Position Error Signal Research Articles

Adaptive Regulation of Time Varying Disturbances in a Tape Storage System

Archival demands for digital tape storage systems require smaller spacing between data tracks on the magnetic tape to allow for more data storage on a single cartridge. Increasing track density while maintaining track following provides a challenge for the servo control of the tape drive. In particular, interchangeable tape cartridges and time-varying tape reel diameters during operation cause variable disturbances during servo control. In this paper we show how a tape servo control algorithm can be regulated to handle changes in disturbance dynamics during servo operation of the tape drive. The approach is based on adaptive regulation by exploiting a Youla-Kucera parameterization of the servo controller and an explicit minimization of the Position Error Signal (PES) during servo operation in real-time. Theoretical results on the stability of the feedback system and real-time application results on an Linear Tape-Open (LTO) drive are included in the paper. The result show a significant reduction of the variance of the PES over different tape cartridges and a constant PES variance during a complete reel-in operation of the tape drive.

Limits of Performance in Systems with Periodic Irregular Sampling and Actuation Rates

This paper examines the limits of performance in systems with periodic irregular sampling rate when the actuation is not necessarily synchronized with the sampling. For such a system, three sampling and actuation schemes are considered: when the sampling and control rate are both regular, when they are both irregular, and when the sampling rate is irregular while the control rate is regular. To ascertain the limits of performance of this type of systems under each sampling and actuation scheme, the system is modeled as a linear periodically time-varying (LPTV) system; optimal LQG control design with a variance constraint is applied to find the smallest achievable mean variance of the performance signal subject to a constraint on the mean control effort variance. In addition, to deal with the computational delay of the controller, an innovative discretization method is proposed which does not introduce any extra states into the state space model. The proposed method is exploited to determine the performance of a hard disk drive (HDD) in track-following mode. A simulation study demonstrates that in the presence of 30% irregularity in sampling time, using the irregular sampling and regular control action scheme for the HDD achieves an RMS 3s value of the position error signal (PES) that is 40% smaller than the corresponding value achieved by using a controller provided by our industry partner, in which both the sampling and control rates are irregular.

An Improved Digital Servo Burst Signal Sampling Model for Disk Drives

In order to calculate the servo position error signal (PES) in the servo system, it is necessary to make high precision sampling and FFT calculation for the burst signal. Most designer s adopt 8bits ADC converter in the read write channel servo system of hard drive controller chip 88i6310 to cost down, and both servo system and signal system share the sampling circuit. Because of the quantitative noise disturbing during the sampling process, it is not enough to use 6bits ADC converter for the serve burst signal, and significant error exist for t he burst signal with the FFT calculation. This paper analyzed the existing servo over-sampling model via quantitative error model firstly, and provided the enhanced servo system sampling model. The simulation shows that, the enhanced solution reduces the average quantitative error from 0. 38 LSB to 0.14LSB, and improves the resolution of ADC converter from 9.5 bits to 10.8 bits.

Analytical Expressions for the Readback Signal of Timing-Based Servo Schemes

Timing-based servo (TBS) is a powerful technology widely used in state-of-the-art tape systems for track following. To optimize TBS systems for future operating points with aggressive track-density scaling, an accurate model is needed to characterize how the servo read head geometry, servo format parameters, and magnetic media properties affect the readback signal. In this paper, we derive new analytical closed-form expressions for the readback signal of shielded magneto-resistive (MR) heads scanning over the magnetic transitions of TBS patterns. The expressions, derived via the reciprocity principle, are applicable to arctan-shaped transitions between arbitrarily magnetized areas of a thin-film magnetic medium and are given in the displacement and wavevector domain. We apply these new expressions to analyze isolated transitions as well as multi-transition TBS patterns on longitudinal, non-oriented or perpendicular media. Comparison between experimentally captured servo readback signals and the analytical expressions show an excellent match. By adding electronics and transition jitter noise, the readback signal expressions can be extended to a servo channel model suitable for TBS performance prediction by means of Monte Carlo simulations and/or bounds on the variance of the position-error signal.

Shock and vibration isolation of laptop hard disk drive using rubber mount

Hard disk drives (HDDs) in laptop personal computers (PCs) are subject to impact-induced failure, as well as to operational vibrations. Elastic mounts with cushioning materials are often used to minimize the likelihood of failure in such cases. In this paper, we analyze the dynamic characteristics and shock response of the vibration isolation systems supporting HDDs. Anti-vibration performance is investigated via position error signal and acceleration transmissibility. Shock response is obtained from a lumped parameter model, based on the nonlinearity of the rubber mounts. Based on the results, we propose guidelines for shock and vibration isolation systems, including a dual rubber mount design.

Topology optimization of actuator arms in hard disk drives for reducing bending resonance-induced off-tracks

The objective of this work is to find an optimal actuator arm configuration in hard disk drives (HDD's) for sufficiently small arm bending-induced off-track error by maximizing the fundamental eigenfrequency of arm bending modes subject to a constraint on the mass moment of inertia of arms. By applying the topology optimization method for the purpose, an arm configuration having two balancing holes instead of a single balancing hole in conventional designs was obtained. It is numerically shown that an optimized arm configuration substantially increases the arm bending resonant frequency with little change in the mass moment of inertia of arms in comparison with conventional designs having a single balancing hole. Finally, the performance of an optimized actuator arm is verified by showing that the eigenfrequencies associated with arm bending modes are increased by about 100 Hz and the off-track error by measuring the position error signal (PES) from actual sample drives can be reduced by as much as 25%.

Adaptive digital demodulation of sinusoidal encoder signal for positioning control of spiral servo track writing

An adaptive digital demodulation method for a sinusoidal encoder signal is proposed for fast and accurate analysis of spiral servo track writing (SSTW). The proposed methods combine adaptive signal correction and digital interpolation. In addition, digital phase-shifted sinusoid (PSS) interpolation is proposed in order to acquire precise position information. The adaptive signal correction method compensates for a fast-motion non-ideal encoder signal. The digital PSS can be simply implemented and can resolve the switching noise of conventional analog methods. The proposed adaptive digital demodulation scheme can be simply digitalized and can improve the position control in SSTW. From the simulation and experimental results, we have validated the proposed methods, which show noticeable improvement in terms of the Lissajous error and the position error signal.

Servo-Pattern Design and Track-Following Control for Nanometer Head Positioning on Flexible Tape Media

Achieving multi-Terabyte capacity in tape cartridges requires a substantially higher track density than that available in present systems, and hence a significantly higher positioning accuracy is required of the track-following servo in tape drives. In this paper, advanced concepts are considered for several elements of a tape system that enhance the track-following servo performance to reach nanometer positioning accuracy. We introduce a novel method for optimizing the geometry of servo patterns in a timing-based servo system. The design criterion aims to minimize the measurement error in the position-error signal (PES) yielded by a digital synchronous servo channel. A flangeless tape path is adopted to mitigate high-frequency components of the lateral tape motion. The track-following servo controller, which is designed based on the H∞ approach, takes into account the measured plant transfer function, the disturbance characteristics of the tape path, and the properties of servo channel. These elements are combined to investigate the track-following performance achievable with a new high-SNR magnetic tape based on perpendicularly-oriented BaFe particles. With this setup, a record closed-loop track-following performance of less than 14 nm PES standard deviation is demonstrated.

A Minimum Parameter Adaptive Approach for Rejecting Multiple Narrow-Band Disturbances With Application to Hard Disk Drives

Many servo systems are subjected to narrow-band disturbances that generate vibrations at multiple frequencies. One example is the track-following control in a hard disk drive (HDD) system, where the airflow-excited disk and actuator vibrations introduce strong and uncertain spectral peaks to the position error signal. Such narrow-band vibrations differ in each product and can appear at frequencies above the bandwidth of the control system. We present a feedback control scheme that adaptively enhances the servo performance at multiple unknown frequencies, while maintaining the baseline servo loop shape. A minimum parameter model of the disturbance is first introduced, followed by the construction of a novel adaptive multiple narrow-band disturbance observer for selective disturbance cancellation. Evaluation of the proposed algorithm is performed on a simulated HDD benchmark problem.

Optimal $H_{\infty}$ Control Design and Implementation of Hard Disk Drives With Irregular Sampling Rates

Missing position error signal sampling data in hard disk drives (HDDs) results in their servo systems having irregular sampling rates (ISRs). With the natural periodicity of HDDs, which is related to the disk rotation, HDD servo systems with ISRs can be modeled as linear periodically time-varying systems. Based on our previous results, an explicit minimum entropy <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> controller for HDD servos with ISRs via discrete Riccati equations is first obtained. The resulting controller is subsequently demonstrated to be periodically time-varying and implementable. Simulation and experimental studies, which have been carried out on a set of hard disk drives, demonstrate that the proposed control synthesis technique is able to handle ISRs and can be used to conveniently design a loop-shaping track-following servo that achieves the robust performance of a desired error rejection function for disturbance attenuation. Experimental results on ten actual 2.5" hard disk drives show around 27% improvement of the 3σ PES was obtained by the proposed control algorithm.

Fast servo signal acquisition in tape drives using servo and data channels

In a tape drive, the head assembly hosting servo readers and data writers/readers must initially be moved to a target position, so that information can be written to or read from the desired tracks. In tape systems with flangeless rollers, as recently introduced in IBM’s tape drives for improved performance and extended drive and tape lifetime, the initial positioning of the servo readers over the servo bands, from which the position-error signal for steady-state track-following operation is obtained, has to be fast. This requirement is dictated by the nonnegligible lateral tape motion experienced in tape paths with flangeless rollers. In this paper, a method is proposed to utilize both the servo readers and the data readers in a drive to detect the presence of a valid servo signal. If the servo readers are not positioned over a servo band, the distance between servo readers and servo band can thereby be continuously estimated by identifying the data reader detecting the presence of a servo signal. This information is then provided to a control element that controls the joint operation of a coarse stepper motor and a voice-coil motor based fine actuator with large stroke in a feedback or feed-forward configuration to achieve fast servo reader positioning. In the feedback-based approach, all data channels are continuously monitored to detect the presence of a valid servo signal and a pseudo position-error signal is generated and fed back to the control unit to drive the actuator towards the target position. In the feed-forward approach, a trajectory for the actuator is determined as soon as the servo band is in the capture range of the fine actuator and a control signal is generated so that the servo reader rapidly lands over the servo band with optimum control effort.

Aerodynamic Pressure Fluctuations Associated With Flow-Induced Vibration of the Head Gimbals Assembly Inside a Hard Disk Drive

This paper presents an experimental investigation and numerical simulation on the spectrum-spatial characteristics of aerodynamic pressure fluctuations around the head gimbals assembly (HGA) in a working hard disk drive (HDD). The pressure fluctuations are measured through tiny holes on the HDD top cover above the HGA. The positioning error signal and servo system sensitivity in the working HDD are measured as well to evaluate the slider off-track vibration. Comparisons between the spectra of pressure fluctuations and the slider off-track vibration show that, the pressure fluctuations around the HGA are highly associated with the slider off-track vibration in terms of principal peaks in the spectra, especially at 1.83 kHz and 2.54 kHz. The results also show that the spatial coherence of pressure fluctuation around the HGA remains high at frequencies around the principal peaks in the spectra, which has been further confirmed by the numerical simulation based on a two-dimensional large eddy simulation model. It is concluded that the flow-induced HGA vibrations in an HDD can be detected and evaluated through the pressure fluctuations by a sensor around the HGA.

Add-on type loop shaping controller design for high precision track following in hard disc drives (HDDs)

In this study, the authors present a systematic design method of the add-on type loop shaping scheme for high precision track following with plant modelling perturbations in HDDs. The proposed controller minimises the standard deviation of the position error signal (PES) of certain closed-loop transfer function while satisfying the robust stability condition in HDDs. Compared with the conventional H ∞ control, the proposed method yields estimated plant states, which can be used for estimator-based controller design. Also, the proposed control method can be implemented in a parallel manner by performing a partial-fraction expansion for easy implementation. The effectiveness and practicality of our work is demonstrated through various experimental results.

Probabilistic Robust Approach for Discrete Multi-objective Control of Track-Following Servo Systems in Hard Disk Drives

This paper deals with the problem of different uncertainties in discrete time track following control of read/write head in hard disk drives (HDD). A multi-objective robust controller is designed which minimizes the worst case root mean square (RMS) value of the positioning error signal (PES) subject to the closed-loop stability in the presence of parametric and dynamic uncertainties. A sequential algorithm based on ellipsoid iteration is utilized to handle parametric uncertainty. Dynamic uncertainty is also represented as linear fractional transformation (LFT) and by the virtue of small gain theorem, the stability of closed-loop system is guaranteed. In this design, two sources of conservatism are avoided: embedding the original non-linear parametric uncertainty into affine structure (converting the original uncertain system into a polytopic uncertain system) and using a single Lyapunov matrix to test all the objectives. The resulting controller has much better track following performance compared to the classical robust approaches which tends to higher storage capacity of HDDs. Simulation as well as experimental results verify the effectiveness of the designed controller.

Thermal Analysis and Design of Disk-Spindle Radial Repeatable Runout in Spinning Data Storage Devices

This paper presents structure thermal mechanics analysis of spindle disk assembly used in spinning magnetic data storage hard disk drives (HDDs). With the view toward understanding the underlying physics and to minimize the corresponding repeatable run-out (RRO) of track following position error signal (PES) in high track per inch (TPI) magnetic disk drives, analytical representation of thermal expansion mismatch between disk and spindle hub structure formulated in form of operators and finite element analyses (FEA) are employed in this paper. Parameter studies with analyses taken at different operational temperatures suggest that RRO can be minimized significantly when location of spindle notch, which is can be the internal feature used to position bearings to the hub structure, is properly located. RRO harmonics resulted from the thermal expansion mismatch and structure misalignments having ramping characteristics from disk outer diameter to inner diameter are studied and concluded with simple algebraic expression related to number of fasteners used in disk-spindle assembly.

Low Frequency Vibration Detection and Compensation in Hard Disk Drive

The paper proposes a control scheme for detecting and compensating the low-frequency vibrations in hard disk drive systems. A vibration detector is proposed, utilizing the down-sampled position error signal (PES). Due to the performance limitation in loop shaping, attenuating low-frequency vibration is usually penalized by high frequency disturbance amplification. This paper proposes an adaptive scheme to intelligently adjust a vibration reduction factor, such that when vibration level is low, the servo loop maintains its normal operation mode, and little or no compensation is carried out. Experiments on hard disk drives on a vibration shaker validated the effectiveness of the proposed compensation scheme.

Model Prediction Based Dual-Stage Actuator Control in Discrete-Time Domain

This paper presents a voice coil motor (VCM) prediction based dual-stage actuator (DSA) control for fast seek and settling times. In the conventional control systems, VCM follows a reference signal, whereas a micro-actuator (MA) seeks a VCM position error signal (VPES) defined by an error between the reference signal and an estimated VCM position. In other words, VCM firstly seeks the reference signal and MA reduces the residual error (equivalently, VPES). In the conventional method, the calculated VPES was utilized in the next sampling period. But, the VPES is not the same as the VPES to be actually compensated because the VPES was calculated under the assumption that VCM doesn't move. And, the error made seek and settle times slow in transient responses. In order to reduce the error, this paper suggests a new control scheme on the basis of VCM model prediction. From the test results, it is verified that the seek and settling times are obviously improved. Moreover, it is shown that frequency responses are also advanced through a maximum peak criteria.

High Frequency Radial Mode Vibration in Hard Disk Drive

In this paper, a 3.5-in computer hard disk drive consisting of a rotating-shaft fluid dynamic bearing spindle motor and 4-disk spinning at 15 000 rpm under external linear vibration is analyzed by commercial finite element analysis software. Predicted frequency response functions of the head-disk off-track motion per unit input acceleration are compared to measured hard disk drive position-error signals on a shaker table. Both theoretical and experimental results show a huge disk pack radial motion near 1600 Hz under linear in-plane vibration. The mode shape analysis indicates that the large radial motion without noticeable disk bending is mainly due to the bending of the motor bracket and shaft. The characteristics of the high frequency radial mode are discussed with the variation of fluid dynamic bearing parameters with the verification of experimental results.

Adaptive Neural Network Control of Hard Disk Drives With Hysteresis Friction Nonlinearity

In this brief, an adaptive neural network (NN) friction compensator is presented for servo control of hard disk drives (HDDs). The existence of the hysteresis friction nonlinearity from pivot bearing, which is represented as the LuGre hysteresis friction model here, increases the position error signal of read-write head and deteriorates the performance of HDD servo systems. To compensate for the effect of the hysteresis friction nonlinearity, NN is adopted to approximate its unknown bounding function. With the proposed control, all the closed-loop signals are ensured to be bounded while the tracking error converges into a neighborhood of zero. Comprehensive comparisons between the conventional proportional-integral-derivative control (without friction compensator) and the proposed adaptive NN control (with friction compensator) are provided in experiment results. It is shown that the proposed control can mitigate the effect of the hysteresis friction nonlinearity and improve the track seeking performance.

Linear protrusion structure on carriage-arm surface for reduction of flow-induced carriage vibration in hard disk drives

One of the major contributors to head positioning errors is carriage vibration in low frequency due to an air flow caused by disk rotation. It is necessary to suppress the disturbance for Hard Disk Drives (HDDs) to have more capacity. We experimentally studied a reduction in the flow-induced carriage vibration using linear protrusion structures by putting wires on carriage arms. This study was carried out using 2.5 inch HDD which has high rotational speed of 10,000 rpm. We measured position error signals (PES) and compared with a conventional carriage. From the experimental results, we found that the linear protrusion structure was effective to reduce the carriage vibration. Leading edge wire configuration and 2 linear protrusion configuration improved average non-repeatable position errors (NRPE) by 6.9% and 6.4%, respectively.