Preliminary design and development of a two degrees of freedom passive compliant prosthetic wrist with switchable stiffness

Amputees wearing a transradial prosthesis are compelled to perform compensatory movements of the shoulder, elbow and trunk, while reaching objects in the environment, due to the lack of degrees of freedom (DoF) in their artificial arm. These unnatural movements yield to articulation injuries in the long run, which could be reduced/eliminated by adding an articulated compliant wrist. Several passive compliant wrists are commercially available, however they are manually adjustable and present a unique level of stiffness. These features make them not very functional when it comes to use them in a broad range of daily activities. In this paper we present the design, preliminary development and characterization of an innovative two DoFs prosthetic wrist. The movements allowed to the hand mounted on the wrist are flexion/extension and abduction/adduction. The wrist is provided with two switchable levels of passive compliance that can be selected by means of a linear actuation system. A preliminary prototype was developed and characterized in terms of stiffness response.