Controller Design and Analysis of Dual-stage Hard Disk Drives in the Presence of Micro-Actuator Stroke Limitation

Abstract

With the continuous increase in storage capacity, the width of a hard disk drive (HDD) data track is decreasing, and the traditional single-stage actuation system is insufficient for such high-precision actuation. Hence, a secondary high-bandwidth piezoelectric (PZT) type micro-actuator is widely used to enhance HDD's precision servo tracking capability. However, the micro-actuator is typically prone to actuator saturation, which limits the achievable closed-loop sensitivity performance of a controller design. Hence, any undesired high-magnitude disturbance might cause an aggressive controller to demand high stroke usage from the actuator system, which can lead to saturation of the micro-actuator. This work presents an analysis method to predict the micro-actuator stroke usage for a given controller by taking advantage of the stochastic interpretation of H2 system norm. Measured data from a Seagate HDD are used to model several disturbance environments and calibrate the proposed prediction model. Then the idea of predicting the micro-actuator stroke usage is used to explore a mixed H2/H-infinity controller synthesis method to avoid saturation of the micro-actuator in a dual-stage actuator system while maximizing the closed-loop disturbance rejection performance. Finally, a two-step nonlinear analysis method is also discussed to determine the worst-case disturbance rejection performance under micro-actuator saturation by utilizing the concept of bounded nonlinearity. Necessary mathematical proofs are provided to support the proposed analysis method and a numerical example is discussed with a validation process via numerical simulation.