A hybrid observer for high performance brushless DC motor drives

Brushless DC motor drive systems are used in a wide variety of applications. These drives may be classified as being one of two types: sinusoidal drives in which there are no low-frequency harmonics in the current waveforms and no low-frequency torque ripple; and nonsinusoidal drives in which there is considerable low-frequency harmonic content, both in the current and torque waveforms. Although sinusoidal drives feature superior performance, they are generally more expensive since rotor position must be sensed on a continuous basis, thus requiring an optical encoder or a resolver, whereas relatively inexpensive Hall-effect sensors may be used for nonsinusoidal drives. In this paper, a straightforward hybrid observer is set forth which enables rotor position to be estimated on a continuous basis using information available from the Hall-effect sensors. The proposed observer is experimentally shown to perform just as well as an optical encoder for steady-state conditions and nearly as well as the optical encoder during transient conditions. The proposed scheme provides designers with a new option for rotor position sensing, one which offers an excellent compromise between accuracy and expense.