A design method of a multi-rate servo controller using H-infinity

A method of designing a multi-rate servo controller for a HDD that operates at a higher sampling frequency than that of the position error signal (PES) is proposed. Multi-rate sampling has an advantage for increasing the track density of a HDD without increasing the overhead of the servo information on the disk. A plant model for a design with a higher sampling rate is obtained by adding a zero-interpolator at the output of the position error signal. The controller is calculated by the H∞ method to suppress the noise at the PES sampling frequency that is created by the use of multi-rate sampling. The servo bandwidth and margins of the method are studied for 2.5″ and 3.5″ HDD's, and for a HDD with a dual-stage actuator. The multi-rate sampling is especially effective for the dual-stage actuator because the MEMS actuator's transfer function is very clean.