A Compliant Robotic Wrist Orthosis Driven by Twisted String Actuators

Robotic rehabilitation is advantageous as it allows for dynamic exercise routines that are more accurate and efficient than human-led routines. However, existing robotic wrist rehabilitation devices are often bulky and are tethered in rehabilitation centers. It is difficult to realize compact wearable wrist orthosis that is capable of inducing three degrees of freedom (DOF) of the wrist. This paper presents the first wearable wrist orthosis with three DOF driven by twisted string actuators (TSAs) using both stiff fishing lines and compliant super-coiled polymer (SCP) strings. The design considerations of the robotic wrist orthosis are provided in detail. Experiments are performed to characterize the compliance and force production of TSAs. The device is capable of inducing pronation or supination, flexion or extension, and abduction or adduction, with a range of 117.9 degrees, 115.5 degrees, and 73.4 degrees respectively. In addition, it is demonstrated that the device can fully induce wrist movement of a human subject without the need of activating the human subject's muscles.