Design and Testing of a Novel, High-Performance Two DoF Prosthetic Wrist

Extrinsically powered prosthetic wrists have the potential to offer significant improvements to the functionality and dexterity of a prosthetic hand. They can also reduce a user’s overreliance on their intact limb and help prevent injury from overuse of upper limb (both intact and residual) and trunk joints. Despite these potential advantages, there are very few prosthetic wrist options that are commercially available and these devices are not commonly used by prosthetic hand users due to several factors including inadequate performance specifications. In this paper, we first seek to establish the target specifications for a prosthetic wrist suitable for both median men and women. We then complete a comprehensive review of the state-of-the-art in extrinsically powered prosthetic wrists in the research, commercial, and patent literature. This review determines that no existing prosthetic wrist meets the target specifications due to the presence of actuators and transmissions that do not offer sufficient torque density, power density, and specific power. In order to address this challenge and produce a prosthesis that achieves target specifications, we next review the performance of existing actuators and transmissions and determine that Brushless DC motors with planetary gearboxes and ball screws offer the best potential to achieve the target specifications. We then present the design of a novel two Degree of Freedom parallel kinematic prosthetic wrist that incorporates this actuator-transmission combination. This first iteration of the proposed prosthetic wrist meets the target torque, speed, and weight but does not meet the target dimensions or range of motion yet. We propose design improvements in subsequent iterations that could lead to a prosthetic wrist that meets all the target specifications of torque, speed, weight, and volume.