Search

Written-in signal quality for heat-assisted magnetic recording (HAMR) is dominated by the hot spot thermal profile on the media. The thermal profile is determined by the head and media design, the laser power, and the near-field transducer (NFT) tip to media spacing (NMS). NMS keeps changing during the writing process due to protrusions generated by the laser-related heat sources, on top of the usual head-to-media spacing change due to thermal fly-height control and writing current. In this paper, data pattern writing process is modeled as a dynamic system consisting of NFT protrusions of different time constants, energy transfer efficiency versus NMS, media heat transient process, and power-to-signal conversion function. The parameters of this model are identified by minimizing the differences between the model output and the measured read-back signal magnitude of an experimental data pattern written on an HAMR drive. The Gaussian-Newton method is used to find the parameters of the non-linear system. Using the identified parameters, our model is able to predict the written-in signal magnitude, given the writing conditions such as laser power, laser current overshoot, and various thermal actuator operating conditions.

Hard disk drives (HDD) have been widely used in video/audio devices; for such applications the acoustic level of HDD is a key performance index. This paper proposes a two-degree-of-freedom (TDOF) seeking controller for minimizing the seeking acoustics. An acoustics factor is included in the reference trajectory generation so that the acoustics level can be adjusted via seek design to make a compromise between the access time and the acoustics level. With the systematic design of seeking controller, the acoustics in HDDs will be greatly reduced without significant increase of the seeking time. The effectiveness of the proposed control scheme is verified by both simulation and experimental studies.

This paper investigates a factorization approach to sensitivity loop shaping for disturbance rejection in hard disk drives (HDDs). The advantage of the factorization approach is that the system sensitivity function can be expressed explicitly in terms of a unique design parameter. This greatly simplifies the control design process to make the system sensitivity function match a chosen target sensitivity function with guaranteed stability. By decomposing the controller structure, the design parameter is revealed to behave as a plug-in disturbance filter, the design of which is then presented to suppress the dominant disturbances at some specific frequencies. It is also shown that based on the nominal control system the proposed disturbance filter can reduce the sensitivity gains at specific frequencies without worsening the neighbouring sensitivity gains. Simulation together with implementation results demonstrate that the proposed method can effectively suppress the disturbances around the servo bandwidth and accordingly offers a superior tracking accuracy in comparison with other existing filters.

This paper proposes a design method of discrete-time optimal reset control with the reset time instants prespecified. With a base linear system designed conventionally, the discrete-time reset law design aims at improving the system transient responses. The design method can guarantee system stability, and the controller is obtained by solving Riccati equations. The proposed design method is applied to short-span and long-span track seeking of a hard disk drive servo system. Experimental results show that the proposed design is much more capable of improving transient response than traditional control design methods.

Narrow-band position error at midfrequencies around the open-loop crossover frequency cannot be effectively reduced by using a conventional peak filter, because the attenuation of sensitivity gains has to be traded off with the associated decrease of phase margin. This paper presents a general second-order filter that can reject narrow-band disturbances at any frequency range. The filter zero is designed to minimally degrade the closed-loop system stability and obtain a smooth sensitivity curve around the disturbance frequency. The paper presents a nonlinear optimization procedure for selecting the filter parameters so that the statistical position error is minimized. Experimental results of a piezoelectrically actuated head positioning control system on a spinstand demonstrate that the add-on filter can further reduce the midfrequency nonrepeatable runout of the position error signal by 8% and preserve the stability margin of the original feedback control system

The dual-actuator hard disk drive with two voice coil motor actuators mounted in the diagonal corners of the baseplate allows faster data access with relatively slow spindle speed. However, the mechanical interaction between the actuators tends to affect the head positioning accuracy. In this paper, we use the circle-fit modal testing method to identify the dual-input dual-output frequency response model of the dual-actuator plant. We discuss the decentralized control scheme for the dual-actuator tracking servo. The servo loop decomposition reveals that the interaction induces narrow-band vibrations near the crossover frequency. We propose a phase-lead interaction filter with a slight change to the loop stability to suppress the vibrations. Experimental results on a dual-actuator prototype show that the position error signal 6/spl sigma/ value due to interaction vibrations is reduced by 61% with the interaction filter in combination with the decentralized control scheme.

The dual-actuator hard disk drive (HDD) with two actuators mounted in the diagonal corners of the baseplate allows faster data access with relatively slower spindle speed. However, the mechanical interaction between the actuators tends to affect the head positioning accuracy. In this paper, the dual-actuator structure is formulated as a dual-input dual-output (DIDO) system and then identified in frequency domain. Based on the interaction models, the feedforward decoupling control (FDC) is developed to compensate for the vibration due to interaction. Experimental results on a dual-actuator prototype demonstrate that the position error signal (PES) value due to interaction is reduced by 72% even under the severest condition.

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.