Discrete-time output feedback nonlinear control for combined low-and high-frequency disturbance compensation

We present a discrete-time output feedback nonlinear control algorithm for reference tracking under both broad-band disturbances at low frequencies and narrow-band disturbances at high frequencies. A discrete-time nonlinear damping backstepping controller with an extended state observer is proposed to track the desired output and to compensate for low-frequency broad-band disturbances. A narrow-band disturbance observer is constructed for rejecting narrow-band high-frequency disturbances. This combination method gives us the merit that both broad-band disturbances at low frequencies and narrow-band disturbances at high frequencies can be simultaneously compensated.