Characterization of lateral tape motion and disturbances in the servo position error signal of a linear tape drive

Lateral tape displacement in a tape drive is believed to be one of the critical limiting factors in achieving high track density in tape storage media. For improving track density, servo control systems can be designed on the basis of information from a position error signal to improve the track following capabilities in a tape drive. In this paper, we consider the characterization of lateral tape motion and additional disturbances that contribute to the position error signal. From a commercially available tape drive, the position error signal is measured electronically and lateral tape motion of the magnetic tape is measured using an optical edge sensor. This information is used to characterize the filtering effects of the disturbance rejection transfer function that determines the lateral tape motion and disturbances present in the position error signal. The disturbance rejection transfer function is characterized by two different methods. In the first method, we model the disturbance rejection transfer function as a simple high pass filter. Using lateral motion measurements as an input to this system, the position error signal is simulated and compared with the measured position error signal. In the second method, system identification methods are used to estimate a detailed high order filter that models the essential components of the disturbance rejection transfer function. Using these models, the position error signal is simulated and predicted and the effects of additional disturbances such as the servo track variability can be separated from the effects of the lateral tape motion. It is shown that the position error signal exhibits contributions from lateral tape motion and additional disturbances that differ significantly at higher frequencies.