Track Seeking Research Articles

AN OBSERVER FOR RUNOUT OBSERVATION DURING TRACK SEEKING FOR THE TWO-ACTUATOR OPTICAL DISK DRIVE

Runout observation in optical disk drive is a common research interest, but very few researches were able to address the runout problem during the track seeking process. This study investigated the possibility of using an on-line observer to estimate the disk runout during track accessing. A model based feedback algorithm for the track-accessing servo is used as the base for the design. The observer in the controller not only helps to provide the necessary feedback information during accessing, but also made it possible to measure the disk runout in the removable media. The experimental results showed that proper tuning of the controller clearly provide the track wobbling information that will make possible further investigation on how to eliminate the runout effect in the accessing process.

Novel method for minimizing track seeking residual vibrations of hard disk drives

Track access time is an important parameter for high-performance hard disk drives (HDDs). Presently, the resonances of the head actuator caused by the pivot-bearing stiffness are an even more urgent problem, since these resonances have become the main design factor that limits further reduction of track access time and improvement of the servo control bandwidth in HDDs. Our objective in this paper is to investigate, from a mechanics and dynamics viewpoint, the influence of the driving force waveforms on residual vibrations and to identify the characteristics of the waveform, which will lead to zero or very low residual vibrations. Using a simplified model of a head actuator assembly and finite-element simulation, we show that the residual vibration of the head actuator assembly depends strongly on the waveform and the time duration of the driving impulse force. If suitable waveform and time duration of the seeking impulse force are appropriately chosen, very low track seeking residual vibrations and quick track access can be achieved.

Track seek control for hard disk dual-stage servo systems

A new decoupled track-seeking controller for a dual-stage servo system of a magnetic disk drive has been developed. The controller is applied to the dual-stage system with a push-pull piezoelectric (PZT) micro-actuator and enables high-speed one-track seeking and short-span track seeking. A one-track seeking controller was designed in three steps: (1) a voice coil motor (VCM) feedforward controller was designed to prevent overshoot in response of VCM actuator; (2) a feedforward loop from the VCM reference signal to the PZT actuator was designed in order to decouple the dual-stage actuator system; (3) a feedforward controller for the PZT actuator was designed such that an output PES (position error signal) only follows a PES reference signal. In the design of the short-span seeking controller, the PZT actuator's stroke limit was considered. Simulations and experiments show that the proposed dual-stage track-seeking controller performs better than the conventional single VCM servo.

Head actuator dynamics

It is common in the present hard disk drives that there exist strong quasi-rigid (QR) system vibration modes between 3 and 5 kHz, which involve coupled vibrations of the slider, suspension, arm and bearing assembly. The system modes are caused by the stiffness of the ball-bearing assembly, and they are particularly troublesome for the servo system. Therefore, a thorough understanding of the dynamic behaviour of the head actuator especially the QR modes, is essential to control the vibration in future high-performance disk drives. In this paper the finite element model of a full head actuator is established. The detailed QR body modes are investigated and the transient response simulating the track seeking and settling is also involved. It is found that due to the stiffness of the pivot bearing in the axial and radial directions, the head actuator exhibits a series of QR body motions, among which the translation in the lateral direction and rotation about the longitudinal direction might contribute to the track misregistration (TMR). In the transient response simulating the track seeking, besides the QR modes, the suspension sway mode and first torsion mode might also be excited. Owing to the material damping, the sway mode attenuates very rapidly. The QR modes may be dampened gradually by the damping in the pivot bearing.

Head actuator dynamics

It is common in the present hard disk drives that there exist strong quasi-rigid (QR) system vibration modes between 3 and 5 kHz, which involve coupled vibrations of the slider, suspension, arm and bearing assembly. The system modes are caused by the stiffness of the ball-bearing assembly, and they are particularly troublesome for the servo system. Therefore, a thorough understanding of the dynamic behaviour of the head actuator especially the QR modes, is essential to control the vibration in future high-performance disk drives. In this paper the finite element model of a full head actuator is established. The detailed QR body modes are investigated and the transient response simulating the track seeking and settling is also involved. It is found that due to the stiffness of the pivot bearing in the axial and radial directions, the head actuator exhibits a series of QR body motions, among which the translation in the lateral direction and rotation about the longitudinal direction might contribute to the track misregistration (TMR). In the transient response simulating the track seeking, besides the QR modes, the suspension sway mode and first torsion mode might also be excited. Owing to the material damping, the sway mode attenuates very rapidly. The QR modes may be dampened gradually by the damping in the pivot bearing.

Track Seek Dynamics of HDD Suspension System Considering Air Bearing Effects

Recently, almost all hard disk drives employ the rotary actuator system. The performance of an HDD depends on the accuracy and speed of tracking motion. We study the dynamics of head-suspension assembly during track seek. We develop the numerical analysis program to study the dynamic characteristics of HDD suspension system considering the air bearing effects. The track seek simulation by using the developed program helps to estimate the effect of the suspension vibration on the air bearing dynamics. We calculate the behaviour of the air bearing for the given track seek profile and calculate the positioning error during track seek process due to the lateral deflection of the suspension.

Discrete-time sliding mode proximate time optimal seek control of hard disk drives

The paper presents a new proximate time optimal seek control approach with discrete-time variable structure control theory for a hard disk drive (HDD) servo. It incorporates the time optimal control idea into the time-varying sliding mode control design and derives a class of proximate time optimal control algorithms with robustness. The proposed approach provides one unified framework for both track seeking and track following controller design, it achieves smooth transition from seek control to track-following control naturally, and it eliminates the common chattering and overshoot problem as well. A systematic method is developed for the choice of controller parameters. Simulation results are demonstrated to show the robustness of the proposed method to disturbances, unmodelled dynamics and parameter variations with near time optimal performance.

Effect of air bearing design on wear of diamond-like carbon coated proximity recording sliders

The effect of air bearing design on the wear behavior of diamond-like carbon (DLC) coated sliders is investigated as a function of “interference height” and calculated contact force for sub-ambient pressure proximity recording sliders with contact stiffness of 1 and 0.25 mN/nm, respectively. The wear rate of DLC coated sliders was measured using Raman spectroscopy during track seeking of 1-h duration on a spin-stand and 1000-h duration in a 2.5-in. hard disk drive. In addition, the effect of air bearing design on flying height variations caused by a change in the static gimbal attitude and a change in altitude was estimated. The results show that wear of DLC coated sliders is proportional to the calculated contact force and that wear of the DLC overcoat levels off after 10km sliding distance regardless of slider design. Furthermore, we found that the amount of wear of the DLC overcoat for track seeking tests of 1-h duration is almost the same as that for a track seeking test of 1000-h duration.

Wear Measurements of Diamond-Like Carbon Coated Proximity Recording Sliders Using Raman Spectroscopy

The wear behavior of diamond-like carbon (DLC) coated sliders for proximity recording is investigated as a function of sliding distance, spindle speed and slider “interference height” using Raman spectroscopy. Both constant speed drag tests simulating track following and “slew” tests simulating track seeking were performed. The results indicate that track seeking increases the wear rate of DLC coated sliders dramatically during the first stages of a seek test. The results also show that wear of the DLC layer depends strongly on sliding distance and interference height of the slider. [S0742-4787(00)03801-7]

Two-degree-of-freedom control with robust feedback control for hard disk servo systems

A two-degree-of freedom (2 DOF) control structure is proposed for read/write head servo systems of hard disk drives (HDDs). This structure is applied to both track seeking and track following, and makes mode switches found in conventional HDD servo systems unnecessary. Two innovative features introduced in the paper are: 1) a new method for generating the reference signal for track seeking and 2) two robust feedback control schemes for rejection of disturbances; one scheme uses a disturbance observer (DOB), and the other uses adaptive robust control (ARC). Simulation results show that the 2 DOF structure with ARC provides better performance than either the conventional servo system with mode switches or the 2 DOF structure with DOB. Experimental results show the advantage of the new reference generation method and the use of the DOB-based robust controller.

Partial contact air bearing characteristics of tripad sliders for proximity recording

Proximity recording introduces some new head/disk interface characteristics as compared to the conventional noncontact recording. The different design philosophy has led to several distinct partial contact tripad air bearing designs, including the shaped rail negative pressure and straight rail positive pressure tripad sliders. The impact of these new characteristics and design differences on the head/disk reliability needs to be thoroughly understood in order to ensure a successful proximity recording. This paper investigates the partial contact air bearing characteristics of three representative tripad slider designs. The contact stiffness is used as a measure of the air bearing compliance. Simulations of the fly height, contact force, altitude sensitivity, substrate waviness induced spacing and contact force modulations, contact take-off track seeking and crash stop dynamics are performed for the three tripad slider designs. The results demonstrate that the negative pressure tripad sliders can be designed such that they can achieve both flat fly height and contact force profiles and, fast take-off while still maintaining a small contact stiffness. However, the air bearing compliance and waviness following performance have to be compromised for the correct partial contact air bearing pitch stiffness.

Partial Contact Air Bearing Characteristics of Tripad Sliders for Proximity Recording

Proximity recording introduces some new head/disk interface characteristics as compared to the conventional noncontact recording. The different design philosophy has led to several distinct partial contact tripad air bearing designs, including the shaped rail negative pressure and straight rail positive pressure tripad sliders. The impact of these new characteristics and design differences on the head/disk reliability needs to be thoroughly understood in order to ensure a successful proximity recording. This paper investigates the partial contact air bearing characteristics of three representative tripad slider designs. The contact stiffness is used as a measure of the air bearing compliance. Simulations of the fly height, contact force, altitude sensitivity, substrate waviness induced spacing and contact force modulations, contact take-off, track seeking and crash stop dynamics are performed for the three tripad slider designs. The results demonstrate that the negative pressure tripad sliders can be designed such that they can achieve both flat fly height and contact force profiles and, fast take-off, while still maintaining a small contact stiffness. However, the air bearing compliance and waviness following performance have to be compromised for the correct partial contact air bearing pitch stiffness.

Dynamic Stability and Spacing Modulation of Sub-25 nm Fly Height Sliders

Designing a reliable sub-25 nm spacing head/disk interface for today’s magnetic hard disk drives demands a greater dynamic stability and a smaller spacing modulation. An air bearing dynamic simulator with multiple features is developed to investigate the dynamic characteristics of three shaped-rail negative pressure sub-25 nm fly height sliders. Various simulations including air bearing stiffness, impulse response, surface roughness induced spacing modulation, bump response, and track seeking dynamics are performed. The results indicate that the roughness induced spacing modulation decreases with the increase of the air bearing stiffness and the decrease of the slider size. The suspension dynamics is integrated into the air bearing dynamics simulation for the track accessing motion by modal analysis. It is concluded that the fly height modulation during a track accessing event is attributed to many factors such as the effective skew angle, the seeking velocity, and the roll motion caused by the inertia of the moving head. The extent of the roll motion effect depends on the air bearing roll stiffness and the level of the inertia force of the moving head. Larger roll stiffness and smaller inertia force produce a smoother track accessing performance.

Track seeking, head-disk spacing variation and head-disk interface failure

Track seeking and the slider-disk space variation during track seeking are becoming more and more serious concerns when technology moves to 0.8-1.5 /spl mu/m gap flying-height. This paper reports results of in-situ experimental investigations of the slider's dynamic flying performance during track seeking. A partial erase method and a slider-disk resistance method were used in the investigation. Results indicate that the suspension's vibration during track seeking plays an important role in the flying height vibration of slider. Furthermore, results indicate that the likely zone of slider-disk impact can be revealed by monitoring the resistance and the VCM current waveform simultaneously. A model is proposed also to explain the relationship of flying particles inside disk chamber, the slider's dynamic performance during track seeking and the flying particles induced HDI failure.

Head positioning control for low sampling rate systems based on two degree-of-freedom control

To enhance formatting efficiency in high density hard disk drives, it is desirable to decrease the number of embedded servo signals. As a consequence, the sampling rate of servo systems has been becoming lower. In this situation, it is difficult to attain fast track seeking by using only a feedback controller. This paper presents a high-speed track seek technique, based on two degree-of-freedom control. A new reference model is developed by focusing on the attainment of fast track seeking through the effective use of a voice coil motor, which functions as a disk head actuator. The model generates reference trajectories, allowing high-speed movement. Also, in order to quickly settle the disk head to a target track under various disturbances, an initial value compensation (IVC) method to minimize a quadratic performance index is developed. Experimental results, using a 3-1/2 inch hard disk drive, confirm the validity of the proposed method.

Particle build-up on flying sliders and mechanism study of disk wear and head-disk interface failure in magnetic disk drives

A novel experiment was designed to promote the understanding of the mechanism of disk wear caused by particulate contamination. Obtained results suggest that the slider picks up particles and accumulates them on its surfaces as it flies above the disk. The instability of the flying slider, especially that induced by track seeking, leads to slider-disk impact which releases the accumulated particles. This results in abrasion amongst the slider, the contaminants, and the disk. The investigation also indicates that the outer two surfaces in a disk-stack and the corresponding sliders have higher likelihood of capturing flying particles. A model has been proposed to explain the observed phenomena.

Effects of seeking velocity on air bearing skew angle, air flow speed and flying performance of sliders with different ABS designs

The effects of seeking velocity on slider's flying performance can be described by the change of air flow speed and air-bearing skew angle. Obtained results indicate that the relative variation of air bearing skew angle, caused by seeking velocity, is significantly higher than that of the air flow speed. Sliders with higher sensitivity to skew angle (like TPC sliders) will correspond to a higher change in flying performance during track seeking. The flying height loss and the increase in rolling rate of TPC slider during the acceleration period of OD seeking increase the likelihood of head-disk impact. Similar, but reduced effects are observed on positive pressure tri-pad. TPC slider and positive pressure tri-pad slider showed contrasting results on the effects of z-height change. Reduction in z-height reduces the flying height of TPC slider but increases that of the positive pressure tri-pad slider. Negative pressure tri-pad is observed to be insensitive to both seeking and z-height change.

High-Speed Positioning Control for Hard Disk Drives Based on Two-Degree-of-Freedom Control.

We present a high-speed track seeking technique based on two-degree-of-freedom (2DOF)control. 2DOF control, which simultaneously uses feedback and feed-forward control, specifies loop performance and response performance independently. Therefore, conventional mode-switching from velocity control to positioning control becomes unnecessary. Focusing on how to effectively use a voice coil motor (VCM), that is, a head actuator, for fast track seeking, a new reference model is also proposed. In this 2DOF control system, a reference model plays an important role in shortening of seek times. Considering the VCM dynamics, this reference model generates a reference trajectory allowing high-speed movement and fast settling to target tracks. Experimental results using a typical hard disk drive indicated the validity of the proposed method, especially for short stroke seeks.

Mode switching control design with initial value compensation and its application to head positioning control on magnetic disk drives

To meet an increasing demand for high performance and large-capacity in magnetic hard disk drives, both fast response and precise positioning are strongly required for the head positioning control. A mode switching control (MSC) system, which includes a track seeking controller, a track following controller, and a switching function, is widely used to meet this requirement. The issue for MSC is to confirm a design method of servo mode switching. This paper proposes the initial value compensation method (IVC). The IVC inputs certain initial values in the controller state variables at mode switching in order to improve the transient response after switching. There are two approaches to determine the values: (1) minimizing a linear-quadratic function of the plant state variables; and (2) cancelling the unfavorable poles of the transfer function between initial values and head position by relocating zeros. Some experimental results are shown to be effective.

Flying height change during seek operation for TPC sliders

The change in the flying height during track seeking is investigated for a TPC slider. A quasi-static approach is used in the numerical simulation of the track seek. We find that at high seek velocities the skew angle effect dominates over the inertia effect. The flying height change is the greatest at the MD. We also found that the deeper etch results in more loss of flying height. Quasi-static measurement is presented. The decrease in the flying height is qualitatively verified through the use of an AE sensor signal in the frequency domain.