Dual-stage Servo Research Articles

Two-Parameter Robust Repetitive Control With Application to a Novel Dual-Stage Actuator for Noncircular Machining

This work presents a robust repetitive controller design for a novel dual-stage actuator system. The dual-stage actuator, which consists of an electrohydraulic actuator for 25-mm-gross motion and a piezoelectric actuator for 40-/spl mu/m fine motion, is designed for noncircular machining application. The controller is designed through a sequence of two single-input-single-output (SISO) designs by exploiting the triangular structure of the two by two actuator system dynamics. The tracking error from the first stage electrohydraulic actuator is used as reference for the second stage piezoelectric actuator. In this master-slave control arrangement, the overall sensitivity function is the product of two sensitivity functions from each actuator's servo loop. Thus, performance is improved at the frequencies where the sensitivity values are already well less than one. In the real-time control implementation, the effects of finite word length are analyzed and addressed via controller order reduction and realization. In an experiment of tracking an automotive cam profile at the rate of 10 cycles per second (600 rpm), the proposed dual-stage servo system generated tracking error of 4-/spl mu/m peak-to-valley and 0.80-/spl mu/m root-mean-square (RMS) value, showing a substantial improvement over the 16 micron peak-to-valley and 2.64-/spl mu/m RMS errors generated by the electrohydraulic servo system alone.

Practical Implementation of a Novel Anti-Windup Scheme in a HDD-Dual-Stage Servo-System

Two novel discrete anti-windup (AW) techniques are applied to a dual-stage actuator of an experimental hard disk drive system. The techniques, one low order, the other full order, employ convex l/sub 2/-performance constraints in combination with linear-matrix-inequality-optimization methods. It is shown that the AW compensators can improve the performance of the nominal dual-stage servo-system when the secondary actuator control signal saturates at its allowable design limits. Also, stability is achieved despite saturation of both the secondary actuator and the voice-coil-motor actuator. Practical results show that the performance of each AW compensator is superior to another well-known ad-hoc AW technique, the internal model control AW scheme. The main contribution of the paper is the application of theoretically rigorous AW methods to an industrially relevant servo system.

Design, fabrication and characterization of single crystal silicon microactuator for hard disk drives

In this paper, a single crystal silicon electrostatic microactuator for hard disk drives' (HDD) dual stage servo systems is presented. The microactuator is mounted between the slider and suspension and drives the slider on which a magnetic head element is attached. The microactuator has electrically isolated microstructures with an aspect ratio of 20:1 directly processed from a single crystal silicon substrate. It consists of a stator attached to the silicon substrate by a bus bar and a movable rotor connected to an anchored central column via spring beams. Both the stator and the rotor are suspended from the silicon substrate. Parallel plate capacitor pairs are used for the electrostatic actuation. When a voltage is applied to the microactuator, the head element will be driven to move in the tracking direction because of the rotational motion of the rotor. The microactuator prototype with overall dimensions 1.4 mm × 1.4 mm × 0.18 mm has been successfully fabricated. The techniques for integration of the microactuator with a head gimbal assembly (HGA) have been investigated. Experiments show that ±0.6 µm equivalent displacement of the head element in the tracking direction is achieved at the driving voltage 28.5 V. Dynamic frequency response testing of the microactuator bonded with the slider has been carried out. The test results demonstrate that the first in-plane resonance frequency of the electrostatic microactuator bonded with the slider is 1.9 kHz and the damping ratio is 2.8%, and the device is equivalent to a single spring mass system up to 50 kHz. The single crystal silicon electrostatic microactuator has a small mass, good thermal stability and dynamic performance, and is feasible to operate as a fine actuator in dual stage servo systems for high density HDD.

Dynamics simulation of MEMS device embedded hard disk drive systems

Currently, hard disk drives (HDD) use rotating disks to store digital data and magnetic recording heads are flying on the disk to read/write data. The recording heads are mounted on a slider–suspension assembly, which makes heads move from one track to another on the disk. The heads movement is controlled by close-loop feedback servo systems. It is well known that dynamic behaviors of head–slider–suspension-assembly (HSA) systems are of great influence on the track per inch capacity of HDD [1, 2]. As the problem is structurally complex, it is usually investigated using experimental methods or finite element simulation models [3]. Furthermore, the dual-stage servo system has been commonly considered as one promising solution to increase the servo bandwidth of the recording positioning system for high TPI HDDS. In particular, MEMS device embedded systems are superior to others in batch-fabrication. However, this dual-stage system has also resulted in more difficulties in predicting HDD dynamic performance. This paper presents the study of the problem using the macromodeling simulation approach. It applies efficient FEM based sub-structuring syntheses (SSS) [4] and fast boundary element method (BEM) approaches incorporated with system dynamics technology to investigate the dynamic characteristics of MEMS actuator embedded HSA systems for HDD.

An Approach to Dual-Stage Servo Design in Computer Disk Drives

A novel design strategy is presented for a dual-stage actuator servo system in magnetic disk drives. Based on the existing decoupled-design strategy, we adopt the idea of the well-known zero-phase error tracking controller to minimize destructive effect resulting when two control loops are combined. Since the zero-phase tracking control requires previews of signals in a feedback loop, a kind of estimation scheme is suggested based on the actuators' dynamic model. The best practical advantage is that we achieve systematically a low-order DSA controller capable of high-accuracy track following as well as of high-speed short-span track seek; besides, the existing single-stage controller is fully utilized. The performance of the proposed method is analyzed and demonstrated through simulation and experiment.

ヘッドサスペンション駆動形マイクロアクチュエータを用いた二段アクチュエータの高帯域化(S62-1 機構・制御(1),S62 情報機器コンピュータメカニクス)

Dual-stage actuators sometimes have a serious problem that the driving force of microactuators excites VCM vibration. To avoid the problem, we designed a suspension-drive piezoelectric micro-actuator to have high resonant frequency and small mass. We installed the microactuator in one of Fujitsu's 3.5-inch commercial drives for evaluation of the dual-stage servo system. The dual-stage actuator system achieved the openloop crossover frequency of 4 kHz.

이중 구동 시스템을 위한 압전 밀리엑추에이터의 제어기 설계

고밀도, 고정밀의 정보저장기를 실현하기 위해서 높은 서보대역이 요구되고 있다 이러한 요구조건을 만족시키기 위해서 이단구동시스템이 도입되었다. 이 시스템은 기존의 VCM을 조동구동기로, 추가적인 밀리엑츄에이터를 미동구동기로 사용하는 것이다. 이를 위해 우리는 압전형 밀리엑츄에이터를 서스펜션 부에 설계하였다 이러한 시스템의 제어를 위하여 PQ 제어 방법을 도입하였으며, 이 방식은 이중입력/단일출력 시스템을 두개의 단일 입출력 시스템으로 단순화시키는 것이다. 이 논문은 PQ 제어 방식을 적용시켜 설계하고. 설계된 제어기의 성능 평가로써 기존의 HDD의 PES을 받아서 외란제어 능력을 평가하는 것이다. To reach high areal density, less track pitch is expected and more servo bandwidth is required. One approach to overcoming the problem is by using dual stage servo system. For this system. we have suggested new milliactuator based on the shear mode of piezoelectric elements to drive the head suspension assembly. In this paper, we introduce milliactuator and controller design method, PQ method. PQ method reduces the controller design problem for DISO (dual-input/single-output) systems to two standard controller design problems for SISO ( single-input/single-output) problems. The first part of PQ method directly addresses the issue of actuator output contribution, and the second part allows the use of traditional loop shaping to achieve the overall system performance. This paper shows how to employ the PQ method to meet aggressive close-loop performance specifications for a disk drive system with a VCM and piezoelectric milliactuator.

Design of flexural pivot for use in hard disc drive actuator

A cross leaf flexural pivot is proposed and designed for use in disc drive actuator to replace the currently used ball bearing system. The static and dynamic performances of the proposed design are investigated using finite element analysis. It is shown that flexural pivot can achieve a translatory stiffness comparable to currently used ball bearing. The tilt stiffness is lower than that of ball bearing. The design is prototyped and the life cycling test shows that the prototype can work for more than 3 million life cycles without any failure. Our experimental results show that the single-stage servo loop using the flexural pivot can achieve the performances of open-loop bandwidth of 630 Hz and closed loop bandwidth of 1100 Hz, with phase margin of 60° and gain margin of 6.6 dB. When attaching an actuated suspension to from a dual stage servo loop, we show that the performances as open-loop bandwidth of 1700 Hz with the phase margin of 45° and gain margin of 7 dB.

Comparison and design of servo controllers for dual-stage actuators in hard disk drives

A systematic design procedure is presented that can be used to construct servo controllers for dual-stage actuation in a hard disk drive. The systematic approach is based on optimal control theory using an H/sub /spl infin//-norm-based control design, where servo design specifications are formulated in terms of relatively simple weighting functions. Different possible servo control strategies are designed and compared on performance and energy consumption. Results based on a piezoelectric milli-actuator from Hutchinson Technology Inc. illustrate the design of a dual-stage servo controller that is able to perform high-density track recording, with a significant reduction in power consumption.

반복 학습 제어를 이용한 NFR 디스크 드라이브의 2단 서보 시스템

Recently, near-field recording (NFR) disk drive schemes have been proposed with a view to increasing recording densities of hard disk drives. Compared with hard disk drives. NFR disk drives have narrower track pitches and are exposed to more severe periodic disturbances resulting from eccentric rotation of the disk. It is difficult to meet servo system design specifications for NFR disk drives with conventional VCM actuators in that the servo system for an NFR disk drive generally requires a feater gain and higher bandwidth. To tackle the problem various dual-stage actuator systems composed of a microactuator mounted on top of a conventional VCM actuator have been proposed. This article deals with the problem of designing a tracking servo system far an NFR disk drive adopting a dual-stage actuator. We summarize design constraints pertaining to the dual-stage servo system and present a new servo scheme using iterative teaming control. We design feedback compensators and an iterative teaming controller for a target plant and verify the validity of the proposed control scheme through a computer simulation.

Frequency response evaluation method for microactuators in dual-stage hard disk servo systems

Much of the interest in dual-stage microactuation for hard disk drives centers on control system design methodology, error rejection improvements, and optimal division of the control effort between the coarse and fine actuators. The authors, instead, place the emphasis on evaluating the secondary actuator mechanics from the point of view of the control system. The effective performance of the mechanical plant can be better assessed when combined with a "target" compensator than it can as a raw mechanical transfer function or frequency response function alone.

Vibration control of a PZT actuated suspension dual-stage servo system using a PZT sensor

Structural resonance modes of the head stack and suspension assembly are one of the major limiting factors for achieving higher head-positioning servo performance in hard disk drives. This paper discusses the vibration suppression control of a PZT actuated suspension dual-stage servo system. A PZT strip on a PZT actuated suspension is utilized as a vibration sensor to control both the voice coil motor (VCM) actuator butterfly mode and the suspension sway mode. The vibration suppression controller is designed using the H/sub /spl infin// control design method. Experimental results show that an attenuation of 15 dB can be achieved for both the suspension sway mode and the VCM butterfly mode, and the rms of the head off-track motion due to airflow excited vibration can be reduced by approximately 30% with the proposed vibration suppression control scheme.

State Initialization of Peak Filter Design in HDD Dual-Stage Servo Control for Smooth Settling Response

Narrow-band disturbance caused by high speed disk rotation is one of the significant contributors to the track misregistration (TMR) in HDD track following. The peak filter design is an effective control method to suppress the narrow-band disturbance, but the activation of peak filter may produce some oscillations in the output response, which would prolong the settling time of HDD servo system. This paper presents a scheme of state initialization to deal with the undesirable transient response caused by control component. The proposed method is applied to an HDD dual-stage servo system with peak filter design. Testing results demonstrate that the settling performance would not be deteriorated when the high-gain filter is properly initialized. This technique would enhance the disturbance attenuation of HDD dual-stage servo system without the degradation of settling performance.

Dual-stage servo with on-slider PZT microactuator for hard disk drives

A dual-stage servo system with an on-slider piezoelectric (PZT) microactuator is developed for future high-density hard disk drives. The parallel control scheme is used in servo design with considerations over the stroke limitation and the hysteresis of the microactuator. Experimental results on a spin-stand with a laser Doppler vibrometer as the position sensor demonstrate that the system has a servo bandwidth higher than 4 kHz and has disturbance rejection capability up to near 5 kHz.

Midfrequency disturbance suppression via micro-actuator in dual-stage HDDs

In hard disk drives (HDDs), it is difficult for the single-stage servo system to suppress the disturbance within the midfrequency band (1-3 kHz) because of the limited servo bandwidth. This article presents a dual-stage control design using a peak filter to deal with the midfrequency narrow-band disturbances caused by disk flutter or windage. The technique of initializing the states of the peak filter is proposed to preserve the smoothness of transient response when the dual-stage servo loop is switched on from track-seeking to track-following. Experimental results demonstrated that the proposed dual-stage control scheme could effectively suppress the midfrequency disturbance without compromising the settling performance. This method would enhance the disturbance attenuation of the dual-stage servo system for higher track density in HDDs.

A push-pull multi-layered piggyback PZT actuator

To increase the recording density of hard disk drives (HDDs), we developed a push–pull multi-layered piggyback PZT actuator that enables fine positioning by a dual-stage servo system. This PZT actuator consists of 31-mode push–pull multi-layered PZT strips and a head suspension. It generates a 1.4-μm effective radial head displacement at 5 V. This displacement is twice that of conventional piggyback actuators. The main resonance frequency of the actuator is higher than 9 kHz, its lifetime is longer than five years, and it has a self-latch property. These features mean that the developed actuator can meet all the requirements for implementation in HDD servo systems, including a track density of 100 kTPI (kilo-tracks per inch). The actuator was implemented in two types of HDDs (A-type and B-type), which reduced the repeatable and non-repeatable positioning errors (by 40 to 45% and 28 to 34%, respectively).

Design and testing of track-following controllers for dual-stage servo systems with PZT actuated suspensions

This paper discusses the design and testing of two track-following controllers for dual-stage servo systems in hard disk drives. The first controller is designed using the μ-synthesis multivariable robust optimal controller design methodology. The second is designed using classical single-input-single-output (SISO) frequency shaping design techniques, based on sensitivity transfer functions decoupling of the dual-stage actuator. The controllers were implemented and tested on a disk drive with a PZT actuated suspension based dual-stage servo system. The position error signal (PES) for the servo system was obtained by measuring the slider displacement using an LDV and injecting simulated track runout. In the experiment, both designs achieved a track-mis-registration (TMR) less than 10 nm.

Design and analysis of dual-stage servo system for high track density HDDs

This paper presents a design method of robust controller using the modified dual feedback controller to improve transient response in track seeking while attaining robust performance and stability in the presence of plant uncertainties. Coupled dynamics model is identified from the Newton–Euler formulation. We use discrete-time LQG/LTR controller design method. Robustness analysis is performed using the μ-analysis method. With the proposed method we are able to obtain an improved transient response. Furthermore, we can achieve the desired sensitivity functions while assuring robustness to the parameter variations to known structured uncertainties. It is demonstrated that the proposed method can be extented to short track-seek control without change of the controller structure. From simulation and experiment results, we confirm the effectiveness of the proposed method.

Development of a Φ-shaped actuated suspension for 100-kTPI hard disk drives

A suspension-based piezoelectric microactuator has been developed for dual-stage servo systems in hard disk drives. The suspension is designed to meet the requirements of high recording density, high shock resistance, high productivity, and a load/unload structure. This paper describes the structure of the actuator and its mechanical characteristics.

Mechatronics of electrostatic microactuators for computer disk drive dual-stage servo systems

A decoupled control design structure and discrete time pole placement design method are proposed for MEMS-based dual-stage servo control design in magnetic disk drives. Dual-stage track following controllers are designed using a decoupled three-step design process: the voice coil motor (VCM) loop design, the microactuator (MA) inner loop design, and the MA outer loop design. Both MIMO (when the MA relative position sensing is available) and SIMO (when the MA relative position sensing is not available) designs are considered. The effect of MA resonance mode variations on the stability and performance of the controllers are analyzed. Self-tuning control and online identification of the MA model are developed to compensate for the variations in the MA's resonance mode.