Lateral Vibration and Read/Write Head Servo Dynamics in Magnetic Tape Transport

Magnetic tape is a flexible mechanical structure having dimensions that are orders of magnitude different in its thickness, width, and length directions. In order to position the tape relative to the read/write head, guides constrain the tape’s lateral motion, but even the modest forces that develop during guiding can cause wear and damage to the tape’s edges. This paper presents a tensioned axially-moving viscoelastic Euler‐Bernoulli beam model used to simulate the tape’s lateral dynamics, the guiding forces, and the position error between the data tracks and the read/write head. Lateral vibration can be excited by disturbances in the form of pack runout, flange impacts, precurvature of the tape in its natural unstressed state, and spiral stacking as tape winds onto the take-up pack. The guide model incorporates nonlinear characteristics including preload and deadbands in displacement and restoring force. A tracking servo model represents the ability of the read/write head’s actuator to track disturbances in the tape’s motion, and the actuator’s motion couples through friction with the tape’s vibration. Low frequency excitation arising from pack runout can excite high frequency position error because of the nonlinear characteristics of the guides and impacts against the pack’s flanges. The contact force developed between the tape and the packs’flanges can be minimized without significantly increasing the position error by judicious selection of the flanges’ taper angle. DOI: 10.1115/1.4000665 In the field of computer data storage, tape libraries are used for long-term archive, backup, and restoration of financial information, medical records, geophysical data, satellite imagery, and electronic intelligence. Such data are stored for decades, often under regulatory requirements, and its value increases with time. During the past fifty years, the volumetric storage density for this technology has grown by six orders of magnitude, partly due to advances in the mechanical design of the path, guides, cartridge, and servo mechatronics 1. Technical roadmaps foresee that the tape’s thickness will decrease to 5 m, the bit length will reduce to 33 nm, and the data track will narrow to 2 m over the next decade. At the same time, the tape’s tension is expected to decrease while the transport speed doubles 2. Such trends place demands on the design of guides, servos, and actuators for the control of lateral vibration and the reduction of position error between the tape and read/write head. Magnetic tape itself is a flexible layered polymeric material with dimensions that differ by orders of magnitude in the thickness, width, and machine directions. The nonlinear forcedeflection characteristics of guides and flange impacts 3 enable low frequency disturbances e.g., from bearing runout to excite high frequency lateral vibration of the tape. Although the read/ write head is actively positioned in order to follow the tape’s vibration, its bandwidth is limited to the lower frequencies. The objective of this paper is to develop a model to support path, guide, media, and servo design in order to reduce the position error between the tape’s data tracks and the read/write head. The vibration of magnetic tape is related to the mechanics of axially-moving materials 4,5 and web transport systems 6,7. Vibration models for magnetic tape include traveling strings and tensioned beams and exact closed form expressions for the response of such systems to arbitrary excitation are available through modal analysis and Green’s function methods 8. Solutions also are available for the displacement of an axially-moving Timoshenko beam having defects in its natural shape, where preformed warpage in the beam’s natural shape can drive undesirable lateral motion 9. The discretization of moving media models and the influence of nonlinearity are discussed in Refs. 10‐12. Edge and surface guiding are two approaches for constraining the lateral vibration of magnetic tape and other moving media. Edge guides establish a lateral constraint on the narrow edge of the tape by its contact against the guide’s flanges—termed buttons—but those forces can lead to excessive heating and wear of the tape’s edge 13. The judicious positioning and choice of a guide’s engagement length are useful approaches to reduce steady-state vibration amplitude 14,15. The propagation of boundary runout disturbances from packs and rollers can be reduced through the use of tilted 16 and deadband 3 guides, the latter of which have finite clearance between the guide’s flanges and the edge of the tape while centered. The high frequency vibration associated with impacts in deadband guides is particularly problematic because of the limited frequency response of the track following servo 17. In the alternative approach of surface guiding, edge contact is avoided entirely, and lateral vibration is controlled by distributing friction forces over the relatively broad face of the tape’s width. Surface guides are particularly useful at higher frequencies while edge guides are useful for lower frequency higher amplitude disturbances. The guides, layout of the transport path, and actuator-servo system synergistically contribute to the alignment of the tape’s data tracks and the magnetic head’s read/write elements. Figure 1 depicts an overview of the model that is developed in Sec. 2 for coupled lateral tape vibration and actuator-servo dynamics. Disturbances from the packs, guides, and imperfections in the tape media itself each excite lateral tape vibration. A general representation of the restoring force provided by a nonlinear edge guide is developed for elements that have a deadband in displacement 1