Control for non-linear compliant actuation of an upper arm exoskeleton

Musculoskeletal diseases of the back and upper extremities are one of the main causes of sick leave in Europe. Exoskeletons are one possible approach to preventive measures. The Biomechatronics Group at Technische Universität Ilmenau is developing an antagonistically actuated exoskeleton with non-linear compliance to support flexion and extension of the elbow in repetitive tasks like in assembly. Here, we present a control strategy to achieve joint stiffness control while benefitting from the advantages of non-linear compliant actuation. We use a decentralized control approach, combining two PID controllers to control joint position and string force and thus, joint stiffness, in the antagonistically acting drive. We show limitations and benefits of this approach through simulation and measurement.