Improved Control for Functional Electrical Stimulation to Restore Walking

Abstract

Functional electrical stimulation (FES) technology carries the potential for restoring walking in patients with paralysis of the legs. The goal is to bring this technology to a large number of people and make them functional walkers. In order to transfer research results into a usable, daily assistive system, the control of FES must be improved. The control of FES systems designed to restore walking uses the principles of robotics. A method that follows the principles of natural control used for walking in the able-bodied is described in this paper. A hierarchical hybrid control (HHC) method has been selected because of the similarities of the flow of information between HHC and natural control of movements. The controller suggested here is a three-level structure. The top, decision level is left with the user, and is exclusively natural. The coordination level uses the finite state model of walking; this represents walking as a sequence of ‘sensing-acting pairs’, called artificial reflexes. These artificial reflexes can be realized by a so-called rule-based control (RBC) that decomposes the control to individual actuator controllers. RBC relates to the process of locomotion and is, therefore, general for all potential users. The third, the actuator, lower level of control is based on a biomechanical model of the neuromusculo-skeletal system at a single joint level. The parameters of the model have to be customized to the user, and they are determined by individual properties (muscle force vs muscle length, velocity of shortening, and recruitment characteristics).