Servo channel

 

The servo channel of a tape system is the signal processing unit that derives essential servo parameters, such as tape velocity, lateral head position, and longitudinal tape position from patterns that are written onto tape during manufacturing time. These so-called timing-based servo (TBS) patterns form dedicated servo bands that straddle the data bands.  For instance, in current LTO drives, five servo bands straddle four data bands. The TBS technology was developed in the mid-1990s for linear tape drives and has been adopted as LTO standard.

In TBS systems, recorded patterns to aid track-following servo consist of transitions with two different azimuthal slopes, as shown in Figure 1. The head lateral position and the tape velocity are derived from the relative timing of pulses generated by a servo reader reading the pattern. TBS patterns also enable the encoding of longitudinal position (LPOS) information by shifting transitions from their nominal position using pulse-position modulation.

TBS servo pattern used in LTO1-LTO6
  • 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.
Synchronous servo channel architecture
  • Figure 2. Synchronous servo channel architecture.

References

[1] High-Performance Servo Channel for Nanometer Head Positioning in Tape Systems
G. Cherubini, J. Jelitto, Proc. 2012 IEEE/ASME Int'l Conf. on Advanced Intelligent Mechatronics "AIM 2012," Kaohsiung, Taiwan (July 2012), pp. 788-795.

[2] Characterization and Detection of PPM-Encoded Servo Signals in Tape Drives, G. Cherubini, R.D. Cideciyan, E. Eleftheriou, J. Jelitto, Proc. 2009 IEEE Pacific Rim Conf. on Communications, Computers and Signal Processing "PacRim '09," Victoria, B.C., Canada, pp. 342-347, August 2009.

[3] Frame Synchronization for PPM-encoded Longitudinal Position Words in Magnetic Tape Storage, G. Cherubini, R. D. Cideciyan, E. Eleftheriou and J. Jelitto, Proc. IEEE GLOBECOM, Hawaii, USA, Dec. 2009.

[4] Fast Servo Signal Acquisition in Tape Drives Using Servo and Data Channels, G. Cherubini, J. Jelitto, K. Tsuruta, Mechatronics Vol. 22, Issue 3, April 2012, pp. 349-360.

[5] High-Precision Skew Estimation in Flangeless Tape Drive Systems by Dual Synchronous Servo Channel, G. Cherubini, J. Jelitto, Proc. 2012 ASME-ISPS / JSME-IIP Joint Int'l Conf. on Micromechatronics for Information and Precision Equipment "MIPE2012," Santa Clara, CA (June 2012), pp. 370-372.

[6] LPOS Symbol Detection by Soft-Output Combining of Dual Synchronous Servo Channels in Tape Drives, G. Cherubini, J. Jelitto, Microsystem Technologies, Vol. 17, No. 5-7, June 2011, pp. 857-862.

[7] Initial resolution of head position and skew uncertainty in control systems for flangeless tape drives, G. Cherubini, J. Jelitto, Angeliki Pantazi, 51st IEEE Conference on Decision and Control, Maui, Hawaii, USA, December 10-13, 2012, pp. 5052-5058.

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