Multirate DC Motor Speed Controller Design for Zero Steady-State Ripple

This paper deals with the design of a multirate DC motor speed controller where speed is sampled at a slower rate than armature current. It is first shown by a design and simulation that multirate linear quadratic regulators produce ripple in the steady-state current fed to the motor. From the multirate LQR controller, we evolve a condition on the gains for zero steady-state ripple. We obtain the gains for the new controller by matching, in a least squares sense, the discrete time step response of speed (at the slow rate) with that obtained from the earlier designed multirate LQR. The performance of the new regulator is then studied by simulation. The degree of freedom present in the proposed multirate controller for pole-zero assignment of the slow rate discrete transfer function of speed to the reference input signal is investigated. The proposed controller and its pole-zero assignment properties are extendible to systems similar to the DC-motor speed control system considered in this paper.