Cheol-soon Kim

In situ runout identification in active magnetic bearing system by extended influence coefficient method

In this paper, an efficient, yet easy-to-use, in-situ runout identification scheme, based on the extended influence coefficient method, is presented for active magnetic bearing (AMB) systems. It is shown experimentally that the proposed scheme successfully identifies and eliminates the troublesome runout of a well-balanced AMB system in the laboratory, so that a high-precision spindle system can be achieved, while it is in operation.

VCM design to improve dynamic performance of an actuator in a disk drive

PES design margin of current hard disk drive becomes getting smaller as track density of high-capacity disk drives increases. In the present work, a new actuator design concept is introduced for reducing one of the actuator vibration modes so called a butterfly mode that normally limits expansion of servo bandwidth. The new actuator has single coil with four sections having respective particular directions. By adjusting the length and direction of the sections of the VCM coil, the forces perpendicular to track seeking direction can be balanced with each other, and then the butterfly mode is not excited by the resultant force of VCM. Contribution of the proposed actuator design for the PES management is verified by numerical simulations. As a result, the servo bandwidth is to be increased so that PES can be decreased due to improved dynamics of the proposed method.

Isotropic Optimal Control of Active Magnetic Bearing System

As a new rotor control scheme, isotropic control of weakly anisotropic rotor bearing system in complex state space is proposed, which utilizes the concepts on the eigenstructure of the isotropic rotor system. Advantages of the scheme are that the controlled system always retains isotropic eigenstructure, leading to circular whirling due to unbalance and that it is efficient for control of unbalance response. And the system analysis and controller design becomes simple and yet comprehensive since the order of the matrices treated in the complex domain approach is half of that in the real approach. The control scheme is applied to a rigid rotor-active magnetic bearing system which is digitally controlled and the control performance is investigated experimentally in relation to unbalance response and control energy. It is found that the isotropic optimal control method, which essentially eliminates the backward unbalance response component, is more efficient than the conventional optimal control in that it gives smaller major whirl radius and yet it often requires less control effort.

In-Situ Identification of Active Magnetic Bearing System Using Directional Frequency Response Functions

Complex modal testing is employed to obtain the directional frequency responses of a four-axis active magnetic bearing system. In the test, magnetic bearings are used as exciters while the system is in operation. The directional frequency response estimates are then used to effectively identify the parameters of the active magnetic bearing system. Experimental results show that the directional frequency response function, which is properly defined in the complex domain, is a powerful tool for identification of bearing as well as modal parameters of the system.

Advanced Unloading Analysis Considering Lateral Velocity and Disk RPM Drop in Emergency Parking

A conventional unloading analysis was carried out in no consideration for fast lateral velocity and disk rpm drop during the emergency parking. However, current hard disk drive (HDD) systems need an improved performance in the portable aspect. Therefore, much faster emergency parking is absolutely necessary to protect the system from sudden power-off or an external shock. In case of fast emergency parking, the fast lateral velocity causes an effective skew, and then there is a flying height (FH) loss which does not occur in a normal unload. Also, the FH loss induced by a disk rpm drop occurs at an unloading point in case that a portable small form factor HDD like 1.8 in is suddenly parking from inner diameter to outer diameter. This means that the unloading performance considering the effective skew becomes lower than one of the conventional unloading analysis. In this paper, we propose the advanced unloading analysis that can be applied to the fast emergency parking velocity. To do so, we investigate the FH loss by the fast lateral velocity and the disk rpm drop, and introduce the result of the advanced unloading analysis.

Optimization of wide-bandwidth hard disk drive actuator design using statistical methods

As track density of hard disk drives is intensified, actuator dynamics has emerged as one of the most important design factors not only for high-performance models but also desktop applications. In spite of many different complex physical considerations required for successful wide-bandwidth actuator designs, most current design processes that rely purely upon designers experience-based historical approaches do not effectively account all factors to determine optimum design. A development of design optimization procedure for a high-performance actuator is presented in this paper. A position error signal (PES) estimator employing both numerical model and experimental data is developed for effective and accurate optimization process. Statistically formulated optimization method presented here delivers not only wide-bandwidth actuator design for PES reduction but higher shockproof HDA design. Furthermore the presented method incorporates volume production parameters so that designers are able to predict design tolerance and cost relations.

Analysis of Thermal Flying-Height Control System About Humid Air Condition in Hard Disk Drive

This research investigates to analyze the effect of humid air on TFC system. Required parameters of humid air and flying attitude of slider are calculated. Condensation of air bearing at over the saturation vapor pressure is considered. Heat transfer coefficient for humid air condition is calculated. TFC simulation about humid air is conducted.Copyright

Characterization and Detection of a Free-falling State of a Mobile HDD Using the Electromechanical Analysis in a Rotating Spindle System

This research investigates the electromechanical characteristics of a spindle motor in a free-falling mobile hard disk drive before unexpected shock. Electromechanical simulation includes a time-stepping finite element analysis of the magnetic field in a speed controlled brushless DC motor and dynamic analysis of the stationary and rotating part linked by the fluid dynamic bearing under the free-falling condition. Analysis results show that the dynamic characteristics of the rotating spindle system during free-falling state have an effect on the relative motion between the stationary and rotating part of HDD. It results from the variation of reaction force in the bearing area due to the gravity force exerted on the rotating part of HDD, and the free-falling condition can be detected by observing the signal of the spindle motor and disk-head interface without using an accelerometer.

Dynamic analysis of magnetic disk drive actuators according to temperature variation

The initial internal temperature of a hard disk drive (HDD) differs according to the operating environment, and the various heat sources from the surrounding environment raise the internal temperature of the HDD after operation starts. Such temperature change ranges from 0 °C to 60 °C in the usual operating environment. This affects the characteristics of a pivot bearing that supports the HDD actuator. This paper predicts how the characteristics of pivot bearings change in relation to temperature and how the dynamic characteristics of actuators change according to the change of bearing characteristics. These changes are also verified through experiments.

Computational Fluid Dynamics and Experimental Visualization of Time-Variable Air Flow Pattern Inside Hard Disk Drives

The time-variable air flow pattern inside hard disk drives (HDDs) was analyzed using computational fluid dynamics (CFD) and experimental flow visualization. The existence of a rotating polygonal flow pattern inside an actual HDD was directly confirmed both through simulation and experiment for the first time. Even though earlier studies reported time-invariant polygonal flow pattern inside fully shrouded co-rotating disks, we found that the polygonal flow pattern varied its polygonal number repeatedly with time in not fully shrouded actual HDD. Triangular, tetragonal, pentagonal, and hexagonal flow patterns were observed both in simulation and experimental visualization. We also observed peaks in the power spectrum density of the simulated air velocity and these corresponded to the polygonal flow patterns. It was also found that each of these frequencies basically coincides with the frequency expected by the relation between the polygonal number and the vortex frequency which has been reported by earlier studies using fully shrouded co-rotating disks.