Parametric Optimization of Series Elastic Actuator for Design and Control of Wearable Robotic Ankle Foot Orthosis

Abstract

Ongoing research on wearable rehabilitation robots explores challenges related to actuation and control in human-centric environments. To achieve precise force control and ensure smooth operation of rehabilitation devices, new actuation mechanisms and advanced control methods are continually being analyzed. The task becomes more challenging when the focus is on the human ankle joint. Conventional actuation methods are being replaced by hybrid actuation methods like series elastic actuators to take advantage of both passive and active elements. Selecting parameters for passive elements and choosing control methods remain challenging due to their behavior and limited operational range. This work has been done to investigate the effect of the passive element spring in series elastic actuators and improve their applicability in the rehabilitation domain. Response of the series elastic actuator for torque tracking when subjected to various standard input signals is thoroughly analysed employing conventional Proportional Integral Derivative (PID) control and Robust Integral of Sign of Error (RISE) based advanced control. The emphasis is on understanding how these controls perform when applied to active orthosis for rehabilitation purposes using RMSE and mean error metrics. The PID control stabilizes much more quickly within a time frame of ~0.02 seconds, whereas RISE control for the same input stabilises, taking ~0.48 seconds to start tracking with minimal error. RISE excels for ankle reference inputs, with extremely low RMSE and Mean Error, especially at higher stiffness values. Results obtained from the analysis will aid orthotic designers in designing robotic ankle foot orthosis and implementing control methodology for ankle rehabilitation robots.