CHALLENGES OF CONTROL DESIGN FOR PRECISION SERVO SYSTEM WITH APPLICATION ON HARD DISK DRIVE

Abstract

With the advent of technology of nanoscience and nanotechnology in the past two decades, nanopositioning systems have received a considerable attention from both science and industry. There is an added impetus on design of nanopositioning systems since they have a critical role in achieving speed and accuracy of many devices such as Scanning Probe Microscope (SPM) and Hard Disk Drives (HDD). In this thesis, several challenges on the control design of nano-positioning systems are studied. We mainly focus on the robust control design methodologies. The robustness is in terms of parametric uncertainties of the nominal plant and sampling time variation. The proposed approaches are less conservative and more computational efficient than the conventional robust control design approaches. The HDD servo system is used as an example in the simulation and implementation of these controllers to evaluate the performance of the proposed methods. In nanopositioning systems, the vibration of flexible modes is a major obstacle to achieve higher bandwidth which is required for the demanded performance. An effective method of suppressing these modes in the piezo-electric micro-actuator is presented which is based on the principle of the integral resonant control. Moreover, a multi-objective robust controller is designed which minimizes the worst case root mean square (RMS) value of the positioning error signal (PES) subject to the closed-loop stability in the presence of parametric and dynamic uncertainties. A sequential algorithm based on ellipsoid iteration is utilized to handle the parametric uncertainties. The dynamic uncertainties are also represented as linear fractional transformation (LFT) and by virtue of the small gain theorem, the stability of the closed loop system is guaranteed. The design is less conservative compared to classical robust approaches. Finally, a novel method of discretizing the continuous-time plant dynamics is presented which deals with the systems with irregular sampling rate and regular control update.Irregular sampling is gradually becoming an issue not to be neglected in modern HDDs. In this thesis, a non-sequential probabilistic algorithm based on a relatively new approach called ``scenario oprimization'' is utilized to handle uncertainties with sampling rate in HDD.