A Generalized Disturbance Filter Design and its Applications to a Spinstand Servo System with Microactuator

Narrow-band position error at mid-frequencies around the open-loop crossover frequency can not be effectively reduced using a conventional peak filter, because the attenuation of sensitivity gains has to compromise with the associated decrease of phase margin. This paper presents a general second-order filter that is applicable to reject narrow-band disturbances at any frequency range. The filter zero is designed to minimally degrade the closed-loop system stability and obtain a smooth sensitivity curve around the disturbance frequency. A nonlinear optimization procedure is developed to select the filter parameters such that the statistical position error is minimized. Experimental results of a PZT-actuated head positioning control system on spinstand demonstrate that the add-on filter can further reduce the mid-frequency PES NRRO by 8% and preserve the stability margin of the original control system

I. Introduction

The narrow-band disturbances with spectral energies concentrating at narrow frequency bands commonly exist in a practical servomechanism, e.g., the hard disk drive (HDD) servo system. In HDDs, the track misregistration (TMR) is composed of many factors such as the repeatable runout (RRO) and the nonrepeatable runout (NRRO). Typically, a large portion of the NRRO is contained within narrow frequency bands. In order to meet the requirement for a high track density HDD, the classical loop shaping methods and modern control theories such as and optimal control techniques have been applied to reject the RROs and the narrow-band NRROs [1]. The modern control design based on state-space formulations is an automated design tool, which however often results in an impractically high-order controller. The classical loop shaping methods can provide more intuition and a greater ability to tune designs to achieve performance than the automated tools [2]. The narrow-band disturbance filter proposed in this paper is also based on the classical loop shaping technique.