UC Berkeley

Current upper-limb prosthetic devices remain insufficient to restore the dexterity of human hands. Myoelectric devices driven by surface electromyography electrodes and robotic hands offer flexible operation and multiple grasp types but lack reliable control and inherent feedback. Body-powered devices actuated with Bowden cables and shoulder harnesses, onthe other hand, provide rich kinesthetic feedback and intuitive control but can require large forces to operate with limited degrees of freedom. In device-level comparisons, body-powered prosthesis users do exhibit a number of functional improvements over their myoelectric counterparts, but adoption rates remain low across both prosthesis types. Directed study could reveal the operational aspects which contribute to these observed benefits and drive the development of novel assistive technologies which augment these elements through mechanical or robotic means.

This dissertation explores the mechanisms underlying the performance of body-powered devices as well as ways to innovate on the topology. A desktop haptic test bed is used to identify ideal force and motion thresholds for shoulder-driven body-powered prosthesis operation, and following evaluation reveals that changes in force feedback, not proprioceptive feedback, drive functional gains but changes to both can improve user experience. The inclusion of force-sensitive variable transmissions to body-powered devices is also considered as a means to selectively augment output forces or motions. A pilot study emulating such continuously variable transmissions in simulated grasping trials shows the potential of this approach to reduce both user forces and motions while successfully lifting a wider set of object types. This concept is realized in a physical wearable system that varies its lever arm and corresponding transmission ratio through a rack and pinion mechanism. Testing with human subjects in a real-world grasping task demonstrates the lower operational forcesand motions required by the variable transmission prosthesis but also shows its potential to produce less steady or excessive grasp forces.