Jianbin Nie

Optimal H ∞ Control for Linear Periodically Time-Varying Systems in Hard Disk Drives

Periodicity frequently occurs in hard disk drives (HDDs) whose servo systems with periodic phenomena can be usually modeled as linear periodically time-varying (LPTV) systems. This paper discusses optimal H∞ control synthesis for discrete-time LPTV systems via discrete Riccati equations. First, an explicit minimum entropy H∞ controller for general time-varying systems is obtained. Subsequently, the developed control synthesis algorithm is applied to LPTV systems and it is shown that the resulting controllers are periodic. The proposed control synthesis technique is evaluated through both single and multirate optimal H∞ track-following control designs. The single-rate servo design shows that our proposed control synthesis technique is more numerically robust in calculating optimal H∞ controllers for discrete-time linear time-invariant systems than the MATLAB function of “hinfsyn,” while the multirate servo design validates its ability of synthesizing multirate controllers to achieve the robust performance of a desired error-rejection function. Moreover, an experimental study-in which the developed control synthesis algorithm on a real HDD with missing position error signal sampling data is implemented-further demonstrates its effectiveness in handling LPTV systems with a large period and attaining desirable disturbance attenuation.

Design and Implementation of Dual-Stage Track-Following Control for Hard Disk Drives

This paper discusses the design and implementation of two track-following controllers for dual-stage hard disk drive servo systems. The first controller is designed by combining an outer loop sensitivity-decoupling (SD) controller with an inner loop disturbance observer (DOB). The second is designed by combining mixed H2 /H∞ synthesis techniques with an add-on integral action. The designed controllers were implemented and evaluated on a disk drive with a PZT-actuated suspension-based dual-stage servo system. Position error signal (PES) for the servo system was obtained by measuring the slider displacement with an LDV and injecting a simulated track runout.© 2009 ASME

Enhanced Vibration Suppression in Hard Disk Drives Using Instrumented Suspensions

This paper summarizes work that has been completed to evaluate the use of high-resolution thin-film strain sensors for vibration suppression in hard disk drives (HDDs). In particular, we demonstrate the viability and necessity of symmetrical sensors for cancellation of common non-off-track modes in the measurement. Thin-film ZnO strain sensors have been successfully integrated into instrumented suspension prototypes, and strain signals were extracted from operating disk drives. High-resolution strain sensing allowed us to investigate the nature of airflow excitation of the suspension structure. Experimental measurements were used to construct both ideal and realistic models. Using the ideal model that ignored non-off-track modes, multirate nominal H 2 control synthesis and simulation were employed to evaluate potential improvements in closed-loop vibration suppression in a multisensor framework. These simulations projected improved tracking performance using auxiliary high-resolution strain sensing with increased sample rate. However, when a robust H 2 damping controller was designed using a more realistic model of the available hardware, performance in simulation and experiment was limited by numerous non-off-track modes in the sensor signal.

Optimal H ∞ control for hard disk drives with an irregular sampling rate

This paper discusses the optimal H∞ control syn thesis via discrete Riccati equations for hard disk drives (HDDs) with an irregular sampling rate. A HDD servo system with an irregular sampling rate, caused by missing position error signal sampling data, is modeled as a linear periodically time varying (LPTV) system. An optimal H∞ track-following control algorithm for HDD servo systems with irregular sampling rates is proposed, which is based on the previous H∞ synthesis results for LPTV systems developed by some of the authors. A simulation and implementation study demonstrates that the proposed control synthesis technique is able to handle the irregular sampling rate and can be used to conveniently design a loop-shaping track-following servo that achieves the robust performance of a desired error rejection function for disturbance attenuation. Implementation results on an actual hard disk drive validate the simulation results.

Control design of concentric self-servo track writing systems for hard disk drives

Modern hard disk drives (HDDs) generate the head position error signal (PES) by reading special magnetic patterns called servo patterns, which are written in designated areas on the disk surface known as servo sectors, and measure the position of the head relative to the center of the data track. Servo sectors are typically created during the HDD manufacturing process and are not subsequently overwritten or erased. The process of writing servo sector patterns is known as servo track writing (STW). The accuracy and precision of the servo track writing process plays a crucial role in dictating the ultimate track density and areal storage density of the HDD. There are several mechanisms for performing servo track writing. Conventional servo track writing processes use additional sensing and positioning equipment external to the HDD. In contrast, self-servo track writing processes only use the HDDs reading and writing heads and servo system; thus avoiding many of the manufacturing cost increases and productivity losses associated with conventional servo track writing. This paper presents two novel controller synthesis methodologies for performing concentric self-servo track using a feedforward control structure. In the first methodology, it is assumed that a conventional track-following causal feedback compensator has been designed. A non-causal feedforward compensator, which utilizes the stored error signal that was created while writing the previous track, is subsequently designed using standard H∞ control synthesis techniques, in order to achieve good disturbance attenuation while preventing the propagation of tracking errors from previous tracks. In the second methodology, both the track-following feedback compensator and the feedforward compensator are simultaneously designed via a mixed H2/H∞ control scheme, which involves the solution of a set of linear matrix inequalities. Simulation results confirm that the two proposed control synthesis methodologies prevent error propagation from the previously written tracks and significantly improve self-servo track writing performance.

Control Design of Hard Disk Drive Concentric Self-Servo Track Writing via

This paper considers two novel controller synthesis methodologies using a feedforward control structure for performing concentric self-servo track writing in hard disk drives. In the first methodology, it is assumed that a conventional causal track-following controller has been designed and a non-causal feedforward controller, which utilizes the stored error signal from writing the previous track, is designed using standard H∞ control synthesis techniques, in order to prevent the track errors from propagating and to achieve good disturbance attenuation. In the second methodology, both the track-following feedback controller and the feedforward controller are simultaneously designed via a mixed H2/H∞ control scheme, which involves the solution of a set of linear matrix inequalities and achieves good disturbance attenuation while preventing the propagation of track errors from the previous tracks. Simulation results confirm that the two proposed control synthesis methodologies prevent error propagation from the previously written tracks and significantly improve concentric self-servo track writing performance.

H_{2}

This paper discusses optimal H∞ control synthesis via discrete Riccati equations for discrete linear periodically time-varying (LPTV) systems. Based on the results presented in [1], an explicit minimum entropy H∞ controller for general time-varying systems is obtained. The control synthesis technique is subsequently applied to LPTV systems and it is shown that the resulting controllers are also periodically time varying. In order to demonstrate the effectiveness of the proposed control synthesis technique, both single-rate and multi-rate discrete-time minimum entropy H∞ track-following control designs for hard disk drives are considered. It is shown, via a comprehensive simulation study, that track-following controllers designed using the H∞ synthesis technique proposed in this paper achieve the robust performance of a desired error rejection function. Moreover, as expected, multi-rate controllers has the ability of outperforming their single-rate counterparts.Copyright

and

This work developed a novel application of piezo-electric thin-film sensing technology by incorporating ZnO strain sensors into a hard disk drive (HDD) to support the high-bandwidth, nanometer-level positioning required to achieve terabit-per-square-inch data densities. First, ZnO sensors were fabricated directly onto a HDD suspension component, which was a unique and challenging process since the substrate was made of steel. The instrumented suspension was then incorporated into an experimental HDD, along with a miniaturized instrumentation circuit and PZT actuation elements. The sensors demonstrated excellent sensitivity to structural vibration modes that contribute to displacement in the off-track direction. A simple active mode damping controller was implemented on these instrumented PZT-actuated prototypes to demonstrate suppression of high-frequency vibration modes.

H_{\infty}

This paper describes the integration of thin film ZnO strain sensors onto hard disk drive suspensions for improved vibration suppression for tracking control. Sensor location was designed using an efficient optimization methodology based on linear quadratic gaussian (LQG) control. Sensors were fabricated directly onto steel wafers that were subsequently made into instrumented suspensions. Prototype instrumented suspensions were installed into commercial hard drives and tested. For the first time, a sensing signal was successfully obtained while the suspension was flying on a disk as in normal drive operation. Preliminary models were identified from experimental transfer functions. Nominal H2 control simulations demonstrated improved vibration suppression as a result of both the better resolution and higher sensing rate provided by the sensors.Copyright

Synthesis

Abstract Missing position-error-signal sampling data in hard disk drives (HDDs) results in their servo systems having irregular sampling rates. Due to the natural periodicity of HDDs, which is related to the disk rotation, HDD servo systems with missing samples can be modeled as linear periodically time-varying (LPTV) systems. Based on previous H ∞ control synthesis results for general LPTV systems developed by some of the authors, an optimal H ∞ track-following control algorithm for HDD servo systems with missing samples is proposed. A simulation and implementation study, which has been carried out on a set of hard disk drives, demonstrates that the proposed control synthesis algorithm is able to handle missing samples and can be used to achieve the robust performance of a desired error rejection function for disturbance attenuation. Implementation results on multiple real hard disk drives validate the predicted robust performance.