Tape servo system: Tape transport servo control

Reliable tape transport, a.k.a. reel-to-reel transport, is of fundamental importance to read-channel performance on all data channels during tape read and write operations. The tape transport servo controls the velocity of the tape during read, write and seek operations as well as the tension applied to the tape.

Increasing data rate requirements necessitate the use of higher tape transport velocities. At the same time, data rate matching also imposes the requirement of reliable operation at very low tape velocities.

Furthermore, tight control of tension is necessary for supporting thinner tape, thereby enabling an increase in volumetric storage density, and mitigating lateral tape motion. The reduction in tape thickness renders the tape more fragile and hence more sensitive to tension variations.

The reel-to-reel servo receives estimates of tape velocity and tape longitudinal position from one or more digital synchronous servo channels, which process servo information from dedicated servo bands. Hall sensors are used to obtain tape velocity information prior to initial parameter acquisition by the servo channel.

We are investigating methods to characterize the tape transport system [2013-7, 2013-8] and to mitigate the impairments affecting the performance of the reel-to-reel servomechanism and the quality of readback signals in data channels, which are to be attributed to the variation of tape tension and velocity. For example, variations of tape tension around the nominal value, also called once-around-effects, may be induced by reel eccentricities.

In tape transport, this problem is particularly serious when the reel rotation frequencies are near the resonance frequency determined by the tape path. Tape tension deviations from the nominal value also affect the position error signal and hence the performance of the track-following servo system.

To address these challenges, we investigate advanced control techniques using feedback of velocity and tension signals, as well as techniques to improve the performance of the motors used in the tape transport system. An experimental tape setup with attached tension sensors is shown in Figure 1.

Experimental tape transport setup with attached tension sensors

Figure 1. Experimental tape transport setup with attached tension sensors.

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