Chunling Du

Nonlinear Tracking Control for a Hard Disk Drive Dual-Stage Actuator System

This paper presents a nonlinear tracking control method for a hard disk drive dual-stage actuator (DSA) system that consists of a voice coil motor (VCM) actuator and a piezoelectric (PZT) microactuator. Conventional track seeking controllers for DSA systems were generally designed to enable the VCM actuator to approach the target track without overshoot. However, we observe that this strategy is unable to achieve the minimal settling time when the target tracks are beyond the PZT actuator stroke limit. To further reduce the settling time, we design the VCM actuator controller to yield a closed-loop system with a small damping ratio for a fast rise time and certain allowable overshoot. Then, a composite nonlinear control law is designed for the PZT actuator to reduce the overshoot caused by the VCM actuator as the system output approaches the target track. Experimental results show that the proposed dual-stage servo outperforms the conventional dual-stage servo in short-span seeking and, additionally, achieves better track following accuracy than the VCM only single-stage servo.

Phase lead peak filter method to high TPI servo track writer with microactuators

A servo track writing (STW) platform with dual piezoelectric (PZT) microactuator is proposed for higher track density hard disk drives (HDDs). We mainly deal with the servo control issues to achieve high track density. Vibration and noise analysis based on the measured position error signal (PES) indicates that the PES is significantly disturbed by narrow-band vibrations. Hence, we propose a general second-order phase lead peak filter (PLPF) that is applicable to reject narrow-band disturbances at all frequency ranges. The filter zero is designed to minimally degrade the closed-loop system stability and obtain a smooth sensitivity curve around the disturbance frequency. A feedback controller is employed to stabilize the servo loop and then a PLPF is embedded to further suppress the specific vibrations. The readback position errors are evaluated and 6.4 nm 3sigma value of the true PES NRRO is achieved, corresponding to 395k track-per-inch (TPI) track density

A generalized KYP lemma based control design and application for 425 kTPI servo track writing

The recently developed generalized Kalman-Yakubovic-Popov (KYP) Lemma establishes the equivalence between a frequency domain property and a linear matrix inequality (LMI), and allows to solve a certain class of system design problems with multiple specifications on gain/phase properties over several frequency ranges. This paper applies the KYP lemma in a control design for microactuator to suppress the narrow-band disturbances through shaping sensitivity functions in data storage systems such as servo track writers for hard disk drives (HDDs). The KYP Lemma is applied together with the Youla parameterization approach to allow a convex optimization for achieving the desired specifications of sensitivity functions. Both LDV based experiment and implementation in a servo track writer demonstrate the effectiveness of the design method in rejecting specific frequency disturbances. In particular, the design method improves the track density of the servo track writer with 425k track-per-inch (TPI)

A Factorization Approach to Sensitivity Loop Shaping for Disturbance Rejection in Hard Disk Drives

This paper investigates a factorization approach to sensitivity loop shaping for disturbance rejection in hard disk drives (HDDs). The advantage of the factorization approach is that the system sensitivity function can be expressed explicitly in terms of a unique design parameter. This greatly simplifies the control design process to make the system sensitivity function match a chosen target sensitivity function with guaranteed stability. By decomposing the controller structure, the design parameter is revealed to behave as a plug-in disturbance filter, the design of which is then presented to suppress the dominant disturbances at some specific frequencies. It is also shown that based on the nominal control system the proposed disturbance filter can reduce the sensitivity gains at specific frequencies without worsening the neighbouring sensitivity gains. Simulation together with implementation results demonstrate that the proposed method can effectively suppress the disturbances around the servo bandwidth and accordingly offers a superior tracking accuracy in comparison with other existing filters.

High Bandwidth Control Design and Implementation for a Dual-Stage Actuation System With a Microthermal Actuator

This paper is devoted to the control design and implementation of the dual-stage actuation system with a newly developed microthermal actuator (MTA) as the secondary actuator. The characterization and modeling of this new MTA is presented for controller design. Its performances such as stroke, time constant, and a good linearity are exhibited. The controller design of the MTA-based dual-stage actuation system is then proposed, and aims to push the bandwidth with the MTA. The simulation and the experimental testing have demonstrated the achievable 6-kHz bandwidth with 60-kHz sampling rate, the MTA displacement of 15 nm at 4 kHz, and 0.1-ms seeking time for 40-nm seeking length.

Dedicated Servo Recording System and Performance Evaluation

The perpendicular magnetic recording (PMR) in hard disk drives is approaching its physical limitation. The emerging technologies, such as heat assisted magnetic recording and microwave assisted magnetic recording have been proposed to record on magnetic media with thermally stable smaller size grains at higher areal density (AD). However, in the media fabrication, achieving well-isolated small size of grains is more challenging than obtaining high Ku material as recording media. Reducing the number of grains per bit is a major path for keeping AD growth of PMR in recent years. To minimize the SNR penalty at a smaller grain number per bit, pushing more on track density is the right approach. With the 2-D magnetic recording (TDMR) readers for inter-track interference cancellation, the off-track read capability is improved significantly for allowing a narrower track read. In the drive working environment, when the external vibration or other mechanical disturbance happens during the writing process, it creates more track squeeze at adjacent tracks and leaves a very narrow track at some locations of the track. When the track width is narrower than the squeeze to death width in the 747 curve, it causes hard failure in the channel. To solve the track squeeze problem, this paper proposes to add an additional magnetic recording layer in between the data recording layer and the soft underlayer of conventional PMR media. This additional recording layer is used to record servo information only. The continuous positioning error signal is able to improve the servo performance and to provide the real-time monitoring of the positioning error. When it is under bad servo conditions, the writing process can be stopped to avoid nontolerable track squeeze. The continuous servo signals are designed to be of moderate intensity at very low frequency, and its impact on data signal has been minimized. The linear density gap between the dedicated servo media and the conventional PMR media is able to be controlled within 3%. As the dedicated servo system keeps only around 100 wedges of track ID and sector ID at the data layer, the surface area saving at the data layer can break even in capacity. The dedicated servo technology together with TDMR readers is the key technology to achieve ultrahigh track density during both writing and reading processes.

A Novel Settling Controller for Dual-Stage Servo Systems

Dual-stage control is the most obvious step for the improvement of hard-disk drive servo systems, and it will most likely be used to support recording densities higher than 1 Tb/in2. In this paper, we discuss the unique characteristics of settling the controller for dual-stage servo systems. A feedforward settling controller is designed to shape the position error signal profile before it is fed into the input of the track following controllers. Initial position and velocity causing transient errors are compensated through the designed feedforward controller for smooth and fast settling. The proposed method is convenient and suitable for real implementation. The simulation and experimental results are presented to demonstrate the design and effectiveness of the method.

Smoothing Position Error Signal Corrupted by Impulsive Disturbances

This paper proposes a filtering method to smooth the position error signal (PES) contaminated by impulsive disturbances in hard disk drives. The impulsive disturbances, observed as sudden changes in PES, may be distributed separately or consecutively. A nonlinear switched filtering method is developed to remove the impulsive disturbances before the position error signal is fed into the servo controller. It turns out that the proposed filtering algorithm is able to effectively remove impulsive disturbances in the corrupted PES with no impact on the stability and the servo performance of the original servo loop.

Vibration analysis and optimal control for self servo track writer

Major vibration sources in self-servo track writing (SSTW) are identified and linear matrix inequality (LMI) approach is applied to design an H/sub 2/ controller for the SSTW. Experimental measurement of the position error signal (PES) is used to determine the spectrum of the PES. The identification of vibration sources is based primarily upon the spectral decomposition of the measured PES. Upon determining the vibration sources, an H/sub 2/ controller together with a feedforward compensator is designed via LMI approach to achieve an optimal error containment for error propagation in the SSTW. In addition, the error containing effectiveness of optimal H/sub 2/ control is compared with that of PID and PD feedback control in the SSTW with the feedforward error compensation. To our knowledge, no existing result involves the methodology proposed here in SSTW.

Three-Stage Control for High Servo Bandwidth and Small Skew Actuation

A three-stage actuation control system is proposed to enable high servo bandwidth track-following and possibly for small skew angle actuation system of hard disk drives. It involves a voice coil motor actuator as a primary actuator and a PZT actuated milliactuator as a secondary actuator. Both of them have the first dominant resonance mode at 5 kHz and the milliactuator has 200 nm stroke. A thermal microactuator having 50 nm stroke is used as a third actuator for a high servo bandwidth. A control strategy with controller design method is proposed to meet the performance requirement of each actuators control loop and the overall control system. More than 6 kHz servo bandwidth is achieved and 430 nm external vibration can be covered by the three-stage actuation system. Simulation and experimental results demonstrate the control performance of the three-stage closed-loop system.