Keeping track: Evolving standards
Steve Scheiber, Contributing Technical Editor -- Test & Measurement World, 8/1/2005 2:00:00 AM
The move toward tracking parts by marking them directly rather than by relying on labels that may come off during manufacturing is gaining momentum. In fact, the US Department of Defense (DoD) MIL-STD 130 now requires manufacturers to mark parts with a unique identifier. Concurrent with this trend, one-dimensional bar codes are being replaced by two-dimensional Data Matrix codes—a collection of black and white dots or squares arranged in a rectangular pattern—that convey more information in less space.
Initially, companies employed direct part marking (DPM) primarily for internal process and inventory control, so the meaning of the marks differed from company to company. As the use of the DPM technique left the manufacturing plant and moved into the supply chain, an open-standard specification arose to ensure readability and avoid misinterpretation of codes as parts passed from one company to another.
Direct-part marks must be both verifiable and readable by vision and inspection systems. In that regard, says Carl Gerst, marketing manager for Cognex's ID Products business unit in Natick, MA, the new print-quality specification (ISO 15415) as well as the original Data Matrix specification (ISO 16022) made some unfortunate assumptions about DPM verification. The specifications assumed that—like paper-based inked bar-codes—DPM codes would predominantly consist of high-contrast, well-formed square cells of equal size.
Techniques challenge standard
When manufacturers turned to different marking techniques, such as lasers, electrochemical etch, and dot-peen, however, the standards proved less than successful. Laser marks on metal are more likely to appear as gray-on-gray than as black-on-white. As a result, verification systems that base their pass/fail decisions on tight contrast measurements may fail DPMs that readers can read with little difficulty.
In addition, dots may be square or round, depending on the marking technique: Dot-peening tends to produce round marks, ink-jet and laser marks can be round or square, and electrochemical etching tends to produce square marks. Also, with the dot-peen technique, the dots are typically smaller than the blank spaces around them. Standards must take these factors into account.
Although the ISO is responsible for issuing the standards, the organization does not create them. That role falls to the Association of Automatic Identification and Mobility (AIM; www.aimglobal.org). Gerst and representatives of other companies have been working with AIM's Technical Steering Committee to revise the Data Matrix standards along more practical lines.
Revisions on the way
Gerst expects the revisions will be released and implemented sometime in Q3. He also anticipates that these standards will eventually be incorporated into the next revision of MIL-STD 130. Even companies that do not work directly with the DoD will likely adopt the standards.
The good news about the evolving Data Matrix standards is that the changes are being orchestrated from the bottom up—that is, by users of the standards who have assessed their limitations. The resulting changes will serve all interested parties better than standards developed in an ivory tower by people without the day-to-day practical experience.
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