The traditional 2D data matrix symbol used for parts identification and tracking within a manufacturing facility has recently come under attack by the Department of Defense (DoD) for reasons related to quality.
The ISO 15415 spec was written for 2-D bar codes with the assumption that marks would be made with high-contrast, even-sized cells to be printed on paper. But suppliers need to laser etch part numbers on metal or other materials, which can degrade the quality of the image and potentially disrupt the supply chain.
To help correct the problem, Cognex Corp. became involved with the Association for Automatic Identification and Mobility (AIM) to help rewrite the direct part mark (DPM) print quality specification. "The problems with 15415 were two basic issues. One was the contrast metric, which assumed high contrast black on white. But when you're doing direct part marking, that's not always true," said Carl Gerst, Cognex's manager of ID product marketing. "It also assumed marks that comprise the code itself would always be square cells, so it looks like a checkerboard. A lot of direct marking methods use square or round dots or maybe a blend in between."
Cognex has presented a concept called "cell separability" that allows the direct part marking verification system -- the system Cognex makes -- to provide a relative assessment of how easy it is to distinguish between a dark cell and a light cell independent of the marking method. The written draft of the spec underwent review late last month (July). It is expected to be issued as a spec by AIM sometime in the fall, Gerst said.
As the industry focuses on the potential of RFID to aid supply chain tracking, it is important to understand the significance of DPM, which is used internally within an organization. "DPM and RFID for the most part are complementary technologies," said Gerst. DPM would be the code applied on a specific part -- a part within an automobile engine such as a crankshaft, for instance -- while RFID tags are put on a crate or pallet that is shipped off to a customer.
The DoD mandates have spurred this initiative, but the new DPM spec will have far-reaching effects across other industries including aerospace, automotive and medical devices.
This article was repurposed from the August 2005 issue of Managing Automation magazine.