Hewlett-Packard Selects ALGOR FEA to Extend Life of Back-up Tape Drive

	Paul Poorman of Hewlett-Packard, shown above, chose ALGOR抯 MES to predict stress levels in the magnetic tape within a back-up tape drive. This information was then used to develop a solution for reducing stresses in the tape.
	HP抯 2/20 Series Tape Library combines one or two LTO Ultrium drives with a 20-tape capacity for a tape archive that functions like a jukebox for data.
	Inside an Ultrium drive, two rollers stabilize the tape as it winds from a cartridge reel, across the tape heads and into a take-up reel.

Data is any company抯 most valuable asset. Archiving systems are essential to ensuring that valuable data is preserved. A wide variety of technologies exists, but tape medium remains a popular choice for data back-up due to its comparatively low cost and large capacity for data storage. Although tape drives are far from new, Linear Tape-Open (LTO) technology is a recent development that was developed jointly by Hewlett-Packard Company (HP), IBM and Seagate. LTO replaces proprietary formats for corporate back-up solutions with an open tape format makes it easier for customers to choose products. 

In the process of designing tape drives to meet the LTO specifications, HP engineers conducted extensive analysis and testing to ensure that their product would stand out among available LTO drives for its reliability. In addition to extensive laboratory testing, HP Mechanical Engineer Paul Poorman used ALGOR抯 FEA-based Mechanical Event Simulation (MES) software to analyze the behavior of the magnetic recording tape as it is wound through the tape drive in order to find a way to reduce wear on the tape and increase durability.

Defining a Standard

Historically, the back-up tape drive industry has been fragmented, and a proliferation of formats and technologies complicated customer buying decisions. LTO was defined as a best-of-breed, open tape format that can better serve multiple market needs and be supported by multiple suppliers, through an open licensing process. LTO maximizes capacity and performance by combining a linear multi-channel, bi-directional tape format already in common usage. It also adds enhancements in the areas of timing-based mechanisms, hardware data compression, optimized track layouts and high-efficiency error correction code. Customers benefit from this format specification through the availability of multiple sources of product and tape cartridges and common tape format specifications for interchange. 

The Ultrium format is the high-capacity, single-reel implementation of LTO, best suited for back-up, restore and archive applications for stand-alone and automated environments. Cartridges have a capacity of up to 200 GB. HP抯 offerings include the Ultrium 230 ?an ultra-high-performance back-up, storing 200 GB in less than 2 hours with an 108 GB/hr transfer rate ?and the 2/20 Series Tape Library which combines one or two Ultrium drives with a 20-tape capacity for a tape archive that functions like a jukebox for data.

Tape Behavior Identified as an Area of Emphasis

In the course of engineering the drive, one of the key areas of emphasis was tape path stability and tape durability. Since the HP drive uses two heads ?one to write the data and the second to read and verify what has been written ?it is crucial that the tape 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. 揥e 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 Poorman, an engineer in HP抯 Boise, Idaho office working on the mechanics of the drive.

The tape consists of a Polyethylene Napthalate (PEN) substrate with a magnetic coating on the front surface and a static-resistant coating on the back. PEN is a ductile and fairly durable material 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 tape heads need to guide the tape without damaging it as it winds through the tape path. 

In order to study the tape抯 behavior, Poorman needed software that could simulate motion, contact between parts in an assembly, large displacement, elastic material behavior and stresses. These requirements were matched by ALGOR抯 FEA-based MES software, which calculates motion and stresses with built-in linear and nonlinear material models. 

揑 started investigating ALGOR because a fellow HP engineer recommended its engineering capabilities,?said Poorman. 揃y trying out a free demo, I discovered that it not only had the analysis capabilities I needed, but also the easiest user interface I抎 ever seen with an FEA package and many useful post-processing tools. The user interface was an important consideration since I didn抰 have time to spend learning to use the software. I needed to be productive with the software immediately.?

Simulating Tape Behavior

Poorman modeled the tape and drive assembly using Superdraw. 揟he ability to work directly with lines enabled me to build a faster-running model by minimizing complex features and possible contact areas,?said Poorman. 揥orking with CAD geometry would have created a more geometrically complex model than I wanted to work with.?

Poorman 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. 揑n early models, it was important to apply constraints to keep the tape in plane,?explained Poorman. 揂s 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 significant strength to the tape, but Poorman was able to model the tape using the tape抯 composite bending modulus rather than modeling all the layers with a composite element. Since the behavior of the tape was Poorman抯 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, however stresses are not calculated for these elements so processing times were greatly reduced. 

The motion of the tape is controlled with prescribed displacements, which wrap the tape around the rollers, where surface-to-surface contact effects were considered. Nodal forces are then activated on the tape抯 free end to pull it into the take-up reel and create stabilizing tension in the tape. 
 

The simulation shows how the tape wraps around the rollers and is pulled into the take-up reel to create stabilizing tension in the tape.

Focusing on where the tape contacts the rollers, Poorman was able to view the motion, deformation and resulting stresses. 揗ES?calculation of the stress levels enabled me to make adjustments and optimize the design,?says Poorman.
 

In deciding how to optimize the design, Poorman consulted with colleagues via the company intranet, publishing images (JPEG) and analysis animations (AVI) of his results. HP抯 proprietary solution to this problem keeps the tape on a straight path while minimizing stresses on the edge of the tape. The first generation of HP Ultrium drives is currently on the market and performing well. 

Hewlett-Packard Company (NYSE:HWP) is a leading global provider of computing and imaging solutions and services with a total revenue of $45.2 billion in its 2001 fiscal year and 88,000 employees worldwide. Paul Poorman holds a Master抯 Degree in Mechanical Engineering from Stanford University and has over 20 years of experience with disk head design.