Nonlinear joint-angle feedback control of electrically stimulated and λ-controlled antagonistic muscle pairs

In order to control human limb movements in neuro-prosthetic systems, a nonlinear, model-based control strategy for torque generation by antagonistic muscle pairs is presented. The controller based on exact linearization methods enables the tracking of reference joint torque profiles and the generation of pre-defined muscular co-contractions. The controller adjusts the desired recruitment levels λ of both muscles that are controlled by two underlying λ-Controllers automatically compensating muscular fatigue. Estimates on λ are obtained from electrical stimulation evoked Electromyography (EMG)-measurements. Short-term increases of the muscular co-contractions are used to achieve an exact tracking of the accelerating joint torque also in the presence of actuation variable constraints which result from the fact that only positive muscular contractions can occur. The linearization-based controller can serve as underlying controller in cascaded control schemes for the joint-angle or velocity. The feasibility of the proposed approach was demonstrated in a simulation study as well as by a joint-angle control experiment for a healthy subject. Concluding, the authors expect that the proposed approach has a great potential for the future control of artificial limb movements also because of the possibility of modulating the system stiffness by controlled co-contractions of the muscles.