Servo Position Error Signal Research Articles

Spin Stand Study of the Longitudinal Field Effect on Servo PES

In a perpendicular recording environment, when reading at an offtrack position, the head can experience a local field in the longitudinal direction from the written track. This field can change head sensitivity depending on the polarity of the written magnet and the offtrack direction. This effect is particularly strong when reading null servo position error signal (PES) pattern, where two adjacent servo tracks are written with opposite polarity. This paper studies the effect of this longitudinal field on the PES signal using a spin stand, which permits multiple PES patterns to be evaluated side-by-side. The result shows that the effect can distort the PES linearity. The extent of its impact depends on the servo PES transition density and head design. As a result, design compromise must be made.

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.

Lithographically patterned servo position error signal patterns in perpendicular disks

For discrete track media to become a viable alternative, it is essential to produce accurate servo patterns in a cost effective manner. This study presents a spin stand analysis and comparison of position error signals generated from various lithographically defined servo patterns and servo bursts written on continuous regions of the same perpendicular recording media. It is demonstrated that the edge variation on the patterned servo elements is much less than that found on conventional servo bursts. In addition, evidence shows that despite some amplitude loss resulting from the removal of magnetic material, patterned servo bursts produce good quality position error signals when compared to servo bursts written on continuous media.

Characterization of lateral tape motion and disturbances in the servo position error signal of a linear tape drive

Lateral tape displacement in a tape drive is believed to be one of the critical limiting factors in achieving high track density in tape storage media. For improving track density, servo control systems can be designed on the basis of information from a position error signal to improve the track following capabilities in a tape drive. In this paper, we consider the characterization of lateral tape motion and additional disturbances that contribute to the position error signal. From a commercially available tape drive, the position error signal is measured electronically and lateral tape motion of the magnetic tape is measured using an optical edge sensor. This information is used to characterize the filtering effects of the disturbance rejection transfer function that determines the lateral tape motion and disturbances present in the position error signal. The disturbance rejection transfer function is characterized by two different methods. In the first method, we model the disturbance rejection transfer function as a simple high pass filter. Using lateral motion measurements as an input to this system, the position error signal is simulated and compared with the measured position error signal. In the second method, system identification methods are used to estimate a detailed high order filter that models the essential components of the disturbance rejection transfer function. Using these models, the position error signal is simulated and predicted and the effects of additional disturbances such as the servo track variability can be separated from the effects of the lateral tape motion. It is shown that the position error signal exhibits contributions from lateral tape motion and additional disturbances that differ significantly at higher frequencies.

Adaptive compensation for position error signal nonlinearity

The magnetoresistive (MR) magnetic head is a poor positioning transducer for a disk file's servo control system because its positioning response is nonlinear with radial displacement. This paper shows how the MR head's poor positioning properties are alleviated by a self-adjusting adaptive algorithm that allows a disk file to linearize its own servo position error signal (PES). The adaptive linearizer uses a nonlinear state estimator whose nonlinearity adjusts to match the nonlinearity of the PES. As the match between the two nonlinearities adaptively improves, the state estimator gives increasingly accurate estimates of the true actuator position.

A closed-form MR tape head PES model

A closed-form phenomenological expression for the convolution of a MR head microtrack profile with a written track profile is derived. Trapezoid shapes with unequal skirt widths are used to approximate both the MR head microtrack profile and the written track profile. The crosstrack profile model result is compared to a measured profile and is shown to accurately reproduce its shape. A servo Position Error Signal (PES) model then is constructed from the crosstrack profile expression. Deviations from PES linearity and gain variations due to MR element microtrack shape variations are studied using the PES model.

Simulation of servo position error signal in perpendicular recording

We describe simulations using a medium model with random grain structure, to study the servo position error signal (PES) in a perpendicular recording system. Under various detection schemes, values of the PES null position jitter and the PES sensitivity variation at the null point are calculated. To evaluate the effect of medium noise on the PES detection, the values are compared to those of media with different average grain in-plane areas and grain size distributions. For both the PES null position jitter and the PES sensitivity variation at the null point, decreasing the average grain in-plane area or the grain size distribution yields smaller fluctuation. The servo PES has a strong dependence on the average-medium grain in-plane area. When PES of longitudinal recording systems is compared with that of perpendicular systems, perpendicular recording yields much better results because of a sharp write-field gradient at the track edge.

Effect of GMR amplitude asymmetry at track edge on servo position error signal

The effect of track-edge amplitude asymmetry on position-error signal, PES, of spin-valve GMR heads was investigated. Cross-track profiles of both amplitude and amplitude asymmetry are showed along with the PES line of the head. The results indicated that amplitude-asymmetry effect from servo burst is finite but not significant enough to alter PES sensitivity, or the slope of PES line. However, it is shown experimentally that the PES linearity is functionally correlated with asymmetry of the servo bursts, which substantiates the track-edge effect of spin-valve heads.

Simulation of the effect of medium noise on servo position error signal detection in longitudinal recording

A medium model with random grain structure and desired grain size distribution is used to study the effect of medium noise on position error signal (PES) signal and sensitivity using 100 Gb/in/sup 2/ parameters in longitudinal media. Three PES detection schemes are compared: (1) average peak voltage; (2) integration of the absolute playback signal; and (3) maximization of the integration of the product of a phase-adjusted square wave with the playback signal. Scheme III is found to have the best output PES sensitivity for every case. The PES, PES sensitivity, and null point position of the RES signal along with their distributions are studied. It is found that the PES sensitivity has a nearly Gaussian distribution. In most cases, the null point position of the PES has a nonzero average and an asymmetric distribution. Media with different grain sizes and size distributions are investigated and compared. The grain size distribution has a small effect on the PES detection. Increasing the average grain size dramatically increases the standard deviation of the PES sensitivity while slightly decreasing the PES sensitivity. The simulation results also show that increasing the medium grain orientation ratio (OR) improves the PES sensitivity.

The impact of GMR baseline popping noise on the servo position error signal of a hard disk drive

Baseline popping noise (BLPN) is one of the major types of instability in a GMR head, and it is often shown as "popping" of the baseline between isolated pulses. From the experiment, BLPN is still a problem in the servo frequency range (10-15 MHz). The offtrack performance of a high BLPN head and the impact of BLPN on a HDD position error signal (PES) are studied in this paper. Generally, burst average amplitude (BAA) rises when the unstable permanent magnet is in the track, especially at the track edge. The head PES is distorted and its linear range could be significantly reduced because of BLPN. A simple model is introduced to analyze it. When the servo tolerance is known, it can be used to scan the head at component level with spin stand.

Analog-to-digital converter algorithms for position error signal estimators

Algorithms are developed that utilize a single analog-to-digital converter in the read channel of a disk drive for dual purposes: for high-rate data detection and for high-resolution servo position error signal estimation. Iterative algorithms exploit the lower clock rate requirement of a servo field, while dithering takes advantage of the periodic characteristic of the servo position waveform.

A nonlinearity specification for constructing the servo position error signal

A high performance disk file actuator servo requires a position transducer whose response is linear with displacement. Two concerns that detrimentally affect positional linearity are the nonlinear response of the burst detector to burst amplitude and the nonuniform sensitivity of the magnetic read element across its pole faces. Nonuniform field sensitivity is of particular concern with the advent of magneto resistive heads for embedded servo systems, and this paper gives specification for both for good actuator positioning.