Feedback control of foot eversion in the adaptive peroneal stimulator

The limited ability to dorsiflex the foot, known as drop foot, can be treated by functional electrical stimulation. Therein, undesired foot eversion/inversion is a common problem which is usually corrected by tedious manual repositioning of the electrodes. We address this issue by presenting a feedback-control solution featuring three major contributions: (1) an algorithm for inertial sensor-based foot-to-ground angle measurement with periodic drift correction; (2) a three-electrode setup that allows distribution of an overall stimulation intensity to the tibialis anterior muscle and to the superficial peroneal nerve that innervates the fibularis longus muscle, thus decoupling dorsiflexion and eversion control; (3) a run-to-run controller and an iterative learning controller, both of which use step-by-step learning to achieve desired eversion foot-to-ground angles. Experiments with a chronic drop foot patient demonstrate compensation of undesired eversion/inversion within at most two steps, while dorsiflexion angle trajectories are not affected.