Targeting 2-D code reading and verification

By Tim Pastore

Reading of two-dimensional (2-D) codes directly marked on parts is a big emerging application area for machine vision in general and smart cameras in particular. After parts have been marked, they can be tracked as they pass along the manufacturing process using smart-camera readers. Although symbols such as data matrix codes offer built-in error correction, which can compensate for as much as 20% of possible damage to the mark, consistently high read rates can only be reliably attained if a mark of sufficient initial quality is made and if the mark quality remains consistent throughout the manufacturing process. To accomplish this, a mark verifier-as opposed to a reader-is needed to show that a mark can be read and to determine the mark quality.

In 1996, the Association for Automatic Identification and Mobility published a set of criteria, known as the AIM Bar Code Print Quality Guideline, to allow verification systems to grade a symbol by degree of acceptability (A to F). Developed for high-contrast paper labels, this type of verification provides a basic quantitative measure of print quality and allows printed 2-D codes to be checked against a quality standard.

Dot-peening is an effective way to mark Data Matrix codes on automotive parts. In this example of a readable, but poorly marked, part, the dot size changes due to pressure variations. Using the AIM print quality verification standard for grading, this mark receives perfect grades (left). DPM Verification running on a HawkEye DPM Verifier can detect the problem and give the mark a much lower grade (right).

In direct-part-marking (DPM) applications, both the marking process and the material marked are important. In some plastic or metal laser-marked parts, for example, the Data Matrix marks may be very low in contrast. Such marks should not be rejected on this criterion alone, since no higher-contrast marking may be attainable, and direct part mark readers should have no problem reading such low-contrast marks.

Additionally, the AIM print growth measurement only shows if the data matrix cells are too large or too small along the border of the code, not inside the data matrix itself, and cannot directly detect any cell misplacement. For example when using a dot-peen marker, any movement of the part and any wear or inconsistent pressure or motion of the stylus may result in dots that are fused together or incorrectly marked within the code.

Special standards created

Overall, the AIM print-quality standard either causes false alarms or misses important marking problems when applied to direct part marks. For these reasons, both SEMI and the International Aerospace Quality Group (IAQG) created special mark-verification standards to determine direct part mark quality for laser-marked silicon wafers and dot-peened aerospace parts, respectively. The IAQG’s standard allows for the dot-placement accuracy and angle of distortion of the mark to be measured. Both of these standards address some of the limitations of the AIM specification, but are not broadly applicable to different types of materials, marking process, or industries.

Recognizing this, RVSI Acuity CiMatrix has developed a set of direct-mark quality measurements that are more widely applicable to the requirements of specific processes and applications. Until recently, this DPM verification technology was only available on PC-based systems (DMxAutoID+ high-performance readers/verifiers and DMxVerifier+ off-line verifiers).

At the May 2004 Vision Show East (Boston, MA, USA), RVSI demonstrated its HawkEye DPM Verifier, a smart-camera reader with built-in DPM verification capabilities. The system also supports legacy AIM, ISO, and IAQG verification standards. Using such a smart-camera-based verification system, full DPM verification can be implemented at each marking station to guarantee good initial mark quality, at the entry point of each line to ensure that marks can be read, and at the end of each line to guarantee that marks can be read at any follow-on production steps.

TIM PASTORE is a business development manager at RVSI Acuity CiMatrix, Nashua, NH, USA; www.rvsi.com.

Company Info

Association for Automatic Identification and Mobility, Warrendale, PA, USA
International Aerospace Quality Group www.iaqg.sae.org/iaqg
RVSI Acuity CiMatrix, Nashua, NH, USA www.rvsi.com
SEMI, Washington, DC, USA www.semi.org