Rejection of limit cycles induced from disturbance observers in motion control

A disturbance observer (DOB) is generally introduced into motion control systems to eliminate the unwanted disturbance and plant uncertainty. Due to the quantization error from the sensors and D/A converter, DOB-based controller might generate limit cycles which severely decreases positioning accuracy as the system reaches steady-state positioning. This paper presents a sufficient condition for removing limit cycles and explores the condition to design controller parameters. In addition, since the parameters based on the condition make the system sluggish, an adaptive mechanism is introduced not only to maintain the system performance but also to eliminate limit cycles. The experimental results validate the analytical results and also illustrate the effectiveness of the proposed method.