Input-output nonlinearities and time delays increase tracking errors in hand grasp neuroprostheses.

Tracking tasks were designed to study how different input-output relationships (nonlinearities and time delays) would affect the performance of functional neuromuscular stimulation (FNS) hand grasp neuroprostheses. Simulated hand grasp neuroprostheses and real hand grasp neuroprostheses with and without closed-loop control were evaluated, with the subjects adjusting the input to the system so that the output would match a visual target track at three different bandwidths (able bodied subjects with the simulated systems and neuroprosthesis users with the real systems). For both systems, the tracking error increased as the input-output nonlinearity increased. Other factors that affected tracking performance were the target bandwidth and delays in the neuroprosthesis. The results support the hypothesis that hand grasp neuroprostheses with linear input-output properties and no delays will be controlled more accurately.