Torsional Pneumatic Actuator Based on Pre-Twisted Pneumatic Tubes for Soft Robotic Manipulators

The human forearm produces torsional motions that are essential in daily activities such as opening doors or containers. It has a large range of motion (ROM), can produce large torques, can move bidirectionally, and can stiffen at will. Implementing such motions in a rigid robotic manipulator is easy due to the inherently rotational motions of electric motors, but pneumatic actuators do not inherently produce rotational motions. Indeed, most torsional pneumatic actuators are capable of unidirectional or bidirectional motions but do not have the antagonistic actuation capabilities necessary to have variable stiffness. Furthermore, their actuation characteristics are very different from those of the human forearm. This article proposes a torsional pneumatic actuator based on pretwisted pneumatic tubes that uses a hybrid hard-soft structure. These pneumatic tubes can be installed at an offset from the rotation axis such that antagonistic tubes can be installed on a single actuator. The actuator is capable of bidirectional torsional motions, can change its torsional stiffness, and has an actuation performance comparable to that of the human forearm. It was implemented in a soft robotic manipulator with seven degrees of freedom with a wide ROM and is capable of rotating objects while manipulating them.