Track-following servo control

Achieving tape cartridge capacities of 10 TB and beyond requires a substantial increase in the track density of future tape drives. Hence, the track-following servo control needs to achieve a significantly improved positioning accuracy. The basic function of the track-following control system is to reduce the misalignment between the tape and the recording head created by lateral motion of the flexible medium. Lateral tape motion (LTM) arises primarily from imperfections in the tape guide rollers and reels, such as run-outs, eccentricities and other tape path imperfections. A promising technique to enable higher track densities is the removal of the flanges from the rollers that are used to guide the tape through the tape path and across the read/write head (see Figure 1). The introduction of flangeless tape paths has led to a significant increase in drive and tape lifetime, as well as to a significant reduction in high-frequency LTM [1]. However, removal of the flanges results in an increase in the amplitude of lateral tape excursions that in turn cause a substantial skew between the read/write head and the tape. To compensate for both LTM and the tape skew, current flangeless tape drives use a two-degree-of-freedom head positioning system that has both translational and rotational degrees of freedom.

• Figure 1. Tape path with recording head and flangeless rollers.

The tape servo system typically consists of a tape transport servo part, which is responsible for maintaining a constant tape velocity, and a track-follow part, which controls the lateral/rotational position of the head with respect to the tape (see Figure 2). The pre-formatted servo pattern and two active servo channels provide tape-to-head position and skew angle information to the track-follow control system.

The track-follow controller must combine good disturbance rejection capabilities, low sensitivity to measurement noise and robustness. The usage of robust control design approaches that take into account the LTM disturbance characteristics and experimentally obtained system models have enabled the demonstration of nanometer-scale accuracy in the head positioning [2-4].

Furthermore, we are exploring control schemes that provide enhanced performance in the presence of stationary and/or time-varying periodic components of the LTM disturbances that originate from the rollers and reels [5-6]. Performance can be further enhanced by mitigating the effects of tape stacking irregularities, called stack shifts, using either an active tape guiding mechanism [7] or feedforward control based on LTM information provided by optical edge sensors [5-6].

• Figure 2. Tape transport and track-follow control systems.

The challenge of improving the track-following accuracy in tape systems is compounded by the wide range of conditions in which tape drives must operate. For example, magnetic tape is used extensively for the collection of seismic data on ships used for oil exploration. In such environments the tape drive is exposed to external vibrations giving rise to additional track-following disturbances that must be compensated for by the control system.

We are developing schemes for vibration rejection using information from micro electro-mechanical (MEMS) based accelerometers to enhance the controller performance [8]. Feedforward control schemes that utilize the acceleration measurements can provide substantial improvement in the track-follow performance under vibration conditions.

Figure 3 shows an experimental set-up that is used to test the track follow performance of such a scheme during the application of external vibrations. 

• Figure 3. Experimental vibration test system.

References

[1] Scaling Tape-Recording Areal Densities to 100 Gbit/in²,
A. Argumedo, D. Berman, R. Biskeborn, G. Cherubini, R. Cideciyan, E. Eleftheriou, W. Häberle, D. Hellman, W. Imaino, J. Jelitto, K. Judd, P.-O. Jubert, M.A. Lantz, G.M. McClelland, T. Mittelholzer,S. Narayan, S. Ölçer,
IBM J. Res. Develop. (Storage Technologies and Systems) 52(4/5) (2008).

[2] 29.5 Gb/in² Recording Areal Density on Barium Ferrite Tape,
G. Cherubini, R.D. Cideciyan, L. Dellmann, E. Eleftheriou, W. Haeberle, J. Jelitto, V. Kartik, M. Lantz, S. Oelcer, A. Pantazi, H. Rothuizen, D. Berman, W. Imaino, P.-O. Jubert, G. McClelland, P. Koeppe, K. Tsuruta, T. Harasawa, Y. Murata, A. Musha, H. Noguchi, H. Ohtsu, O. Shimizu, and R. Suzuki,
IEEE Transactions on Magnetics 47(1) (2011).

[3] 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.

[4] Servo-Pattern Design and Track-Following Control for Nanometer Head Positioning on Flexible Tape Media,
M. A. Lantz, G. Cherubini, A. Pantazi, J. Jelitto,
IEEE Transactions on Control Systems Technology 20(2), 369-381 (2012).

[5] Track-following in tape storage: Lateral tape motion and control,
A. Pantazi, J. Jelitto, N. Bui, and E. Eleftheriou,
Mechatronics 22(3), 361–367 (2012).

[6] Track-Follow Control for Tape Storage,
A. Pantazi, J. Jelitto, N. Bui, E. Eleftheriou,
IFAC Mechatronics Symposium, Boston, USA, Sep 2010.

[7] Active Tape Guiding,
A. Pantazi, M. A. Lantz, W. Haeberle, W. Imaino, J. Jelitto, E. Eleftheriou,
Proc. 20th ASME Annual Conf. on Information Storage & Processing Systems 2010 "ISPS," Santa Clara, CA, (June 2010) 304-306.

[8] Vibration compensation in tape drive track following using multiple accelerometers,
A. Pantazi and M. Lantz,
Proc. 6th IFAC Symposium on Mechatronic Systems 2013, Hangzhou, China, (April 2013), pp. 506-510.

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