Brushless DC Motor (BLDCM) systems, which consist of a permanent magnet synchronous motor (PMSM), an inverter, and a position sensor mounted on the motor shaft, have recently become widely used in many industrial applications. However, when trying to make a high precision control for the BLDCM, such as making the steady state error less than 0.01% of the rating speed , the effect of the small torque ripple which does not appear in the DC Motors cannot be ignored. This torque ripple in BLDCM is mainly due to fluctuations of the field distribution, the influence of the switching dead time and the cogging torque. The offset error and the unbalanced gain between each phase of the current detector also contribute to the periodical disturbance. This paper presents a new control method for small vibration suppression. We propose a new disturbance torque model which consists of several kinds of periodical components and a DC component. As the state space observer theory can be applied to this model, an observer based approach is used to estimate the amplitude and the phase of these periodical disturbances assuming that the frequency is known. The estimated torque is fed to the current reference to suppress the vibration. This method can he applied not only to a constant speed control system but also to a variable speed control system.Experimental results show the validity of the proposal method.

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