A Symmetric Bidirectional Soft Rotary Actuator with a Bendable Shaft Driven by Extension-Type Pneumatic Actuators

Soft actuators capable of flexible motion have been actively studied and developed. Soft actuators that can extend and bend have been developed; however, soft rotary actuators have rarely been reported. In our previous studies, a soft rotary actuator (SRA) that generates rotational motion by constraining extension-type soft actuators (EFPAs) in a spiral shape was developed. However, the SRA had the limitation of rotating only in one direction, either clockwise or counterclockwise. In this paper, we aim to develop a symmetric bidirectional soft rotary actuator (SB-SRA) that achieves comparable rotational performance in both clockwise and counterclockwise directions while maintaining the flexibility of polymeric materials, such as rubber, and allowing static hysteresis adjustment. In particular, arranging several EFPAs so that they do not interfere with each other during extension enables the SRA to generate both clockwise and counterclockwise rotational motion. Unlike previously reported unidirectional soft rotary actuators, and unlike rigid antagonistic bidirectional joints that require bearings and a dedicated brake, the proposed SB-SRA achieves symmetric bidirectional rotation with a bendable rotational axis and enables static hysteresis compensation using only the opposing actuators. The results show that the maximum rotation angle and torque of the SB-SRA are approximately 105° and 0.51 Nm, respectively, under an input pressure of 500 kPa. The maximum directional differences between clockwise and counterclockwise operation were 3.3% for the rotation angle and 2.4% for the torque, demonstrating approximately symmetric output characteristics. In addition, static hysteresis adjustment can be achieved by varying the supply pressures applied to the opposing EFPAs.