On Low-Velocity Compensation of Brushless DC Servo in the Absence of Friction Model

Friction compensation is usually realized based on a friction model; however, the induced undercompensation or overcompensation, which can lead to steady-error or limit-cycle phenomenon, respectively, is inevitable. A novel straightforward reference compensation scheme for the brushless dc servo system independent of the friction model is presented to reject the aftereffect caused by the friction instead of its original mechanics. Neither the accurate friction model nor any estimation of some specific characteristic parameters is needed in this design. Moreover, the inexact velocity estimation information is not as crucial as it is in the traditional compensation methods. A linear control law is proposed based on a reduced-order observer, which could be readily implemented in low-cost embedded fixed-point chips. Two guidelines for the selection of control parameters are presented in terms of delay sensitivity and limit-cycle avoidance. The approach can be applied to the cases with multiple disturbance torque sources except the friction one, where the friction-model-based strategy is difficult to take effect. Finally, comparative mathematical simulation and practical experiment results are used to validate the effectiveness of the proposed method. Substantial low-velocity performance improvements are achieved in the hardware experiments.