Comments on "Sliding Mode Closed-Loop Control of FES: Controlling the Shank Movement

There are some essential problems with the arguments presented in the above paper about the design of a sliding-mode controller for functional electrical simulation (FES) induced control of knee-joint angle. In this note, we show that applying some approximations in derivation of the control law violates the reaching condition and could introduce some parasitic unmodeled dynamics in the sliding-mode control loop. Therefore, the proposed controller cannot force the system into a sliding-mode regime, and its ability of producing a robust control loop with good tracking performance is theoretically under question.