xiao Guo

A nonlinear track following controller for hard disk drives

Most of the HDD track following controller are linear controllers. This paper presents a nonlinear control scheme for track following. The controller mainly includes the parts: Tracking Differentiator (TD), Extended State Observer (ESO) and Nonlinear PID (NLPID). Simulation results and experimental results show that the design performs superior to the linear design.

HAMR Writing Process Model-Based Compensation of Laser-Induced Transients

An established model is presented, which includes relative slow write head protrusion, fast near-field transducer (NFT) only protrusion, and heat transient process for heat-assisted magnetic recording writing process. Based on this model, equations are derived to calculate the write head and NFT only protrusions as a function of writing duty cycle. Then, we demonstrate how to compensate for the protrusion-caused NFT-to-media spacing change by thermal fly-height control for three different writing conditions of duty cycle from low to high, namely, repeatable run-out field write, servo pattern write, and user data field write. Finally, we present how to compensate for the impacts caused by fast transient processes, which include NFT only protrusion and heat transient process, within each servo wedge or data sector. There are two methods presented, pole placement method and deconvolution method. Both the methods produce very similar input laser profiles, which will cause approximately step change at the output signal magnitude responses from the simulation.

Modelling and compensation for laser induced transients to improve HAMR writing quality

A model including NFT slow and fast protrusions and heat transient in HAMR writing is given. The effects of transients on different types of writing are discussed. Based on the model, a pole placement method is proposed to design the parameters for overshoot circuit to handle fast protrusion and heat transient. The alternative method is a discrete solution using deconvolution to generate a table for add-on laser current. The effectiveness for both methods is demonstrated in simulation results. Slow protrusion can be compensated by the fly height delta produced by heater control. The heater current is obtained by deconvolution method to achieve a constant fly height for all writing samples.

External Disturbance Rejection by Use of an Add-On Nonlinear Controller in HDD Servo Systems

A novel control method with an add-on nonlinear feedback controller is proposed in this paper to work together with the existing linear controller to deal with external disturbance in hard disk drive (HDD) systems. In the proposed control scheme, the capability of disturbance rejection is boosted when the tracking error of the control system gets larger due to the external disturbances. The stability of the proposed scheme has been proved, and chassis tests on several commercial notebook PC models showed that the proposed method is effective to handle wide-band vibration. Moreover, the implementation of the proposed scheme is very simple and may be even simpler than the linear controller.

Position-Error-Based Shock Protection in HDDs

This paper studies off-track write protection in HDDs. The system data integrity requirement is formulated based on the head/media and data track pitch configurations and a servo-mechanical system track misregistration (TMR) under various conditions. A write fault protection scheme is formulated based on position estimation through the current and predicted future PES (position error signal). The write fault trigger threshold determination and its impact on the system trade-offs are qualitatively discussed. The shock protection method applies to both conventional magnetic recording (CMR) and shingled magnetic recording (SMR).Copyright

A Robust Servo Demodulation Algorithm to Compensate for Head Velocity Induced Error

The fast growing areal density and continuous increasing demand of IOPS and throughput in hard disk drive present great challenges to servo systems on the performance of the track-follow accuracy and seek time. A high bandwidth servo system is required to meet the demand. However, the performance of the servo systems is ultimately limited by the resolution of the position signal which is demodulated form embedded servo sector. Inside the servo sector, the servo track address combining with the servo bursts provide a global position signal of read/write head. The servo track address, which is commonly encoded as gray code, provides coarse position information. Whereas the servo bursts carries the position error signal (PES) to determine the fine position information. There are multiple servo pattern schemes that have been used in hard disk drive, for example, amplitude based servo pattern, phase pattern, and dual frequency pattern [1]. The servo track address and servo bursts span multiple magnetic cycles. The head position is assumed to be unchanged when it travels through the servo sector. The assumption is valid when head is track-following or seeking at low velocity. However, when the head seeks with high velocity, the assumption induces large PE demodulation error, since the head can move as much as a few tracks when it travels through the servo sector. Thereby, it prevents us to use high servo bandwidth for pushing seeking performance. To tackle this problem, we proposed a robust PE demodulation algorithm which can eliminate the head velocity introduced error. The new demodulation algorithm can be generally applied to any servo pattern schemes. The paper is organized as follows. The servo burst modeling and problem formulations are introduced in section I. The velocity based PE demodulation algorithm is presented in section II. The paper concludes with simulation and experiment results.Copyright