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.