Evaluation of a Fuzzy-Based Impedance Control Strategy on a Powered Lower Exoskeleton

This paper comprehensively presents the analysis, design, and control of a wearable lower limb exoskeleton intended to enhance human performance and support load-carrying. The exoskeleton is powered at hip and knee joints to provide maximum joint torques of 74 Nm for the joint flexion/extension augmentation and load support. Typical issues regarding the implementation of the exoskeleton, such as mechanical design, sensory system, distributed embedded system, and high-speed networked control architecture are briefly presented. In order to control the coupled human-robot system, a new fuzzy-based impedance control strategy, previously developed by the authors, is used to provide assistive torques by regulating the desired impedance between the exoskeleton and a wearer's limb according to a specific motion speed. The effect of human behaviours on the change of impedance parameters across variable walking speeds is adopted to design the fuzzy rules for the control strategy. As a result, the fuzzy-based impedance regulation is separately designed for swing and stance walking phases to adapt to the change. The control performance of the designed exoskeleton evaluated on a bench-testing over different ranges of walking speeds (about 0.3-1.2 m/s) have demonstrated that, resulting interaction torque, human-exoskeleton tracking error, and electrical power consumption are significantly reduced as compared to a traditional impedance control. Besides that, an average of 72.3 % of the load is transferred to the ground by the exoskeleton during the stance phase of walking. The developed control strategy on the lower exoskeleton has the potential to increase comfort and adaptation to users during daily use.