Tape Path Research Articles

Tape transport control with suppression of time-varying tension disturbances

Disturbance suppression in the transport system of tape drives is essential for achieving high volumetric densities in tape storage, as improved tension control will enable operation with thinner tape media. In addition, tension variations lead to dimensional changes in the cross-track direction of the tape that must be accounted for in the track width budget. Hence, better tension control enables a higher track density. Furthermore, fluctuations of the tape velocity and tension around the nominal values impact the performance of the data channels, and consequently also the achievable areal recording density. In a tape transport system, disturbances are mainly induced by reel eccentricities that lead to time-varying tension disturbances. In this paper, the tension disturbances are first characterized using a strain-gauge sensor included in one of the rollers that determine the tape path. Then, a second characterization method that does not rely on a strain-gauge sensor is introduced. The new method is based on an estimate of the tension variation signal, which is derived from the difference of head lateral position measurements obtained by reading servo patterns on two adjacent servo bands. A tension feedback system is proposed, which uses time-varying controllers that adapt their frequency characteristics depending on the longitudinal tape position. The closed-loop system is designed to enhance disturbance suppression at the reel rotation frequencies. Experimental results are presented to illustrate the performance of the proposed tape transport control system.

Lateral Friction Behavior of a Thin, Tensioned Tape Wrapped Over a Grooved Roller: Experiments and Theory

Effects of friction forces on the lateral dynamics of a magnetic recording tape, wrapped around a grooved roller are investigated experimentally and theoretically. Tape is modeled as a viscoelastic, tensioned beam subjected to belt-wrap pressure and friction forces. Including the effects of stick and slip and velocity dependence of the friction force render the tape's equation of motion nonlinear. In the experiments, tape was wrapped under tension around a grooved roller in a customized tape path. The tape running speed along the axial direction was set to zero, thus only the lateral effects were studied. The grooved roller was attached to an actuator, which moved the roller across the tape. Tests were performed in slow and fast actuation modes. The slow mode was used to identify an effective static, or breakaway, friction coefficient. In the fast mode, the roller was actuated with a 50 Hz sinusoid. The same effective friction coefficient was deduced from the fast actuation mode tests. This test mode also revealed a periodic stick–slip phenomenon. The stick-to-slip and slip-to-stick transitions occurred when the tape vibration speed matched the roller actuation speed. Both experiments and theory show that upon slip, tape vibrates primarily at its natural frequency, and vibrations are attenuated relatively fast due to frictional and internal damping. This work also shows that an effective friction coefficient can be described that captures the complex interactions in lateral tape motion (LTM) over a grooved roller.

High-Rate Skew Estimation for Tape Systems

Abstract: In several recent magnetic tape drive designs, an actuator with two degrees of freedom has been adopted for joint skew- and track-following control. This enables the removal of flanges from guide rollers in the tape path, thus avoiding debris build-up, tape-edge damage, and high-frequency lateral tape motion. Skew compensation is needed to keep the head perpendicular to the direction of tape motion to enable read-while-write functionality in the presence of large lateral tape excursions originating from the removal of roller flanges. In this paper, we present a novel skew estimator that generates skew estimates at a high rate for an extended range of allowable tape-to-head skew values. A skew estimate is given by the sum of a fractional and an integer part, extracted from information provided by two parallel synchronous servo channels operating on timing-based servo patterns. The fractional part is obtained from the accurate measure of timing intervals between dibit correlation peaks, whereas the integer part is derived from longitudinal position information. The new method was implemented in an FPGA and verified experimentally using a commercial tape drive.

Mechanics of lateral positioning of a translating tape due to tilted rollers: Theory and experiments

A mechanics based model to describe the lateral positioning of a thin, tensioned, translating tape over a tilted roller is introduced, based on the assumption that the transport velocity of the tape should match the surface velocity of the roller when there is sufficient traction. It is shown that this condition requires the slope of the neutral axis of the tape and the slope of the centerline of the tilted roller to be the same over the wrapped segment. An extension of this model is discussed including the possibility of circumferential and lateral sliding, depending on the velocity difference between the tape and the roller. The new model is incorporated into a generalized model of a tape path that consists of numerous rollers as well as the appropriate boundary conditions for the take-up and supply reel dynamics. The nonlinear equation of motion is solved numerically, and the steady state solution is found by an implicit time stepping algorithm. An experimental setup with one tilting roller, two or three nearly ideally oriented rollers and two reels is used for verification of the model. The effects of roller tilt angle, tape wrap angle, and the lengths of the free-tape spans upstream and downstream of the tilted roller on the steady state lateral tape position are investigated experimentally and by simulations. The experiments show that the circumferential position of the wrap on the upstream side of a tilted roller has the strongest effect on pushing the tape in the lateral direction. The total wrap angle around the roller has a smaller effect. It was also shown that the tape segments upstream and downstream of the tilted roller interact, and the combined effect results in a different overall lateral tape response in steady state.

Tape transport control based on sensor fusion

Reliable and precise tape transport is of fundamental importance to achieve larger volumetric recording densities in tape storage systems. The performance of the tape transport control system, which is affected by variations in the tape velocity and tension, impacts the write and read quality of the data tracks and eventually the achievable areal recording density. During operation in cruise velocity mode, disturbances in velocity and tension may be induced for example by tape reel eccentricities. This problem is particularly serious when the reel rotation frequencies are close to the resonance frequency determined by the tape path. Typically, the tape velocity is estimated at the tape head from a servo signal, which is obtained by reading pre-formatted servo information, and used for tape transport control during cruise mode. In such a scheme, velocity disturbances observed at the head cannot be attributed to an individual reel. Hence no targeted disturbance suppression can take place at the individual tape reels. However, Hall sensors are typically included in tape drives to obtain additional tape velocity information from the individual reels. This information is used to achieve proper tape transport operation in the absence of valid velocity estimates from the pre-formatted servo information, for example during tape acceleration. In this paper, we present a novel control scheme for tape transport that uses velocity measurements from three sources, the Hall sensors at the tape reels and the servo channel that yields velocity estimates at the head. Specifically, characterization of the multiple-input multiple-output (MIMO) tape transport system provides an accurate system model and enables an optimized two-sensor control design. Furthermore, H∞ filtering is employed to perform sensor fusion and the resulting state estimates are used in the feedback control of the two reels. Experimental results are presented to illustrate the behavior and performance of the proposed tape transport control system.

Coupling between the in-plane and lateral tape dynamics in high capacity linear tape transport systems

Lateral tape motion (LTM) is defined as the lateral deviation of the tape from its prescribed strain-free state path. The effects of tape, roller, guide or reel imperfections on the LTM are reasonably well understood. Yet, the effects of disturbances in the longitudinal (in-plane) direction, which can couple into LTM, have not been well described. Longitudinal tape vibrations can be due to, but not limited to a) tension dynamics due to servo control of the tape reels, and/or tension impulses due to the unwinding of tape layers that experience sticking. The problem is further complicated due to the uncertainty of the tape length on the downstream side, as a result of the “floating layers” in the take-up reel. In this work, the equations of motion of the longitudinal and lateral tape motion are derived from first principles. The coupling due to non-linear longitudinal strain is considered. The equations of motion are solved by using the finite element method, and an explicit time integration algorithm. The entire tape path is modeled directly, where the interaction of the tape with the recording head and the guides are represented as concentrated forces, and moments. The effects of disturbances, typical for a tape transport system, on the coupling or lack there off are investigated.

Lateral Tape Motion Control With Robust Performance Evaluation Based on RBode Plot

Magnetic tapes are still widely used in data storage facilities. A novel approach is presented for reducing the lateral tape motion (LTM) at the reel and thus for preventing the impacts between tape and reel flanges, which can easily damage the fragile tape edges. To measure the LTM, photonic sensors are employed, which have to be calibrated to compensate for nonlinearities and for the shift of the tape path between sensor probes while the reel winds and unwinds. Moreover, since a sensor cannot be placed as close to the reel as desired, the lateral tape position at the reel is estimated by extrapolating from two sensors. Actuation is provided by a tilted rotary guide. In this paper, first, the identified dynamics at the two sensors are combined to estimate the dynamics at the tape nip, and a complex proportional-integral-lead controller is implemented to compensate for the LTM at the extrapolated location. Then, the robust performance characteristics of the implemented controller is evaluated using the robust Bode plot, which accounts for the uncertainties embedded in the used system identification method based on frequency response data. Finally, the experimental results show a significant reduction above 60% in LTM at the reel.

Servo-Pattern Design and Track-Following Control for Nanometer Head Positioning on Flexible Tape Media

Achieving multi-Terabyte capacity in tape cartridges requires a substantially higher track density than that available in present systems, and hence a significantly higher positioning accuracy is required of the track-following servo in tape drives. In this paper, advanced concepts are considered for several elements of a tape system that enhance the track-following servo performance to reach nanometer positioning accuracy. We introduce a novel method for optimizing the geometry of servo patterns in a timing-based servo system. The design criterion aims to minimize the measurement error in the position-error signal (PES) yielded by a digital synchronous servo channel. A flangeless tape path is adopted to mitigate high-frequency components of the lateral tape motion. The track-following servo controller, which is designed based on the H∞ approach, takes into account the measured plant transfer function, the disturbance characteristics of the tape path, and the properties of servo channel. These elements are combined to investigate the track-following performance achievable with a new high-SNR magnetic tape based on perpendicularly-oriented BaFe particles. With this setup, a record closed-loop track-following performance of less than 14 nm PES standard deviation is demonstrated.

Track-Following High Frequency Lateral Motion of Flexible Magnetic Media With Sub-100 nm Positioning Error

Lateral motion of the magnetic tape in data storage applications misaligns the flexible medium relative to the recording head and reduces the potential for decreasing data track width and spacing, and thereby the achievable data storage density. In order to reduce the effect of misalignment between the tape and the recording head, the head is mounted on a track-following actuator that can compensate for the position error up to a frequency that is governed partly by its mechanical resonances; this bandwidth is typically 1 kHz or less for conventional voice coil-based actuators. This paper introduces the design and implementation of a track-following actuator that is piezo-electrically actuated in order to potentially achieve a significant increase of bandwidth and positioning precision. A proof-of-concept is prototyped and experimental results are presented. An H∞ controller is designed and implemented to compensate for typical disturbances such as periodic runout of packs and rollers, or non-periodic flange impacts. Preliminary closed loop experimental results with an experimental low lateral motion tape path, and conventional servo format and media demonstrate a reduction in positioning error standard deviation to 74.5 nm, with simulations indicating a potential for further improvement to 20.6 nm with the use of improved media and servo format.

Dynamic tape path alternation with novel rotary guider

To enable higher recording density and data access accuracy, controlling lateral tape motion within high precision specifications through calibrated tape tension control is one of many key focuses for extra high density magnetic tape drives. This paper presents a novel rotary actuation mechatronic system which regulates travelling tape tension in a modern tape drive by dynamically alternating its tape path through closed-loop controls. Effectiveness of the system was demonstrated through calibrated laboratory parametric studies from which variation of travelling tape tension was found to be significantly reduced. The work described herein suggests engineering feasibility of using flangeless guiders in modern advanced tape drives as well as tape tension control without regulating tape drive’s driving motors.

Combined Feedforward/Feedback Control for Tape Head Track-Following Servo Systems

AbstractIn order to remain competitive with other data storage technologies, the tape industry is planning to significantly increase track densities on tape. Higher track densities require narrower track pitch and more precise positioning of the read/write heads. Hence, improved track-following servo systems for the head assembly are needed. Longitudinal transport of the tape along the tape path is subject to lateral disturbances that cause displacement between the head and the desired track and hence increase position error. This paper proposes a combined feedforward/feedback control scheme to improve the track-following performance of the tape head positioning system in the presence of lateral tape motion (LTM). In this architecture, the feedback controller is designed to guarantee system stability and take into account the lower frequency components in the disturbances. The feedforward controller requires as its input a prediction of the lateral tape motion displacement (LTMD) at the head assembly. We propose a least squares based algorithm to predict the LTMD at the head from a history of upstream and downstream LTMD measurements. Compared to using feedback-only control, the combined feedforward/feedback controller enables the head assembly to be positioned more accurately over the desired track.

Nanoscale Track-follow Performance for Flexible Tape Media

AbstractThe positioning accuracy achievable by track-follow servo for flexible tape media is investigated using advanced concepts for several elements of the system. First, the servo pattern geometry in the timing-based servo system is designed in conjunction with a digital synchronous servo channel to minimize the measurement error in the position-error signal. Second, a flangeless tape path is introduced to mitigate high-frequency components of the lateral tape motion. Finally, a track-follow controller is designed based on the H∞ approach, taking into account the disturbance and noise characteristics of tape path and servo-channel and the measured plant transfer function. Combining these elements with a new high SNR magnetic tape based on perpendicularly-oriented BaFe particles, we demonstrate a closed-loop track-follow performance with a standard deviation of the position-error signal of less than 15 nm.

Characterization of Long-Length, MOCVD-Derived REBCO Coated Conductors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A leading approach to the fabrication of long-length, high-performance <formula formulatype="inline"><tex Notation="TeX">${\rm REBa}_{2}{\hbox {Cu}}_{3}{\hbox {O}}_{7}$</tex></formula> (REBCO) coated conductor is by metal-organic chemical vapor deposition (MOCVD) of REBCO on buffered templates. Templates are produced by ion beam assisted deposition of textured MgO onto polished metal substrates. The overall performance of MOCVD coated conductors achieved to date is impressive, but further improvement is desired. We have used a coordinated set of characterization techniques to identify the underlying causes for critical current (Ic) performance variations in long-length MOCVD conductors. Using electron microscopy and Raman spectroscopy, we studied tape specimens from specially designed experiments performed in SuperPower's MOCVD manufacturing equipment with its six-track “helix” tape path. We find that in multi-pass depositions used to produce thicker REBCO films, the REBCO phase uniformity and texture quality in the first pass play key roles in pass-to-pass microstructure evolution, with nucleation of second phase particles in the first layer promoting misoriented grains that propagate through subsequent layers. These misoriented grains, many growing in close proximity with second phase particles, present current-blocking obstacles that limit Ic performance. Our results show that achieving more uniform deposition in the very first deposited layer plays a critical role that in turn leads to reduced misoriented grain content and REBCO lattice disorder in the second and subsequent layers of the REBCO film. </para>

Attenuation of lateral tape motion due to frictional interaction with a cylindrical guide

Lateral tape motion (LTM) is a function of tape path and tape properties, design parameters such as speed, tension, and friction between tape and tape path components. This investigation deals with the LTM of a tape moving over the surface of a cylindrical guide. Starting from the equation of motion for a moving tape on a cylindrical guide surface, the effects of friction, tape properties and design parameters are examined by calculating the ratio of LTM before and after a cylindrical guide. Attenuation of LTM is found to be mainly dependent on the guide radius and the wrap angle.

A Model for Magnetic Tape/Guide Friction Reduction by Laser Surface Texturing

The friction coefficient between a magnetic tape and a guide in a tape path can be minimized by creating micro dimples on the guide surface with laser surface texturing. The dimples enhance the formation of an air bearing and reduce the friction coefficient between the tape and the guide due to the increased spacing. A model is presented to optimize the geometry of the surface texturing parameters to maximize the average air bearing pressure and minimize the tape/guide friction coefficient.

Investigation of roller interactions with flexible tape medium

Lateral tape displacement and roller axial runout are measured on an experimental tape transport. The effect of roller axial runout on lateral displacement of tape is investigated. Tape lateral displacement is found to be generated at the roller surface, and is observed to be correlated to roller axial runout. A number of typical roller types are investigated to determine the mechanism causing lateral tape displacement. Edge contact and friction are each shown to play a role in the creation of lateral tape displacement in the tape path.

Control and operation of reel-to-reel tape drives without tension transducer

This paper presents the control and operation of reel-to-reel tape drives without any tension transducer. First, the tape drive model is validated while operating in the read/write or the velocity mode. Utilizing the coupled nature of the velocity-tension loop and the fact that the system is completely observable, the tension errors are estimated based on the validated model using state estimation techniques. The tension error estimates do not correlate well with the actual tension errors primarily due to the periodic disturbances caused by reel eccentricities during steady running and are not used for feedback purposes. In this paper, the controller design for such transports for the velocity and the position loops are presented. Due to lack of forced air in the tape path in such drives, there is considerable stiction/friction during start-up when the position loop is used. Feed forward currents which are functions of desired tension at the head and tape velocity are used for proper friction compensation. For systems using two tachometers, one for each reel, a linear velocity difference technique is presented which shuts down the drive in case of excessive or low tape tension and thus preserves tape integrity. Finally, test results are presented to illustrate the effectiveness of the design techniques.

Tape edge study in a linear tape drive with single-flanged guides

Improved tape guiding and tape dimensional stability are essential for magnetic tape linear recoding formats to take advantage of vastly increased track density and thereby achieve higher storage capacities. Tape guiding is dependent on numerous parameters, such as type of the guides and tape path geometry, quality of virgin tape edge, drive operating parameters (e.g., tape speed and tape tension), mechanical properties of the tape, and tape geometry (e.g., cupping and curvature). The objective of the present study is to evaluate guiding and tribological performance of single-flanged guides with porous air bearings in a linear tape drive. A comparison of guiding performance of the dual flanged stationary guides and single-flanged guides with porous air bearings is performed. The effect of tape path geometry, drive operating conditions (speed and tension) and tape edge quality of factory-slit tapes on tape guiding are evaluated during short-term tests. A lateral force measurement technique is used to measure the force exerted by the tape edge on the guide flange. A technique for the lateral tape motion measurement is used to study the effect of continuous sliding on tape guiding. Wear tests up to 5000 cycles are conducted and coefficient of friction and lateral tape motion are monitored to study the effect of drive operating conditions (speed and tension), edge quality of factory-slit tapes and tape thickness on tape guiding. Optical microscopy, atomic force microscopy and scanning electron microscopy are employed to study and quantify the quality of tape edge.

Wear of tribo-elements of video tape recorders

The helical scan magnetic recording equipment, like the video tape recorders (VTR), consist of many tribo-elements. The wear of these tribo-elements is an essential problem affecting the reliability of the equipment. The rotating magnetic head in high-speed rubbing with the magnetic tape needs a self-cleaning effect by mild wear. A simple trial of reducing the head wear often brings partial recession, friction polymer and brown stain. The summary of the current study of head wear is given by dividing it into the head factors, the tape factors, the system factors and the atmosphere factors. The computational analysis of head contour change caused by wear is compared with the experimental results. The outline of the tribology of motor, capstan and tape path in the VTR is introduced.

Spontaneous micromagnetic oscillation in a thin-film head

Reported are experiments on a spontaneous micromagnetic oscillation found in a thin-film tape head excited by a typical write current. The effect is revealed as an additive low-frequency signal (154 to 300 KHz), written into the tape when writing with a higher frequency (e.g., 2.5 MHz). It is shown to be associated with writing by cross reading/writing with the two heads. The source is micromagnetic as demonstrated by detecting an induced EMF in the coil when exciting the head outside the tape path.