Perceptual and control properties of a haptic upper-limb prosthetic interface

Current upper-limb prostheses fail to meet user expectation. Common complaints are the large control forces for body-powered prostheses (BPP) and the lack of proprioceptive feedback in myo-electric prostheses (MEP). This study investigates sensitivity and control accuracy and feedback sensitivity of a haptic (master-slave) interface that combines BPP-like control for a MEP utilizing low control forces in the presence of proprioceptive feedback. This first step focuses on static forces. One experiment focused on the just noticeable differences (JND) and the Weber fraction (WF) of the shoulder, and a second on force control. JND and WF were determined by a two-alternative-forcedchoice-method at 4 forces levels (2, 4, 6, and 8 N). Force control was evaluated by a visual matching task and blind reproduction task at the same force levels, and force error (FE) and force variability (FV) were obtained. WF results (7% for 2 N, 3% for 4 N, 3% for 6 N and 2% for 8 N) indicated a level of sensitivity comparable to human weight perception. FE and FV values were small enough as not to affect usability when grasping objects. We conclude that forces of 2-10 N are sufficient to operate an externally powered prosthesis while maintaining a sufficient level of proprioceptive feedback.