Observer-based fractional-order dynamic terminal sliding mode control of active shock absorbing prostheses for lower limb amputees

Abstract

Recent biomedical engineering developments have empowered prosthetic devices to evolve from purely mechanical devices to more sophisticated controlled devices, allowing amputees to perform advanced locomotion modes such as passing stairs and walking on sloped surfaces. However, the strongly coupled nonlinear system dynamics make it difficult for the lower-limb prosthesis (LLP) to adapt to complex tasks and isolate the vibrations and acceleration from the residual limb soft tissue. In this regard, realizing the potential of active LLPs to increase user mobility and efficiency requires reliable, stable, and intuitive control strategies to provide a comfortable gait quality. In this study, a fractional-order dynamic terminal sliding mode controller (FDTSMC) is proposed to effectively isolate the residual limb soft tissue from the vibrations and acceleration arising from the pylon and foot. The proposed sliding surfaces guarantee the fast finite-time system states’ convergence, and the chattering is remarkably alleviated. Furthermore, since from the practical viewpoint, the actuators are non-ideal and are affected by dead-zone and hysteresis that degrade the LLP’s performance, an observer is augmented with the control system to estimate the lumped uncertainties and compensate for the effects of model nonlinear dynamics and disturbances. The closed-loop system stability is ensured in terms of Lyapunov concept. Comparative performance investigations in ideal and non-ideal situations are carried out, and the proposed control scheme’s favorable gait shock absorption performance over observer-based conventional SMC and dynamic SMC approaches is revealed.