Non-repeatable Runout Research Articles

Reduction of the Influence of Non-Repeatable Run-Out in X-Y Plane of Machining Surface

In recent years, the quality of the machined surfaces of molds for optical or medical instruments has needed improvement, and the fabrication of mirrored surfaces by means of cutting only has been gaining in importance. In order to obtain smooth surfaces, it is necessary to reduce various vibrations in the machine tool during the machining process. Many factors cause vibration in a machine tool, including feed mechanisms, pumps, and chatter. A high-speed spindle for precision machining is one source of vibration, but it is a challenge to avoid the non-repeatable run-out (NRRO) of a spindle. This study has developed an excitation system that can reduce the influence of non-repeatable run-out on machining surfaces. This paper presents a newly-developed excitation system with an excitation unit for each the X and Y direction. The excitation units consist of a voice coil motor and leaf springs, fixed on a spindle head. The tool run-out and vibration of the spindle head are measured by a displacement sensor and an acceleration sensor fixed on the spindle head, and their NRRO components are obtained through extraction using band-pass filters. By using these NRRO components as feedback signals, the excitation unit generates the force to cancel the NRRO of the tool. In order to determine the performance of the developed system, experimental evaluation was performed on a vertical 3-axis machining center. Since the spindle used for evaluation had three bearings, the measured NRRO of the tool had three peaks in the frequency domain. First, we conducted evaluation experiments under non-machining conditions, and all NRRO peaks were reduced by applying the developed system. Furthermore, there was no interference of the excitation units in the X and Y directions. Next, we evaluated the influence of the NRRO of the tool on the machining surface under finish machining conditions. The reduction in NRRO components in the measured surface profiles was observed through feedback. It was concluded that the developed system can reduce the influence of NRRO on machining surfaces.

Reduction of the influence on machining surface caused by tool non-repeatable run-out of rolling bearing spindle for machine tools (Improvement of machining surface quality with an excitation system)

Reduction of the influence on machining surface caused by tool non-repeatable run-out of rolling bearing spindle for machine tools (Improvement of machining surface quality with an excitation system)

Reduction of the influence on machining surface caused by tool non-repeatable run-out of rolling bearing spindle for machine tools (Development of an excitation system)

Reduction of the influence on machining surface caused by tool non-repeatable run-out of rolling bearing spindle for machine tools (Development of an excitation system)

Hydrostatic ionic liquid-lubricated fluid film bearing for a rotational electron-beam lithography system

Rotational electron-beam mastering (REBM) systems have been studied with the aim of achieving the high data density necessary to facilitate the fabrication of next-generation optical data media, such as holographic storage. This study reports the design and testing of an ionic liquid (IL)-lubricated hydrostatic spindle system comprising an IL bearing, IL supply pump, and ionic magnetic fluid seal; and its outgassing performance under high-vacuum conditions. An inner vacuum chamber pressure of approximately 10−4 Pa was maintained during rotation of the spindle system. The outgassed products, as measured by a quadrupole mass spectrometer, were primarily generated from the air components in the lubricant IL. The non-repeatable runout in the radial direction, which is an important parameter of REBM accuracy, was 100 nm for a rotational speed of less than 130 min−1. The proposed method can be used for a 100-nm scale REBM device.

A Prototype Excitation System to Reduce the Influence on Machining Surface Caused by Tool Non-Repeatable Run-Out

A Prototype Excitation System to Reduce the Influence on Machining Surface Caused by Tool Non-Repeatable Run-Out

Robust shaft design to compensate deformation in the hub press-fitting and disk clamping process of 2.5″ HDDs

We investigated deformation of the outer diameter of a shaft due to the hub press-fitting and disk clamping processes associated with a 2.5? hard disk drive. We propose a new robust shaft design to minimize the effect of deformation on the outer diameter of the shaft. We numerically show the effect of deformation on the shaft due to the pressure, stiffness, and damping coefficients of fluid dynamic bearings (FDBs), and the critical mass and excitation response of the rotor-bearing system. We also experimentally measured the axial non-repeatable runout and the amplitude at the half speed whirl frequency of FDBs with both conventional and proposed designs. Through these tests we confirm that the proposed design improves the static and dynamic performance of the FDBs and rotor-bearing system.

Effect of an hourglass-shaped sleeve on the performance of the fluid dynamic bearings of a HDD spindle motor

This research investigated the characteristics of fluid dynamic bearings (FDBs) in a HDD spindle motor with an hourglass-shaped sleeve. We demonstrated experimentally that the hourglass-shaped sleeve generated through the ball-sizing process is a major source of large repeatable runout and non-repeatable runout in a HDD spindle system. We also numerically proved the effect of hourglass-shaped sleeves on pressure, friction torque, stiffness and damping coefficients, critical mass, and shock response. Finally, we proposed a robust design for FDBs with hourglass-shaped groove depths to compensate for the decrease in the static and dynamic performance of FDBs with hourglass-shaped sleeves. The proposed hourglass-shaped groove depth improves the performance of FDBs with both straight and hourglass-shaped sleeves.

Rolling bearing-suspended spindle run-out control using repetitive control and adaptive feedforward cancellation

The wettability, friction, adhesion, and optical characteristics of a surface are affected by its micro-topography. In turn, the characteristics of a surface micro-topography can be determined by controlling the trajectory of a cutting tool mounted on a spindle. However, this method is constrained by the tool’s cutting accuracy, which is limited by the run-out error and thermal behavior of the spindle. In this work, an electromagnetic actuator (EMA) is installed at the end of the spindle to compensate for the spindle run-out error and the thermal displacement in real time. The spindle system model includes a cross-coupled stiffness and damping term for the ball bearings. Repetitive controller (RC) reduces the repeatable run-out (RRO) while adaptive feedforward cancellation (AFC) reduces the non-repeatable run-out (NRRO). The control system uses a zero phase error tracking controller (ZPETC) to compensate for its total phase delay. Experimental results show a 70% reduction in run-out error at rotation speeds of 6,000 rpm and 12,000 rpm. The thermal displacement in the radial direction is regulated within a range of 1 µm for more than 3 hours.

Development of Optimized Adaptive Feed-Forward Cancellation with Damping Function for Head Positioning System in Hard Disk Drives

This paper presents an adaptive feed-forward cancellation (AFC) technique with a damping function based on optimized AFC. The proposed AFC includes a mechanical resonance characteristic as an internal model that is realized by using the damping function and that achieves the best performance in the frequency domain. This makes it possible to apply the proposed AFC to suppress not only repeatable run-out (RRO) but also non-repeatable run-out (NRRO) of the head positioning system in hard disk drives. Simulation results showed that the proposed AFC can suppress NRRO such as disk flutter vibration and that the stability of the feedback loop was improved by the damping function.

Modal Analysis of Optical Disk Rotating at 15,000 rpm Close to a Rigid Wall

The rotational speed of optical disks must be above 15,000 rpm owing to the required high data-transfer rates. At these speeds, the airflow between the rotating disk and a rigid wall is important when identifying the dynamic characteristics of the disk in a slim optical disk drive. This airflow is simulated using a numerical model and examined by a closed-form modal analysis. The calculated dynamic characteristics of the rotating disk can be used to predict the nonrepeatable runout for the servo controller design.

Two-Period Repetitive and Adaptive Control for Repeatable and Nonrepeatable Runout Compensation in Disk Drive Track Following

This paper presents the design and implementation of an integrated two-period repetitive and adaptive control scheme to reject both repeatable and nonrepeatable disturbances in the track-following servo control of a hard disk drive read-write head. A baseline linear quadratic Gaussian controller is augmented by a plug-in repetitive controller to reject periodic disturbances with two periods, one that is synchronized with the disk rotation and the other that is not, and an additional adaptive-Q control scheme to reject the remaining aperiodic and random disturbances. The adaptive-Q control algorithm uses the well-known result that all stabilizing controllers for a plant can be synthesized by conveniently parameterized augmentation to a nominal controller. The Q-filter is restricted to a finite impulse response filter, which minimizes the root-mean-square value of the track-following error. Experimental results show substantial performance improvement for the two-stage augmented controller over the single-stage repetitive or adaptive-Q control acting alone.

Design of Herringbone Groove Disk Damper for Effective Suppression of Axial Vibration of Disk in Hard Disk Drive

A herringbone groove pattern is applied to a plane disk damper of a hard disk drive to reduce the axial vibration of the disk rotating at high speed. According to the design parameters such as mean radius r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> , ridge-groove ratio B, and spiral angle β , the performance of the herringbone groove disk damper is studied numerically in terms of the stiffness and the damping coefficients and the ratio of the axial stiffness to the frictional torque. The dynamic coefficients are calculated from the result of computational fluid dynamics solution of a simple hard disk drive model using four-point central difference scheme. To confirm the numerical results, the nonrepeatable run-out of the disk is measured when a plane or a herringbone groove disk damper is used. It is found that a herringbone groove disk damper increases the stiffness as well as the damping coefficient of the air film by 20% more than those of a plane disk damper.

2D Nano-Motion Actuator for Precise Track Following

The authors proposed a new actuator for track following on a spin-stand that evaluated magnetic heads and media for high density magnetic recording with high speed. The new actuator was named “Nano-Motion Actuator (NMA)” by the authors. At the present time, effect of azimuth angle which causes between a center line of a head slider and a tangential direction of the track is increasingly actualized as a track pitch of the head becomes narrow. Therefore, if a discrete track media (DTM) will be put to practical use, the effect of the azimuth angle will be actualized more clearly. Because, DTM will have large RRO (Repeatable Run-Out) that is caused by eccentricity error between a medium and a hub of an air-spindle. Furthermore, NRRO (Non-Repeatable Run-Out) which is caused by mechanical vibration of the air-spindle, flutter of the medium, turbulence around a HGA (Head gimbals Assembly) and so on is overlapped with the RRO. Especially in case of the large NRRO, since the azimuth angle will rapidly change, compensations of the azimuth angle should be absolutely necessary. Therefore, precision positioning actuator with high speed on an X-Y plane which is coplanar will be required the evaluation of the high density magnetic recording. We proposed a new actuator which was consisted of a NMA mechanism and a translation mechanism. The translation mechanism was composed of a stacked piezoelectric that was supported by two elastic springs. The new actuator that was called “2D Nano-Motion Actuator (2D NMA)” could move within 10 square micrometer and be positioned by nanometer resolution with high speed.

Accurate Runout Measurement for HDD Spinning Motors and Disks

As hard disk drive (HDD) areal density increases, its track width becomes smaller and smaller and so is non-repeatable runout. HDD industry needs more accurate and better resolution runout measurements of spinning spindle motors and media platters in both axial and radial directions. This paper introduces a new system how to precisely measure the runout of HDD spinning disks and motors through synchronously acquiring the rotor position signal and the displacements in axial or radial directions. In order to minimize the synchronizing error between the rotor position and the displacement signal, a high resolution counter is adopted instead of the conventional phase-lock loop method. With Laser Doppler Vibrometer and proper signal processing, the proposed runout system can precisely measure the runout of the HDD spinning disks and motors with 1 nm resolution and 0.2% accuracy with a proper sampling rate. It can provide an effective and accurate means to measure the runout of high areal density HDDs, in particular the next generation HDDs, such as, pattern media HDDs and HAMR HDDs.

Written-In RRO and NRRO Characteristics in Mobile Hard Disk Drives

The characteristics of repeatable runout (RRO) and nonrepeatable runout (NRRO) of hard disk drives (HDDs) have to be determined for higher tracks per inch since the static positional accuracy has almost reached its limit. To break this limit, it is necessary to identify precisely these track misregistration characteristics. We experimentally estimated the actual component of disturbances in both servo-writing and the actual drive using a mixture of helium and air. In the study described in this paper, windage or flow-induced vibration in servo-writing was the dominant component as a torque disturbance in the drive measurement. We then found that written-in RRO was key to understanding the characteristics of disturbances. As a result, the most dominant component in NRRO and even in written-in RRO is torque disturbance followed by positional disturbance for mobile HDDs.

Probabilistic Analysis of Off-Track Capability Assuming Geometric Track Misregistration Model for Higher Track Density Disk Drives

Head offset capability is a key parameter for achieving higher track density. We analyze it by using a probabilistic approach. The error rate in head offset positions is calculated using core width of write and read head and mechanical runout. The nonrepeatable runout (NRRO) experiment shows the reasonableness of a Gaussian distribution assumption. The readback signal depends on geometrical overlapped write and read core width. The error rate is calculated by integration of positioning probabilistic density. We also study side erasure phenomena of multiple adjacent track writing by a similar probabilistic approach. The calculation shows the mechanical NRRO affects side erasure in million times adjacent track writing. On these offset-capabilities, the calculated and experimental results are well correlated. These results show this probabilistic modeling is appropriate.

Control Design for Self-Servo Track Writing Using a State-Space Disturbance Observer

Self-servo track writing is a process of writing servo bursts on a subsequent track using servo bursts from the previously written track as a reference. A fundamental obstacle in self-servo track writing is radial error propagation due to written-in repeatable runout (RRO) in the seed track and external disturbances. In this paper, we introduce a control scheme for the self-servo track writing process using a state-space disturbance observer. A linear quadratic Gaussian/loop transfer recovery controller with a state-space disturbance observer is designed for feedback control. In addition, feedforward control for the reference correction signal is designed to prevent radial error propagation. From simulation results, it turns out that the nonrepeatable runout (NRRO) component is a major source of AC track squeeze. The proposed disturbance observer is very effective in reducing the effect of the NRRO. The proposed control scheme is validated by simulation results which show improvement in the standard deviation and the AC track squeeze of the written tracks.

나노급 정밀 구동을 위한 스핀들 모터-척 시스템 설계

The STW(servo track writing) system which is the process of writing servo signals on disks before assembling in drives uses the spindle motor-chuck mechanism to realize low cost because the spindle motor-chuck mechanism has merit which can simultaneously write multi-disk by piling up disks in hub. Therefore, when the spindle motor-chuck mechanism of horizontal type operates in high rotation speed it is necessary to reduce the effect of RRO(repeatable run-out) and NRRO(non-repeatable run-out) to achieve the high precision accuracy of nano-meter level during the STW process. In this paper, we analyzed that the slip in assembly surfaces can be caused by the mechanical tolerance and clamping force in hub-chuck mechanism and can affect NRRO performance. We designed springs for centering and clamping considering centrifugal force by the rotation speed and assembly condition. The experimental result showed NRRO performance improves about 30 % than case of weak clamping force. The result shows that the optimal design of the spindle motor-chuck mechanism can effectively reduce the effect of NRRO and RRO in STW process.

A Neural-Networks-Based Adaptive Disturbance Rejection Method and Its Application to the Control of Hard Disk Drives

This paper presents a neural-networks-based disturbance rejection adaptive scheme for dealing with repeatable and nonrepeatable runout simultaneously. The effectiveness of this method is demonstrated empirically on a commercial hard disk drive where the adaptive disturbance rejector is added to a baseline linear time-invariant (LTI) controller. The adaptive scheme can be broken into two subsystems: one subsystem is designed to suppress the repeatable runout (RRO) and the other to attenuate the residual disturbance and nonrepeatable runout (NRRO) by the use of radial basis functions. Two different methods for RRO suppression are employed in conjunction with the neural-networks-based NRRO rejector. The first one is an adaptive feedforward disturbance rejection scheme. The second is a repetitive controller. In both cases the neural modeled disturbance rejector is adapted online further increasing the track-following performance by as much as 6.4%. Experimental results of the schemes at various locations of the hard drive are included to demonstrate the general applicability of the approach on commercial drives. The total reduction of the error during track-following is measured to be as much as 25.4% respect to the baseline LTI controller.

ナノモーションアクチュエータに対するPID制御器の設計(機械力学,計測,自動制御)

A conventional spin-stand system which used to evaluate read-write performance of magnetic recording heads and media can not allow thermal drift of its mechanism and NRRO (non-repeatable run out) of its air spindle motor. Therefore, we proposed a precise track following actuator which applied the spin-stand to construct a piggyback system. The actuator called Nano Motion Actuator (NMA) by the authors consists of a piezoelectric element and a displacement amplifying mechanism. In this paper, a design method of a PID controller to realize a specified gain crossover frequency for the NMA is proposed. At first, we discussed about a model matching problem of an open loop transfer function and decided PID gains from physical parameters of the NMA. Next, based on a loop shaping method, the properties of the frequency response of the open loop transfer function were improved. The effectiveness of the proposed method is illustrated by the simulation results and experimental results.