Talke Lab Head/Tape Interface Research

Modern tape drives in the computer field are useful as inexpensive backup devices for the home user as well as terabyte-capacity, robotic tape libraries for the large industrial or governmental user. There are two basic kinds of tape transports: linear systems (as in a music cassette drive, see Figure 1) and helical-scan systems (as in a VCR, see Figures 4 and 5). In linear systems, the head is basically fixed and the tape moves in a straight line past the head. In a helical-scan system, there are one or two stationary cylindrical drum sections with a rotating section which carries two or more heads. The tape is wrapped around the drums at a small angle causing the head to make slanted tracks on the tape.

Linear			Helical-scan
Figure 1	Figure 2	Figure 3	Figure 4	Figure 5

In high density magnetic tape recording systems, the typical head/tape spacing is on the order of 30-60 nm. At spacing of this order of magnitude, asperities on the medium surfaces are in contact with the magnetic head, thereby causing wear of both the head and the medium. Wear reduces the life of the magnetic head/medium interface and is undesirable from the point of view of system reliability.

Current research in the Talke tape lab consists of two topics closely related to the increasing of track density. The first is a theoretical and experimental study of the way tape moves in a drive, in particular, we are interested in how the tape moves laterally, perpendicular to the axial (long) tape direction. This lateral tape motion is a limiting factor for how thin the track widths are allowed to be. If lateral tape motion exceeds 10-15% of the track width, read and write errors will increase dramatically. Experimentally, this motion is measured with either an optical probe, or magnetically.

The second study is also related to increases in tape track density, but also to the current decrease in tape width (<9um). It is an experimental and theoretical investigation into the wear of the tape edge. Some current tape drives use small, usually ceramic 'pinching' guides to keep the tape from moving laterally when close to the head. These guides press on the edge of the tape with a small force and cause the edge to wear. With increased usage of tape and track widths getting smaller, the possibility of edge wear becoming significant is there. At the sub-micron track width level, a whole new array of challenges meets tape in the next 5-10 years.

Previously, the mechanics and tribology of both kinds of tape drives have been studied in the Talke lab using both numeric simulations and experimental investigations. The interface between the head and the tape is governed by the interplay of fluid dynamics mechanisms generating a pressure between the head and the tape which tends to push the tape away from the head and the tape's mechanical behavior as a thin shell which reacts to the pressure load as well as to the tape tension and inertial forces. Bleed slots (Figures 2 and 3) are designed into the head to control the pressure generation and maintain head/tape contact. The actual contact of the head and the tape is controlled by the behavior of surface asperities on the tape, which maintain a mean separation of the head and tape on the microscopic level even when the two are nominally in contact.

In order to determine the compliance of the medium surface, three-color interferometry was used. Elastic asperity compliance curves were obtained to evaluate changes in surface roughness due to burnishing, determine mechanical properties of flexible medium and analyze differences between single layer and dual layer metal particle tapes. The spacing of the head/tape interface was measured using replica glass heads and three-color interferometry. Glass heads with different island widths and zero and nine-degree skew angles were used to investigate the effect of head contour on the head/tape spacing. In addition, different metal particle tapes were used to study the effect of tape compliance, tape speed and tape tension.

Wear of the magnetic head materials and magnetic tapes was also studied using wear tests with wear bars as well as nano-scratch test. The nano-scratch tests were found to be useful in characterizing wear of various head materials. Hardness of head materials was also measured using the so-called "nano-indentation" method. From the results we have observed the microstructure influences the hardness substantially. Some head materials with high hardness were found to exhibit higher wear rates than head materials with low values of hardness.

Below are descriptions of some of the projects we are currently engaged in.

• HIGH FREQUENCY LATERAL TAPE MOTION (>1000Hz)
• LATERAL MOTION AND EDGE WEAR OF MAGNETIC TAPE
• MEASUREMENT OF CROSS-TRACK MOTION OF MAGNETIC TAPE

HIGH FREQUENCY LATERAL TAPE MOTION (>1000Hz)

Researcher: Ryan Taylor, Graduate Student, CMRR & MAE Dept.
Advisor: Frank E. Talke, Professor, CMRR & MAE Dept.

For years, there have been increases in the linear density of magnetic tape. These improvements have enabled developers to delay the necesity for increases in track density. With disk areal densities well above that of tape, tape technology is now pushed to such a point where increasing track density is inevitable. This study is to investigate how the tape is moving. Lateral Vibrations cause motions over 1 kHz that are not trackable by bandwidth limited servo actuators. Lateral motion spectrum measurements are given for tape moving on a porous air bearing guided tape drive.

LATERAL MOTION AND EDGE WEAR OF MAGNETIC TAPE

Researcher: Jason Wang, Graduate Student, CMRR & MAE Dept.
Researcher: Ryan Taylor, Graduate Student, CMRR & MAE Dept.
Advisor: Frank E. Talke, Professor, CMRR & MAE Dept.

Lateral motion and edge wear are measured on magnetic tape, and the relationship between tape edge wear and tape lateral motion is investigated. Instrumentation for measuring tape edge force is described. Tape edge force is measured at the tape guiding position using a force-calibrated cantilever spring and a linear optical probe. Typical measurements of tape lateral motion are given as a function of tape tension, tape direction, and path position. A technique for measuring tape edge wear is introduced and wear measurements are presented as a function of load and the number of passes.

MEASUREMENT OF CROSS-TRACK MOTION OF MAGNETIC TAPE

Researcher: Ryan Taylor, Graduate Student, CMRR & MAE Dept.
Advisor: Frank E. Talke, Professor, CMRR & MAE Dept.

In a magnetic tape drive system, cross-track (lateral) motion of the tape can cause the read/write head to move away from a desired data track and cause a track misregistration (TMR) error. TMR errors are degrading to system performance and so must be avoided to improve the recording density in tape drives. Two instruments, a laser doppler anemometer and an MTI fotonic edge probe were used to measure the cross-track motion of magnetic tape in tape drives. These two instruments are evaluated based upon their bandwidth capabilities, resolution, accuracy, and ease of use. Several measurements of cross-track tape motion are shown and discussed for various drives.