Model-free super-twisting terminal sliding mode controller using sliding mode disturbance observer for n-DOF upper-limb rehabilitation exoskeleton with backlash hysteresis

AbstractThis paper proposes a model-free super-twisting terminal sliding mode controller (MF-STTSMC) for the n-DOF upper-limb rehabilitation exoskeleton in the presence of external disturbance and backlash hysteresis. This ultra-local model-based control scheme consists of a sliding mode disturbance observer (SMDO) and a super-twisting non-singular fast terminal sliding mode controller (ST-NFTSMC). The ultra-local model is used to approximate the origin complex exoskeleton system in a short sliding time window, which avoids the requirement for accurate modelling and decreases the difficulty of controller design. To compensate for the lumped uncertainties, the SMDO is constructed to achieve finite-time observation and zero estimation error. Besides, to realise finite-time convergence of sliding surface and tracking error with less chattering, the ST-NFTSMC is proposed by employing a super-twisting algorithm. Furthermore, the stability of a closed-loop system with MF-STTSMC is ensured by using the Lyapunov theorem. Finally, to validate the proposed strategy, co-simulations of 7-DOF iReHave upper-limb exoskeleton are realised by using SolidWorks, SimMechanics, and Matlab/Simulink. Compared to time-delay estimation-based intelligent proportional-derivative control (TDE-iPD), α-variable adaptive model-free controller (α-AMFC), and inclusive and enhanced time delay controller (IETDC), the higher performances of the proposed MF-STTSMC are demonstrated.Keywords: Model-free controlsliding mode disturbance observersuper-twisting terminal sliding mode control7-DOF iReHave upper-limb exoskeleton Disclosure statementNo potential conflict of interest was reported by the author(s).Additional informationFundingThe work was partially supported by the National Natural Science Foundation of China [grant number 62173182, 61773212], and the International Science & Technology Cooperation Program of China [grant number 2021YFE0102700].