Short seeking and settling control for single actuator and dual-stage actuator hard disk drive servo systems

This dissertation is concerned with short seeking control and settling control for single actuator and dual-stage actuator hard disk drive (HDD) systems.;Short seeking control is for moving the read/write head across a few tracks to the target track quickly and precisely. Two-degree-of-freedom (2DOF) control is a popular method for short seeking control of hard disk drive, which requires design and implementation of reference trajectory and feed-forward controllers. In this dissertation, several new designs for short seeking control are proposed and systematically developed for enhanced performance with a small amount of real time computation.;First, short seeking control by optimal finite-step feedforward control is proposed for singe-actuator hard disk drives. In this method, the short seeking control design is formulated as an optimization problem, where the feedforward control input is of finite time steps and is calculated offline to minimize a cost function. By incorporating the tracking error, control efforts and jerk in the cost function, the optimal finite-step feed-forward controller can achieve desired short seeking without using any reference trajectory. As a consequence, real time computation and memories can be saved in its implementation.;Secondly, multi-rate short seeking control by initial value adjustment (IVA) is proposed for singe-actuator hard disk drives. This method, which uses a multi-rate track following controller as the feedback controller, tunes the initial values of the feedback controller for short seeking. The proposed method significantly saves real time computation in two aspects: first, the multi-rate scheme inherently saves computation by updating the slow-component controller at slow rate; second, by tuning the initial values of the feedback controller, the desired transient characteristics in short-seeking can be obtained without the use of feed-forward controller or reference trajectory, which implies no real time computation for feed-forward control is needed.;Settling control is often used when the controller is switched from track seeking to track following for a fast and smooth transition. A novel settling control method by nonlinear control scheme is proposed. The control law during settling consists of two parts: a linear component which is the same as the track following controller, and a time varying nonlinear component which introduces a time-varying damping as the head approaches the target track, such that it provides a low damping when the head is far from the target for a fast seek, and a high damping as the head approaches the target to reduce overshoot. Thus, a fast response and precise positioning can be simultaneously achieved in settling.;Dual-stage actuation has been recognized as a promising candidate to expand the servo bandwidth and thus improve the servo performance. This dissertation also proposes 2 and investigates several novel methods for short seeking and settling control of dual-stage actuator HDDs, which have a coarse actuator (voice coil motor, VCM) and a fine actuator (piezoelectric actuator, PZT). First, a dual stage short seeking control method by initial value adjustment (IVA) is proposed. This method, which adjusts the initial values of the dual stage track following controllers for short seeking, makes the use of the feed-forward controller and reference trajectory unnecessary. As the fine actuator (PZT) has a stroke limit of only a few micrometers, a two-stage IVA scheme, which takes into consideration the saturation properties of the PZT actuator, is proposed for short seeking and settling control when the seeking distance exceeds the PZT stroke limit: (1) the coarse actuator (VCM) is utilized in the first stage (beyond PZT stroke limit), and IVA is designed to yield a high-speed seeking; (2) the PZT actuator is switched on when the read/write head approaches the target track in the second stage (within the PZT stroke limit), and IVA is used to produce a fast and smooth settling performance. Finally, multi-rate dual stage short seeking control by IVA is proposed for computation saving. It significantly saves real time computation in two ways: first, computation is saved by multi-rate scheme which updates the controller for the low bandwidth actuator (VCM) at a slow rate; second, initial value adjustment of the dual-stage feedback controller can achieve excellent short seeking performance without using any feed-forward controller or reference trajectory.;Simulation and experimental results have verified the effectiveness of each of the proposed methods.