Direct design approach of discrete PI controller for hard disk drive spindle motors

As the area density of hard disk drives (HDD) increases, the speed of spinning media disks should be very stable. Now each HDD employs a sensorless brushless DC (BLDC) motor to spin the media platters at a constant speed. However, the drag torque of the spinning disk varies as the slider with writer and reader heads moves in or out to seek the targeted tracks, which will cause the spinning speed fluctuation. In order to reduce the amplitude of the speed fluctuation, a proportional-integral (PI) close-loop speed controller for the spindle motor should be employed. But few publications introduced the proper design of such a speed controller. This paper develops a direct approach to design the HDD spinning speed controller. Firstly, the special features of HDD spindle motors are analyzed and a small signal equivalent model of the spindle BLDC motor is proposed based on HDD's constant spinning speed. Then a discrete PI speed controller is introduced and the design of its PI gains is studied. At last, MATLAB simulation and experimental results are presented to check the effectiveness of the proposed spindle motor model and the design methodology of the discrete PI speed controller.