Bandwidth estimation of HDD actuators using time delay identification

A method to estimate the maximum achievable bandwidth of HDD actuators without closing the servo loop is presented. The actuator dynamics are characterized by the time delay created by the unstable zeros of the actuator dynamics. The achievable bandwidth is a simple function of the time delay and the sampling frequency. The feedback controller, including a notch filter, can be designed simultaneously. Effects of dynamics variation of the actuator are also considered, and a method to identify the time delay caused by the variation is provided by using the recursive least square method. By applying this method to a single-stage and a dual-stage actuator system of HDDs, it is observed that the variations of the high frequency modes play an important role in the bandwidth improvement, especially of the dual-stage actuator.