Variable sampling rate controller design for brushless DC motor

The brushless DC (BLDC) motor uses a few Hall sensors to determine the position of the rotor and to subsequently drive the motor. Without additional hardware, the velocity measurement depends on the consecutive sensor outputs. Therefore, the speed measurement frequency in a BLDC motor is velocity dependent. Conventional approach for the BLDC motor control uses shunt resistor to allow constant current sampling rate. However, the sampling time for the velocity measurement is still variable. To solve this problem, this paper proposes a variable sampling frequency observer (VSFO) for the velocity estimation. A constant sampling rate model-based controller can then be build around the proposed VSFO. The paper also presents a conservative controller design procedure to guarantee system stability. Experimental results showed that the proposed control is effective. The VSFO provides an access to accurate velocity measurement. It also offers additional freedom in the control design.