Rejection of nonlinear disturbance torque in servomechanism with repetitive control

In this paper, a repetitive control method is presented for the rejection of significant nonlinear disturbance torque in servomechanisms. The disturbance torque dealt with is assumed to be periodic in position, which is the main cause for the degradation of the system steady-state precision. A sufficient condition for stability is given. The existence of periodic steady-state error is proved, thus the repetitive control approach can be directly applied to the rejection of disturbance torque coupled to position. It should be noted that this property is also necessary for other learning algorithms based on the period-invariance property of the system. Simulation results show that with the proposed method, the steady-state error due to the disturbance torque can be effectively reduced.