Gravity-Balancing Leg Orthosis and Its Performance Evaluation
IEEE Transactions on Robotics
Complementary Stability and Loop Shaping for Improved Human–Robot Interaction
IEEE Transactions on Robotics
Lower Extremity Exoskeletons and Active Orthoses: Challenges and State-of-the-Art
IEEE Transactions on Robotics
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Limited research has been done on exoskeletons to enable faster movements of the lower extremities. An exoskeletonâ聙聶s mechanism can actually hinder agility by adding weight, inertia and friction to the legs; compensating inertia through control is particularly difficult due to instability issues. The added inertia will reduce the natural frequency of the legs, probably leading to lower step frequency during walking. We present a control method that produces an approximate compensation of an exoskeletonâ聙聶s inertia. The aim is making the natural frequency of the exoskeleton-assisted leg larger than that of the unaided leg. The method uses admittance control to compensate for the weight and friction of the exoskeleton. Inertia compensation is emulated by adding a feedback loop consisting of low-pass filtered acceleration multiplied by a negative gain. This gain simulates negative inertia in the low-frequency range. We tested the controller on a statically supported, single-degree-of-freedom exoskeleton that assists swing movements of the leg. Subjects performed movement sequences, first unassisted and then using the exoskeleton, in the context of a computer-based task resembling a race. With zero inertia compensation, the steady-state frequency of the leg swing was consistently reduced. Adding inertia compensation enabled subjects to recover their normal frequency of swing.