Control algorithms for prevention of impacts in rehabilitation systems

Safety is one of the most important requirements in robotic rehabilitation systems. Vulnerable users may get injured or feel pain by a sudden impact, i.e. large assistive torque or fast change in the assistive torque from the rehabilitation systems. In automated rehabilitation systems, the desired torque is determined in real-time based on measurements that represent the patient´s status. Abrupt changes of the desired torque command may cause the impact on the patient during the operation of the rehabilitation system. In this paper, a proxy is placed between the desired position and current position of the actuator as a safety buffer to prevent impacts from the rehabilitation system. The actuator tracks the proxy as accurate as possible with a sliding mode controller, which compensates for the modeling uncertainties and nonlinearities in the actuator. The performance of the proposed control algorithm has been verified by experiments with a compact series elastic actuator installed at the knee joint of an orthotic.