Feedforward Control Using Sampled-Data Polynomial for Track Seeking in Hard Disk Drives

To decrease the seek time of hard disk drives, a feedforward control method was developed by using a sampled-data polynomial. The sampled-data polynomial satisfies the boundary conditions that include the characteristics of the zero-order hold, and compensates for the discretization error caused by the zero-order hold without the need for complicated calculations. Therefore, the feedforward control using the sampled-data polynomial enables real-time calculation and does not require lookup tables which need a large amount of memory. The parameters of the sampled-data polynomial are designed by using shock-response-spectrum analysis to minimize the settling vibrations caused by the feedforward control inputs. When the proposed method was applied on a hard disk drive, it significantly reduced the amount of tracking error in the seek control and also reduced the seek time.     

Minimization of unobservable oscillation in repeatable run-out for magnetic-head positioning system of hard disk drives

A positioning control system for a magnetic head in hard disk drives (HDDs) is a sampled-data control system that has a sampler and a hold. As a result, the control system has unobservable oscillations in a repeatable run-out (RROs) whose frequencies are above the Nyquist frequency. These unobservable oscillations may lead to destruction of user data in HDDs. To solve this problem, this paper presents a minimization method for the unobservable oscillation in the RROs. This minimization method focuses on a relationship between RRO frequencies and a sampling frequency of the sampled-data control system. To estimate amounts of the unobservable RROs beyond the Nyquist frequency, this study employs two types of unobservable magnitudes: a maximum value based magnitude, and a root-mean-square value based magnitude. By using these unobservable magnitudes, we can choose the best suited sampling frequency to minimize the unobservable oscillations in RROs of HDDs.     

Head-positioning control using resonant modes in hard disk drives

The best way to enhance the input/output (I/O) performance of a hard disk drive is by increasing the spindle speed. Therefore, the effect of windage vibrations caused by the airflow increases as the spindle speed increases. The servo bandwidth is limited by the primary resonant frequency of the mechanical system. However, the frequencies of the windage vibrations are higher than the primary resonant frequency. Accordingly, these frequencies are also above the servo bandwidth and are too high to be controlled by a conventional control system. In response to this problem, we have developed two methods for designing a servo control system that can suppress the windage vibrations. One method uses a stable mechanical resonant mode, and the other uses a stable resonant mode created by a digital filter. By using these methods, the head-positioning system can control the vibrations above the frequency of the primary resonant mode and the servo bandwidth. Application of these methods to actual hard disk drives showed that they can greatly decrease the windage vibrations, in which the peak frequency is about six times the open-loop gain 0-dB crossover frequency.     

Disturbance suppression beyond Nyquist frequency in hard disk drives

In conventional hard disk drives, a control system compensates for vibration in which the frequency is higher than the Nyquist frequency by using a multi-rate filter that decreases the gain above the Nyquist frequency. However, such a control system can only avoid instability and cannot suppress disturbances above the Nyquist frequency. To overcome this problem, a control system design method that suppresses disturbances beyond the Nyquist frequency is proposed. This method uses frequency responses of a controlled object and a digital controller to calculate the gain of the sensitivity function in a sampled-data system without requiring complex calculations involving matrices, and realizes a stable resonant filter that decreases the gain of the sensitivity function above the Nyquist frequency. Significant suppression of the vibrations caused by the disturbances beyond the Nyquist frequency is demonstrated by implementing this method in the head-positioning system of a hard disk drive.     

Trajectory design considering derivative of jerk for head-positioning of disk drive system with mechanical vibration

In this paper, we propose a novel design method of target trajectory for high-speed and high-precision head positioning of a hard disk drive (HDD) system. To realize smooth acceleration and deceleration, the derivative of jerk is considered not to activate any mechanical vibrations. We applied the well-known optimal control theory to fix the initial and terminal conditions. Moreover, we show that various performances can be improved by introducing time-varying weighting coefficients. Some experimental results using a 2.5-in HDD are shown to verify the effectiveness of the proposed method.     

Seek Control to Suppress Vibrations of Hard Disk Drives Using Adaptive Filtering

A method to generate an optimal seek trajectory for a hard disk drive using an adaptive filtering technique is presented in this paper. The method minimizes seek-settling vibrations induced by the actuator vibration modes while maintaining the maximum control input. The cost function to minimize is a sum of square error of the position error signal at settling. Since the error equation is quadratic, a simple least-square algorithm can generate an optimal trajectory and the global optimum is guaranteed. The seek time overhead compared to bang-bang control is characterized by the order of the resulting finite-impulse-response (FIR) filter. The resulting performance is demonstrated by both benchmark simulations and experiments using a hard disk drive.     

Modeling and experimental analysis of the vibrations in hard diskdrives

A combined methodology, based on dynamic modeling and experimental measures, is used to study the vibrations in hard disk drives, in a frequency range up to 1 kHz. A lumped parameter model of the disks/spindle assembly is developed, where only the disk modes coupled with the spindle motion are retained. The proposed model has been validated by comparison with experimental results. Using the model, the sources of axial and radial vibrations measured in a single plate commercial hard disk drive have been explained. Among them are the disk structural resonances, due to compliances in bearings and disks, displacements of the center of mass from the rotation axis and unbalanced magnetic forces. On the other hand, no evidence has been found of the influence of torque disturbances on the measured vibrations     

Learning of Hybrid Fuzzy Controller for the Optical Data Storage Device

A hybrid track-seeking fuzzy controller for an optical disk drive (ODD) is proposed in this paper. The proposed hybrid fuzzy controller (HFC) smoothes the voltage applied to the sled motor and improves the track-seeking efficiency. The HFC consists of two subsystems including an intelligent time switch and a driving force controller. Both subsystems are designed based on fuzzy logic inferences. The main functions of the proposed HFC are to drive the optical head unit (OHU) to the target track neighborhood as fast as possible and smoothly park the OHU in the least time in the target track neighborhood. An automatic learning approach based on genetic algorithms (GAs) is proposed for learning the fuzzy rules for both the intelligent time switch and driving force controller. Modulated orthogonal membership functions are utilized in both fuzzy controllers to improve the GA learning efficiency. The number of parameters needed to parameterize the fuzzy rule base is greatly reduced with the modulated orthogonal membership functions. Compared to the conventional track-seeking controller currently utilized in most ODDs that employ a speed profile as the reference signal for the track-seeking feedback control system, the proposed HFC outperforms the conventional track-seeking control schemes. Experiments are performed to justify the performance comparison.     

Design and analysis of a high bandwidth disk drive servo systemusing an instrumented suspension

Resonance modes in the suspension of hard disk drives limit the closed-loop bandwidth. The bandwidth of the servo can be increased by active vibration control of the resonance modes. This paper considers the optimal placement of strain gauge sensors on a suspension to observe the vibration states of the suspension. Using a finite-element simulation of an actual suspension, a state-space model is identified for the two normal strains and the shear strain at each finite element. The state-space model includes the dynamics of the three primary resonance modes. A numerical search algorithm is used to determine the sensor location and orientation which maximizes the minimum singular value of the observability grammian. With the strain gauge output signal, a multirate inner loop controller is designed to be used with the existing head-positioning system. Simulations and analysis results suggest that use of an instrumented suspension is a viable candidate method for improved disk drive servo performance     

Nonlinear Tracking Control for a Hard Disk Drive Dual-Stage Actuator System

This paper presents a nonlinear tracking control method for a hard disk drive dual-stage actuator (DSA) system that consists of a voice coil motor (VCM) actuator and a piezoelectric (PZT) microactuator. Conventional track seeking controllers for DSA systems were generally designed to enable the VCM actuator to approach the target track without overshoot. However, we observe that this strategy is unable to achieve the minimal settling time when the target tracks are beyond the PZT actuator stroke limit. To further reduce the settling time, we design the VCM actuator controller to yield a closed-loop system with a small damping ratio for a fast rise time and certain allowable overshoot. Then, a composite nonlinear control law is designed for the PZT actuator to reduce the overshoot caused by the VCM actuator as the system output approaches the target track. Experimental results show that the proposed dual-stage servo outperforms the conventional dual-stage servo in short-span seeking and, additionally, achieves better track following accuracy than the VCM only single-stage servo.     

Observability criteria for winding currents in three-phasebrushless DC drives

A rigorous method for applying observability criteria to the estimation of winding currents in brushless DC drives is presented. Observability is related to the drive's operating condition, both motoring and regenerating, and to the pulse-width modulated (PWM) switching strategy being employed. The precise conditions under which motor winding currents can be estimated from measurements of DC link current are clearly defined. A modified single current sensor arrangement, in which switching device currents are measured, is shown to improve the observability of currents with complementary switching, but to have an adverse effect on observability with the basic switching strategy. An arrangement involving two current sensors allows discontinuous estimation of the winding currents for all operating conditions and both switching strategies. The primary advantage of this technique is that measurements by each sensor are made equally on all three phases of the motor     

Settling control and performance of a dual-actuator system for hard disk drives

This paper presents a settling control of a dual-actuator system for hard disk drives. The dual-actuator system consists of a voice coil motor (VCM) as a first stage actuator and a push-pull-type piezo-electric transducer (PZT) as a second-stage actuator. The settling controller is designed in three steps. In the first step, the VCM controller is designed so that the VCM feedback loop has basic performance and appropriate stability. In the second step, the PZT controller and a decoupling filter are designed in order to achieve superior performance of the dual-actuator system. The decoupling filter, which is placed between the PZT controller output and the VCM controller input, is a PZT output estimator so that the PZT actuator output is canceled at the VCM controller input. In the third step, the reference trajectory is designed for fast and smooth settling. In this study, the closed-loop sensitivity function is used as a performance index, and the gain and phase margins of the open-loop characteristic are used as stability measures. Experimental results show that the dual actuator system with the proposed settling controller achieves better performance than a single actuator system with the same VCM and a conventional settling controller.     

光纤陀螺随钻测斜仪钻进中井眼轨迹测量方法研究

为提高光纤陀螺随钻测斜仪的井眼轨迹的测量精度,采用卡尔曼滤波组合的测量方法,对井斜、方位和工具面失准角进行估计。由于姿态失准角的估计精度与其可观测性密切相关,为了提高估计精度,文章分别在匀速、匀加速和匀速转动三种钻进运动状态下,采用分段线性定常系统(PWCS)和奇异值分解(SVD)的方法分析了姿态失准角的可观测性,并分析了钻进中井斜角和转动速率对姿态失准角估计精度的影响。由仿真结果可知,匀速钻进时,方位失准角不可观测;匀加速钻进时,方位失准角可观测且在水平井中可观测性最强,随着井斜角度增大,方位角、井斜角的测量精度逐渐提高;绕轴向匀速转动钻进时,方位失准角的可观测性和估计精度均优于匀速、匀加速钻进状态;角速率由0°/s增加到5°/s时,三个姿态失准角的估计精度均增大并逐渐趋于稳定。文中提出的轴向转动钻进运动可有效提高井眼轨迹的测量精度。     

A mechanical sensorless control system for salient-pole brushlessDC motor with autocalibration of estimated position angles

In this paper, a mechanical sensorless control system is reported for salient-pole brushless DC motor drives. Here, two new methods are proposed for obtaining the position angle, the accuracy of which affects the operation of the switching devices of the inverter that drives the motor. First, the method for estimating the position angle is proposed. Secondly, the correcting method for reducing the errors involved in the estimation of position angle is given. The experimental results show that the estimated position angles are calibrated automatically, and then the proposed sensorless control system can control the speed and the position angles of the motor precisely     

Two-degree-of-freedom control with robust feedback control for harddisk servo systems

A two-degree-of freedom (2 DOF) control structure is proposed for read/write head servo systems of hard disk drives (HDDs). This structure is applied to both track seeking and track following, and makes mode switches found in conventional HDD servo systems unnecessary. Two innovative features introduced in the paper are: 1) a new method for generating the reference signal for track seeking and 2) two robust feedback control schemes for rejection of disturbances; one scheme uses a disturbance observer (DOB), and the other uses adaptive robust control (ARC). Simulation results show that the 2 DOF structure with ARC provides better performance than either the conventional servo system with mode switches or the 2 DOF structure with DOB. Experimental results show the advantage of the new reference generation method and the use of the DOB-based robust controller     

网络存储系统容错编码技术进展

专业的大型磁盘存储系统均发展为包含多块磁盘的大型阵列系统。随着系统中的磁盘数目的不断增加,由磁盘失效引起的数据丢失的可能性越来越大。对于由存储系统中部分磁盘失效所引起的数据丢失的问题,目前业界公认的比较好的解决方案是使用冗余容错编码技术来实现磁盘的容错。在工程实践中,目前广泛应用的编码方法大多局限于双容错阵列码。随着系统规模的进一步加大,3容错甚至更多容错的编码方法已引起研究者的重视。今后的5至10年间,对于3容错或多容错的编码方法的研究将会成为新的热点。     

Compensation for repeatable tracking errors in hard drives usingdiscrete-time repetitive controllers

Presents the repeatable runout (RRO) compensation in hard disk drives (HDDs). To cancel the RRO, we applied repetitive controllers to the tracking system. Through experiments, we obtained a high reduction ratio of from 10 to 45 dB for up to 10th harmonics component of the RRO. Furthermore, by switching filter parameters between the learning state and the steady state, we obtained fast learning of less than two disk rotations     

磁光阵列:一种新型数据存储模式的实现

介绍了一种磁盘与光盘融合数据安全存储的新模式,简称磁光阵列或ORAID（Optical Replicated Arrays of Independent Disks）。ORAID模式包括:将数据同时存储到磁盘模块和光盘模块中;可根据数据的属性信息从磁盘模块中读取数据,在目标磁盘无法被读取时,可根据数据在光盘模块中对应的位置信息,将存储有目标数据的光盘中的数据读入预设的数据缓冲区;将存储有目标磁盘数据的光盘中的数据存储到磁盘模块中更换后的新磁盘中。该模式在使用上像基于硬盘的系统一样方便,读写性能与RAID（Redundant Arrays of Independent Disks）一样;在数据安全方面,无论前端硬盘如何损坏数据都不会丢失,并且在硬盘坏损以及重建过程中保持数据的可提供性;ORAID具有物理层面上的合规性,所存的数据不可更改（WORM,Write Once Read Only）并可在无迁移的情况下长期保存50年以上;ORAID无须额外的备份系统。磁光阵列模式同样适用于固态硬盘与光盘的融合。     

Real-time modeling of power networks

The use of large digital computers in control centers has made it possible to track the changing conditions in the power system with a mathematical model in the computer. This real-time model can be used to assess the security of the present system as well as to check out possible control strategies. In this paper the various steps in constructing the model from the real-time measurements are described. These steps include the determination of the network topology, the estimation of the network state, and the approximate modeling of the unobservable (external) network. This paper also discusses the checks for observability and bad measurements, and the calculation of bus load forecast factors and generator penalty factors.     

RRO Compensation of Hard Disk Drives With Multirate Repetitive Perfect Tracking Control

In this paper, novel repetitive controllers are proposed based on perfect tracking control (PTC) in order to reject high-order repeatable runout (RRO) of hard disk drives. First, the feedback approach of the repetitive PTC (RPTC) is developed with internal model of periodic disturbance. Although this method has performance robustness against small plant variation, the internal model worsens the stability robustness for big modeling error. Then, the feedforward (FF) approach of RPTC is introduced with switching mechanism such that the high-order RRO can be rejected without any sacrifice of the closed-loop characteristics. In both approaches, multirate FF control is utilized to overcome the unstable zero problem of discrete-time plant. Finally, the advantages and disadvantages are demonstrated through simulations and experiments.