Jun Jeong

Repetitive controller design for minimum track misregistration in hard disk drives

Repetitive control is a widely used technique for the compensation of repeatable error in systems that contain rotating mechanisms or repeat a trajectory. Generally, it includes delay chains and a low-pass filter in the positive feedback loop, which generate a periodic signal. The controller has typically been implemented in a plug-in fashion and designed heuristically with the simplest form of the filter. However, this design approach is somewhat ambiguous in the selection of controller parameters because of its influence over nonharmonic frequencies. Also, it leaves the possibility for further improvement. This paper presents an improved design method for the repetitive controller that provides minimum track misregistration (TMR) in a hard disk drive (HDD). For TMR prediction, the method identifies disturbances acting on an HDD and estimates servo performance, using the identification result. We have confirmed the identification and estimation procedure through experiments. In our method, first the basic tracking controller is designed and later the repetitive controller is designed in conjunction with a Q filter. A cost function based on Parsevals theorem, reflecting the servo performance as TMR, is defined. Then the servo performance is estimated from the identified disturbance, and the plant and designed controllers frequency response are modified as necessary by changing the parameters of the controller, whose optimization is carried out with a commercial nonlinear optimization tool. The design strategy facilitates the controller design by providing an accurate estimation for the attainable servo performance and design criteria under the optimization framework.

Improved Air-Gap Control for SIL-Based Near-Field Recording System

We propose a gap servo method to reduce the overshoot in the near-field (NF) region. In general, we use a mode-switching servo which consists of approach, hand-over, and gap-control modes. However, there is a critical problem, such as an overshoot at the turning point from the approach mode to the hand-over mode, which may cause a collision between the SIL and the disk. In this paper, we propose an improved NF gap servo system using an exponential input in the approach mode which can reduce the overshoot and settling time of the mode-switching servo

Improved Gap Control System Using a Disturbance Observer for Near-Field Recording

In a solid immersion lens (SIL) based near-field recoding (NFR) system which is one of the emerging technologies for next-generation optical data storage systems, it is essential that the air gap between the SIL and the rotating disk is maintained at less than 50 nm without collision in order to obtain the proper coupling efficiency of evanescent waves. To fabricate a reliable near-field air gap servo system, various disturbances such as disk vibration, external shock and overshoot have to be considered, and these possible disturbances have to be prevented effectively. We propose an improved gap servo system using a disturbance observer (DOB), which has a reduced overshoot and rejection performance for the previously mentioned disturbances. The effectiveness of the proposed controller is verified by experimentally. The experiment results, show that the overshoot was reduced using the proposed near-field air gap servo system with a DOB. In the case of the ramp approach mode with and without the hand-over mode in the mode-switching servo, the overshoots were decreased to 50.9% and collision was avoided, respectively. In addition, in the case of the modified approach mode with and without the hand-over mode in the mode-switching servo, the overshoot was decreased to 2.9 and 3.7%, respectively. Consequently, the access time was decreased in each approach case without the hand-over mode using the DOB-based controller. In addition, the disturbance rejection performance of the external shock was improved to 9.11%.

Identification of contributors to HDD servo errors by measuring PES only

In this paper, disturbances, the source of TMR being the barrier to high track density of HDD are analyzed quantitatively using PES only without extra sensors. Disturbances are separated into RRO and NRRO, then, NRRO is separated into input torque disturbance, output disturbance, and noise. The effect of each disturbance on PES is compared. It is shown that PES NRRO measured in open-loop and in closed-loop can be used to calculate sensitivity transfer function. Using separated disturbances, PSD of PES RRO and NRRO are predicted for various gain levels in track following servo, and they are compared with PSD of PES measured in experiments. We propose that this can be used to predict servo performance accurately. Also addressed are some discussions on the differences between this work and the former contributing work.

Improved Air Gap Controller for Solid Immersion Lens-Based Near-Field Recording Servo System

The near-field air-gap controller for solid immersion lens (SIL)-based near-field recording (NFR) systems has been required to improve its robustness to dynamic disturbances associated with rotating conditions such as periodic disk resonance and external shock. These dynamic disturbances usually exist in optical disk drive systems as a result of the operating condition. Moreover, SIL based NFR servo systems should be considered to realize stable servo systems because it is essential that the air gap between the SIL and the rotating disk should be less than 100 nm and that the SIL and the disk should be protected. Hence, we propose improved air-gap control methods using internal model principle (IMP)-based and dead-zone nonlinear controllers. To increase air gap control performance with respect to dynamic disturbances, the IMP-based control method, which is one of the improved control methods for eliminating periodic disturbances at some frequencies, is implemented in the original NFR servo system. In addition, in case of anti-shock control performance, the original air gap servo system should be improved in terms of the anti-shock control such that the actuator head with SIL optics has to maintain on extremely small air gap to avoid collision between the SIL and the media when external shock is applied to the NFR system. Consequently, the experimental results show that the gap error signal is reduced by 76% during vibration of the disk using an IMP-based controller, and anti-shock control performance under external shock is improved by 60% using a dead-zone nonlinear controller.

Improved Antishock Air-Gap Control Algorithm with Acceleration Feedforward Control for High-Numerical Aperture Near-Field Storage System Using Solid Immersion Lens

A near-field storage system using a solid immersion lens (SIL) has been studied as a high-density optical disc drive system. The major goal of this research is to improve the robustness of the air-gap controller for a SIL-based near-field recording (NFR) system against dynamic disturbances, such as external shocks. The servo system is essential in near-field (NF) technology because the nanogap distance between the SIL and the disc is 50 nm or less. Also, the air-gap distance must be maintained without collision between the SIL and the disc to detect a stable gap error and read-out signals when an external shock is applied. Therefore, we propose an improved air-gap control algorithm using only an acceleration feedforward controller (AFC) to maintain the air-gap distance without contact for a 4.48 G at 10 ms shock. Thus, the antishock control performance for the SIL-based NF storage system in the presence of external shocks is markedly improved. Furthermore, to enhance the performance of the antishock air-gap control, we use the AFC with a double disturbance observer and a dead-zone nonlinear controller. As a result, the air-gap distance is maintained without contact for a 6.56 G@10 ms shock.

Improved Air Gap Control With Acceleration Feedforward Controller Using Time Delay for Solid Immersion Lens-Based Near-Field Storage System

This paper aims to improve the robustness of the air-gap controller for a solid immersion lens (SIL)-based near-field (NF) storage system against dynamic disturbances, such as external shocks. The robust control system is essential in NF data storage technology because the nanoscale gap distance between the SIL and the disk is 50 nm or less. Also, the air-gap distance must be maintained without physical contact between the SIL and the disk to evaluate the efficient reading and recording signals when an external shock is applied. Therefore, we propose an improved air-gap control algorithm with an acceleration feedforward controller (AFC) using time delay to maintain the nanoscale air-gap distance without collision for a 5.76 G at 10 ms shock. Thus, the anti-shock control performance for the SIL-based NF storage system in the presence of external shocks is markedly improved. Furthermore, to enhance the performance of the anti-shock air-gap control with AFC using time delay, we use the AFC using time delay with a double disturbance observer (DOB). As a result, the air-gap distance is controlled without contact for a 6.85 G@10 ms shock.

On the improvement of tracking servo performance of HDD using accelerometer

In the track following problem of HDD, there are many disturbances such as windage, pivot friction, bias torque from flex cable, RRO, and noise. Especially, the windage is one of the major sources, and its magnitude increases drastically as the disk rotation speed increases. In this paper, a method to attenuate the influence of input torque disturbances is proposed by the use of an accelerometer. The disturbances are estimated from the accelerometer attached at the carriage of VCM actuator, and are feedbacked into the control input in order to attenuate its effect to PES in the multi-rate control framework. Experiment shows that the proposed method effectively decreases the influence of the windage in the low frequency range without increasing the control bandwidth

Repetitive controller design for minimum TMR in HDD

This paper presents a design method for repetitive controller in the sense of minimum TMR. For TMR prediction, disturbances acting on a disk drive are identified and servo performance is estimated using identification result. The validity of the identification and estimation procedure is proved through experiments. As a repetitive controller, an add-on type is adopted, and PID controller is used for base tracking controller. Each controller is optimized for minimum TMR. Whole system performance is simulated and design criterion is suggested. For improved response in both transient and steady state, two kinds of gain-switching method (discrete, continuous) are applied and switching effect also investigated