Tape servo system: Track-following servo control
Achieving tape cartridge capacities of 20 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 to remove the flanges from the rollers that guide the tape through the tape path and across the read/write head (see Figure 1). Introducing 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 [2008-1]. However, removing 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.
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 [2011-1, 2011-2, 2012-7, 2014-1, 2014-3].
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 [2012-1, 2010-8]. Performance can be further enhanced by mitigating the effects of tape stacking irregularities, called stack shifts, using either an active tape guiding mechanism [2010-6] or feedforward control based on LTM information provided by optical edge sensors [2012-1, 2010-8].
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Figure 1. Tape path with recording head and flangeless rollers.