How far can tape guide rollers go? Is a 3-piece design the future?

Computer Technology Review, March, 2004 by Gary Collins

Tape continues to be the cheapest, most convenient way to store large amounts of data. Cartridges of 200 and 300GB in half-inch-wide format are now common. Soon, 500GB and 1TB capacities in one cartridge will be available.

Increasing cartridge capacity is directly connected to increasing the track density. However, track density is primarily dependent on tape path stability. Other factors, such as a wide-band servo with the ability to follow a jittery track, only partially compensate for poor guiding. The servo wouldn't need to be so robust if the track stability were more controlled. Therefore, steady tape guiding is fundamental, and the issues influencing it must be thoroughly understood. Rollers have come to play a primary role in guiding the tape, but how far can rollers go?

Long Paths in the Past

In the IBM 3590 era of long tape paths and large tape drive decks (Figure 1), unevenly stacked tape on the reels had a long time to adjust to smooth level motion over the head. Tape was mainly guided from the cartridge reel to the machine reel over a series of air-bearing guides, called D-bearings. This was completely satisfactory for 36-track tape.

[FIGURE 1 OMITTED]

These guides supported tape on a film of air supplied by tiny air holes in the smooth surface and connected to a small air pump. The tape's bottom edge was registered against a guide edge, usually ceramic (Figure 2). On the top edge, ceramic pads on flexible fingers pressed gently down to register the tape against the bottom guide edge.

This worked fine for wide tracks in the region of 250 microns. However, when tracks become 25 microns--as today's LTO, Quantum, and StorageTek drives are--problems arise. Lateral tape vibrations caused by the tape rubbing against the guiding surfaces become significant. These movements are sometimes more rapid than the tracking servo can follow. Movements above 500-800 hertz are common, and many tracking systems cannot follow them. This leads to significant position error signal, or PES. For example, if a tape moves 10 microns and the head can only follow 9 microns of movement, 1 micron of PES is generated.

The trend toward smaller and cheaper drives prompted the abandonment of hydrostatic bearings--that is, bearings with air supplied to them to float the tape in a static mode. Hydrodynamic bearings were substituted, which develop an air film when the tape gets up to speed and flies over the surface. This was thought to be a cost-effective approach because no air pump was needed. Moreover, the air pump was a space liability in the small and compact 5.5-inch form factor of modern half-inch cartridge tape drives. However, guide vibrations and edge wear problems persisted. The embracing of rollers was the result.

[FIGURE 2 OMITTED]

Tape guide rollers had been used in many tape transport systems, especially video systems. Compact rollers were common in half-inch tape video decks of professional broadcast level. Some manufacturers had as many as 13 rollers in the complex helical scan tape path. Because the tape moved relatively slowly, 3-4 inches/second, severe edge wear did not develop. Rollers were thought safe.

Short Paths Now

However, tapes speeded up. Two meters per second, then four, and then six became common in order to have a high data rate and to read a 600m length cartridge tape in a reasonable amount of time. Even more challenging, tape path lengths became very short. Instead of the ample tape path of about 1m long, as in the 3590 family of tape decks, paths were shortened to about 15mm (Figure 3). This forced the tape coming off the reels to meet the first guide roller at 5mm distance rather than 20-25mm. The result was poor guiding and edge wear.

Severe edge pressure came from the flanges at rollers 1 and 4. It is the job of these rollers to reduce lateral excursions that come from uneven wrapping on the reels, called stagger wrap or scatter wrap. If these could be reduced, the problem of guiding tape would be made much easier. However, rollers 1 and 4 cannot do the whole job. Most manufacturers agree that the tape has to be guided by stages. To this end, various vendors have designed rollers with the outer ones different from the inner ones. They are either wider or have a different surface finish.

[FIGURE 3 OMITTED]

Guiding and Flying

The outer rollers push the tape toward the inner rollers or D bearings next to the head. These outer elements do the brunt of steering the tape. The inner elements do the final guiding and also damp out vibrations normal to the tape plane to present a smooth motion over the head.

The tape flies over the outer rollers, touching the surface only occasionally. One can check this out by examining the outer roller rotation with a strobe light to see that it is irregular and not synchronous with the tape velocity. This slipping allows the tape lateral freedom to adjust to the roller flange's guiding pressure. On the inner elements, it is necessary for the tape to adhere to the surface in order to damp out vibrations.