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Sliding mode control of Functional Electrical Stimulation for knee joint angle tracking


S. Jezernik, P. Inderbitzin, T. Keller, R. Riener

International Workshop on Functional Electrical Stimulation, Vienna, Austria, no. 7, pp. 160-163

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, and was robust to the muscle fatigue. It was derived in a closed-form by considering only a subsystem of the muscle-knee model and using a joint angle-joint velocity sliding surface stabilized by appropriately generated muscle activation that followed from the sliding mode reachability and stabilizability conditions. In our earlier studies, instead of using a closed-form expression for the sliding mode controller, we have only used an approximate sliding mode control law, and have additionally necessitated a cascaded PI controller, which degraded the performance and sometimes caused instability. Simulations with the new controller were carried out in SIMULINK with different knee angle trajectories and different controller parameters, and the results were analyzed. The controller with best parameters was able to track ramps and real knee joint angle walking trajectories with a root-mean-square (RMS) error of about 2 degrees at physiological (fast) speeds.

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% Autogenerated BibTeX entry
@InProceedings { JezEtal:2001:IFA_234,
    author={S. Jezernik and P. Inderbitzin and T. Keller and R. Riener},
    title={{Sliding mode control of Functional Electrical Stimulation
	  for knee joint angle tracking}},
    booktitle={International Workshop on Functional Electrical Stimulation},
    address={Vienna, Austria},
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