Tape drive track following using cascade control

Ultra-precise positioning of the recording head over the data tracks is required to achieve the aggressive track density scaling envisioned for future tape storage systems. The track-follow control system is responsible for reducing the misalignment between the tape and the head created primarily by lateral motion of the flexible medium. The challenge of improving the track-following accuracy becomes even more difficult in the presence of external vibration disturbances. Typically, position error signal (PES) information is measured at the tape head using pre-formatted servo information and is used in a feedback controller during track-following to maintain the position of the read/write elements at the desired position during tape transport. In this paper, a track-follow scheme is introduced in which a high-bandwidth low-noise position sensor is used in conjunction with the position information read back by the head from preformatted servo patterns on the tape. A cascade control configuration is proposed, in which first an inner loop is designed based on the actuator position information provided by the sensor. The high-bandwidth and low-noise characteristics of the sensor enable a high closed-loop bandwidth of the inner system that effectively compensates for external vibration disturbances. Subsequently, using a cascade structure, a PES-based controller is designed that compensates for the lateral tape motion disturbances. Simulation and experimental results are presented that illustrate the performance of the proposed scheme compared to a conventional PES-based track-follow control system.