Servo format optimization

The performance of the track-follow and reel-to-reel servo systems is ultimately limited by the resolution and bandwidth of the velocity and position signals provided by the servo channel, which are in turn related to the geometry of the servo pattern. Figure 1 illustrates the basic format of the timing-based servo (TBS) pattern used in LTO1-LTO6 tape drives.

• Figure 1. TBS servo pattern used in LTO1-LTO6.

To optimize TBS systems for future operating points with aggressive track-density scaling, we developed an analytical framework/model to characterize how the servo format parameters, servo read head geometry and magnetic media properties affect the servo readback signal [1]. Figure 2 shows examples of dibit readback signals for the case of longitudinal, perpendicular, and non-oriented media. 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.

• Figure 2. Dibit servo readback signal for three different media types [1].

For optimal system performance, the servo pattern parameters, e.g., azimuth angle, transition width and period, have to be optimized jointly with the read head geometry, e.g., shield-to-shield distance, magneto-resistive sensor width, reader width, and the magnetic media characteristics.

Figure 3 shows an example of the standard LTO servo pattern written with an azimuth angle of 6 degrees and an experimental pattern written with an angle of 12 degrees.

Our goal is to improve the resolution of the servo signal down to the nanometer scale by jointly optimizing all the components of the servo system. By combining an optimized servo pattern, a prototype perpendicularly oriented BaFe medium, an H-infinity servo controller and a low-disturbance flangeless tape path, we demonstrated closed-loop track-following performance of less than 14 nm PES standard deviation [2].

Figure 3. Experimental servo patterns.

References

[1] Analytical expressions for the readback signal of timing-based servo schemes
S. Furrer, P. Jubert, G. Cherubini, R.D. Cideciyan and M.A. Lantz
Digest of Intermag 2012, Vancouver, Canada (May 2012), HT02.

[2] Nanoscale Track-follow Performance for Flexible Tape Media
M. A. Lantz, A. Pantazi, G. Cherubini and J. Jelitto
Proceedings of the 18th IFAC World Congress, Milano (Italy) August 28 - September 2, 2011. DOI.

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