable resistance. Changes in the resistance between the stator and the end of a coil are converted to a voltage and used for position detection. The resulting sensor-actuator system has good response characteristics, high linearity, and robustness, without reducing flexibility and controllability.