On the sliding mode control for DC servo mechanisms in the presence of unmodeled dynamics

In this paper, via describing function techniques, sliding mode control of DC servo mechanisms is analyzed in the presence of unmodeled dynamics. We first show that, a conventional sliding mode controller with signum function will inevitably generate a limit cycle where a second or higher order unmodeled dynamics exists. A fractional interpolation based smoothing scheme is then proposed to eliminate the limit cycle, and maintain a reasonable tracking precision bound. In particular, a DC servo motor with unmodeled stator and sensor dynamics is taken into consideration. Both theoretical analysis and simulation results verify the effectiveness of the proposed new fractional interpolation control scheme.