Multirate digital control for high track density magnetic disk drives

As track density increases, higher control bandwidth is required to attain sufficient positioning accuracy while suppressing the effect of high frequency disturbances in disk drive controllers/actuators. This paper presents a computation effective fixed order multirate digital control system. The multirate controller outputs control at each control update instant, while the multirate control scheme is evenly computed every measurement sampling period, thereby eliminating the need for very powerful processors for implementation. As the input multiplicity increases, the control command becomes smoother, without losing phase and gain margins, so as to reduce the chance of the actuator flexible mode excitation that should be avoided in high track density disk drive servos. Application results demonstrate how the proposed multirate digital scheme substantially removes the excitation of the actuator arm/suspension flexible mode dynamics due to the discrete stepwise control input.