Harmonious cable actuation mechanism for soft robot joints using a pair of noncircular pulleys

Various slim robots, such as surgical robots or humanoid robot fingers, are remotely actuated using transmission cables. If pull-pull drive is applied to actuate them using circular driving spools regardless of the shape of the joints, the tension of the driving cable becomes difficult to be maintained properly. Fortunately, it is possible to solve such a cable slack problem by providing an appropriate cable actuation length to the joint structure of the robot from the cable driving unit. Therefore, we propose a harmonious nonlinear cable actuation mechanism suitable for driving noncircular shaped joints. The proposed cable driver can mechanically provide the required cable actuation length to suit the angle change of the target joint using a pair of noncircular pulleys without increasing the number of actuators. In this paper, a design methodology of a noncircular pulley that can be applied to pulleyless rolling joints (PR joints) as well as pulleyless hinge joints is shown. Moreover, a practical cable driver is designed for actuating a hyper-redundant discrete bending joint composed of PR joints, and its effectiveness is verified through experiments. This novel cable actuation mechanism using noncircular pulleys or gears is expected to be applicable to various miniature robots such as surgical robots and animal robots of continuum structure in the future.