Sliding Mode Control Application in Knee and Lower-Limb Exoskeletons: A Review

This review systematically surveys Sliding-Mode Control (SMC) strategies applied to knee and lower-limb exoskeletons with direct knee-related control or evaluation, emphasising research published over the last decade (2015–2025). The paper first outlines key control challenges arising from nonlinear joint dynamics, external disturbances, sensor feedback, and uncertainties in human–robot interaction. The review then analyses 126 unique studies, including peer-reviewed journal articles and full conference papers, covering classical SMC, higher-order formulations, observer-based architectures, intelligent adaptive schemes, and optimisation-enhanced approaches. Beyond tracking accuracy and chattering behaviour, the review considers practical aspects relevant to wearable rehabilitation systems, including sensing requirements, real-time implementation, patient-specific adaptation, and human–robot interaction. These aspects highlight current methodological trends, practical limitations, and the gap between control performance and real-world rehabilitation use. The review aims to support the development of more robust, safe, and practically deployable control frameworks for knee and lower-limb exoskeletons in rehabilitation applications.