Modified ADRC for position lower-knee exoskeleton system: Rehabilitation case

Abstract

The goal of this study is to develop a linear disturbance rejection control (ADRC) based on transfer-function approach. The proposed control strategy is applied to control the angular position of knee joint for rehabilitation purpose. The Extended State Observer (ESO) is the core of ADRC strategy and the performance of all ADRC controllers are assessed based on tracking and estimations errors due to controller and observer, respectively. The transfer-based approach of ADRC (TFADRC) is characterized by simplicity and direct control design. A comparison study in performance between conventional linear ADRC (CLADRC) and TFADRC approach has been made. The results based on numerical simulation showed that the proposed approach gives better tracking performance compared to conventional one. Based on Root Mean Square of Error (RMSE) metric, the TFADRC gives less tracking error (0.0205 rad) under load disturbances than that based on CLADRC (0.0547 rad). Moreover, better noise rejection capability can be obtained by TFADRC as compared to the conventional one. However, the price of better performance gained by TFADRC is to actuate higher level of control signal compared to its counterpart.