Isokinetic Muscle Strength Training Strategy of an Ankle Rehabilitation Robot Based on Adaptive Gain and Cascade PID Control

Isokinetic muscle strength training refers to the mode of movement based on constant speed and variable resistance, which can guarantee the maximum resistance and force–distance output of each muscle in different angles of exercise. In this article, we develop an isokinetic muscle strength training strategy based on the adaptive gain and cascade proportion integration differentiation (PID) controller for ankle rehabilitation on our newly developed ankle robotic system. First, a heuristic threshold intention recognition method based on time-series integration was proposed to precisely recognize the motion intention, thus inducing the motion in this direction. Then, an adaptive gain algorithm was developed to provide speed gain for the isokinetic training, to avoid jitter during speed switching. Finally, the isokinetic characteristic was realized by the cascade PID controller, which can control the motion velocity in a given range with fast response speed. Experiments with healthy subjects showed good performance in the smoothness of the control system, the accuracy, and the real-time performance of velocity tracking. By introducing the isokinetic characteristic in ankle rehabilitation, the ankle robot can provide resistance training at a constant speed no matter how much force the patient uses, which is a very functional supplement and improvement for ankle rehabilitation.