LAUSR

Abstract This paper investigates the extended state observer (ESO) based friction compensation at low velocities with only the position measurement. ESO is an effective model-free friction compensation technique and thus is employed in this paper. Based on the describing function analysis, it is revealed that the higher the observer bandwidth is, the larger the velocity feedback gain should be to suppress the limit cycle. However, the available damping provided by the derivative control is restricted when the signal-to-noise ratio of the velocity is low. Under such a condition, the observer bandwidth cannot be high and the friction compensation performance is thus limited. To solve this conflict, a switching control law based on the ESO is proposed to compensate the friction in a fast manner and suppress the limit cycle simultaneously. The switching strategy aims to determine when the disturbance compensation should be added in the control signal to eliminate the friction induced oscillations without making the system response become sluggish. Such an idea is enlightened by the fact that nonlinear modifications to the integral action are always needed in practice. Hardware experiments are performed on a brushless DC motor to validate the effectiveness of the proposed compensation scheme.