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A Novel Sliding Mode Controller for Functional Electrical Stimulation: tracking the lower leg movement.


S. Jezernik

Glasgow, Scotland, University of Glasgow, Invited Talk, Centre for Systems and Control.

An inherently robust closed-loop control strategy called sliding mode (SM)was applied to the problem of knee joint angle tracking by stimulation of the knee extensor and flexor muscles. The controller was synthesized using a state-space model of a human knee and muscles (composed of activation dynamics, muscle dynamics, and biomechanics). The model was experimentally validated in our earlier studies. The derived SM control law provided asymptotic stability of the knee joint angle and velocity. It was derived in a closed-form by considering only a subsystem of the overall muscle-knee state-space model, and by using a joint angle-joint velocity sliding surface stabilized by appropriately generated muscle activation that followed from the sliding mode reachability and stabilizability conditions. The controller was first evaluated in simulations with artificial and physiological knee joint angle trajectories. It was able to track trajectories with a period of 2 s with a mean root-mean-square errors of approximately 2 degrees, which is considered a good performance. It was also shown to be robust to parameter variations of the model. This is important as models for different persons will differ considerably. Further on, the controller was tested in actual experiments on healthy subjects and two SCI patients. The tested movements included steps, ramps, and actual walking trajectories with periods of 1, 2, 4, and 8 seconds. The controller was stable and has achieved the desired tracking with root-mean-square errors ranging from 1.5-5.4 degrees.


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