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.

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 [2012-5].

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.

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.

—IBM scientist Mark A. Lantz

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.

Dibit servo readback signal for three different media types

Figure 2. Dibit servo readback signal for three different media types [2012-5].

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 [2011-1].

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Mark A. Lantz

Mark A. Lantz

IBM Research scientist