A discrete time variable structure controller for a brushless DC motor drive

The design and the microprocessor-based implementation of a variable-structure-strategy (VSS) controller for a brushless DC motor drive are described. The controller is a conventional variable-structure design in the continuous-time domain. However, the microprocessor implementation using a constant sample period implies that full sliding mode is not achieved. The properties of the quasi-sliding that results are explored. It is shown that the sliding line expands into a sliding region, which can be described as a sector. The size of this sector is related to the sampling period and the switching gains. A modified design procedure is proposed for discrete-time VSS design. The design was verified on an experimental set-up, which generated variations in system parameters as well as external load disturbances