Combined Feedback-Feed Forward Control Strategy for a Knee Rehabilitation Device with Soft Actuation

Assistive devices for rehabilitation purposes have gained much attention in robotics research. Using actuation systems that include compliant elements, provide advantages such as natural motions of robotic devices and safety in the interaction with people. These actuation systems are called soft actuators. There are parallel elastic actuators (PEA), series elastic actuators (SEA) and variable stiffness actuators (VSA), which differ among them by the position of the compliant element and the possibility of changing the stiffness online. We have designed a five-bar-linkage knee rehabilitation system which uses the advantages of soft actuation. To accomplish desired tasks in a proper manner, using automatic systems, control strategies are required. In our case, this means to reproduce the desired motions without affecting the patient. In this way, the control system is crucial. In this chapter, we present a combined feedback-feedforward control strategy for the knee rehabilitation device designed. This work was partially presented before in [16], where we discussed the strategy and presented some simulation results. In this chapter we extend the results, presenting experimental trials to validate the performance of the controller and the behavior of the system. The goal of the proposed strategy is to control the system while maintaining the intrinsic softness of the actuators when the patient is in the rehabilitation process. The feedback control strategy acts in a defined threshold to maintain the stiffness of the system, and it is combined with a feed-forward decision control to reject disturbances. The simulations and the experimental results presented are obtained from the analysis of a One-Degree-of-Freedom (DoF) soft actuated system, to allow us to have an insight of the controller and the system, without losing generality.