Playing for Keeps

playing for keeps

Designers of a new tape backup format first had to prove that they could get all the data to line up.

This article was prepared by staff writers in collaboration with outside contributors.

Secure archiving systems ensure that a company's valuable data is kept available. And many companies rely on an old standby, magnetic tape, as their backup medium because it holds a lot of information for the investment.

Tape systems are sold by many manufacturers, and there's the catch: They're not compatible. As in many entrepreneurial businesses, competing formats and technologies have complicated buying decisions.

That's why Hewlett-Packard Co., IBM, and Seagate Technology LLC have jointly put forward a format they call Linear Tape-Open, or LTO, a format to be shared through an open licensing process. That way, the reasoning goes, customers can buy hardware and software from several suppliers, and read all their files, regardless of the brand of equipment they were recorded on. In short, the developers hope to make it easier for customers to choose and use data-storage products.

Hewlett-Packard's LTO offerings include the Ultrium 230 drive, which can store 200 gigabytes in less than two hours, and the 2/20 Series Tape Library, which combines Ultrium drives with a 20-tape capacity for an archive that functions like a jukebox for data.

In engineering the drive, a key area of emphasis was tape path stability and tape durability. Since the HP drive uses two heads—one to write data and the other to read and verify—it is crucial that the tape path remain straight so that the second head reads the data in line with the first. Promoting long tape life was a requirement for creating a reliable product. "We tested the product far beyond what customer tapes would see, looking for any sign of wear or damage to the edges of the tape," explained Paul Poorman, a mechanical engineer in HP's Boise, Idaho, office.

Poorman studied the tape system and analyzed increasingly complex models in Mechanical Event Simulation software from Algor Inc. of Pittsburgh.

Hewlett-Packard's engineers simulated the inside of the Ultrium drive with Algor software to work out the mechanics of the new data-backup medium.

The tape consists of a polyethylene napthalate, or PEN, substrate with a magnetic coating on the front surface and a static-resistant coating on the back. PEN is ductile and durable, with a high yield point and good resistance to breakage even after it yields. However, the magnetic coating is brittle and likely to crack before the substrate shows any sign of damage. The rollers that keep the tape stable and traveling straight across the heads need to guide the tape without damaging it.

To study the tape's behavior, Poorman simulated motion, contact between parts in an assembly, large displacement, elastic material behavior, and stresses.

He started with a simple model consisting only of a 1-inch section of tape with many constraints and prescribed displacements to get a feel for the behavior of the tape. "In early models, it was important to apply constraints to keep the tape in plane," Poorman explained. "As I added more realistic physical phenomena, such as the tape tension, many of those boundary conditions became unnecessary." Over the course of several models, he added complexity, extending the length of the tape to about 10 inches and adding rollers, heads, and edge guides.

The tape was modeled using a user-defined isotropic elastic material with the published properties of coated PEN and a shell element with a thickness of 9 microns. The coatings add strength to the tape. Poorman was able to model the tape using its composite bending modulus.

Since the behavior of the tape was Poorman's main concern, he used kinematic elements on all parts of the model except the tape. Kinematic elements for motion and stress simulation behave dynamically like regular solid elements and can transmit forces, but stresses are not calculated for these elements, so processing times are greatly reduced.