THE APPLICATION of a readable bar code is hardly a new concept for manufacturers. The basic bar code business is 30 years old, and construction of 2-D Data Matrix symbology was formalized about 10 years ago.
There are distinct advantages to these 2-D codes over traditional bar codes, and the reasons for using 2-D Data Matrix bar codes are similar to other data capture applications. It’s a method to improve traceability and higher productivity.
“The data file concept, that is, large amounts of data being encoded into the symbology, provides the necessary information for the most complex tractability requirements,” reports Mike Mendicino, bar code and camera systems product manager at Pepperl+Fuchs.
A built-in Reed-Solomon Error Correction provides for a robust control system, adds Mendicino. Reed-Solomon codes are what data-correction specialists refer to as block-based, error-correcting codes. Reed-Solomon first constructs a polynomial from the data symbols to be transmitted, then sends an oversampled plot of the polynomial, instead of the original symbols themselves. Because of the redundant information contained in the oversampled data, it’s possible to reconstruct the original polynomial and then the data symbols, even in the face of transmission errors, up to a certain degree of error. Besides its use in bar code technology, this process is found in a range of digital communication and storage applications, including storage devices such as tapes, compact disks and DVDs; wireless communications including cellular telephones and microwave links; satellite communications; digital television; and high-speed modems such as ADSL and xDSL.
"It's estimated that some 90% of bar code data capture failures are directly attributable to a poorly applied bar code symbol."
“This feature assures a good reading the first time, every time, and facilitates high-speed reading,” says Mendicino.
Typical applications include pick-and-place machines that first must identify the parts they’re picking. The size of the Data Matrix symbols and high data integrity make this an ideal application, especially for marking small parts. Mendicino says, when tracking PCB parts, 2-D bar codes can be very small and even rectangular with symbols placed at the edge of the PCB.
It was usually the machine user’s job to install this equipment on the machine during installation and commissioning, often in conjunction with an SI. Machine users in industries such as those mentioned are beginning to ask machine builders to design and place the bar code applicator and scanner in their machines.
As this trend grows, it’s also important to understand direct part-marking techniques, which also are increasing in adoption. This involves foregoing labels, and applying the bar code directly on the part. Permanency, use, traceability, size limits, and other factors often prohibit using labels.
If the specifier doesn’t take proper care with the application selection process, the readability of the data captured can be jeopardized. It’s estimated that some 90% of bar code data capture failures are directly attributable to a poorly applied bar code symbol.
Direct-part marking can be applied with either an intrusive or non-intrusive technology. “Intrusive markings alter a part’s surface by abrading, cutting, burning or vaporizing a part’s surface, and are considered controlled defects,” states Kevin Grow in a Purdue University technical evaluation. If done improperly, the process can degrade material properties beyond the point of acceptability. Consequently, some intrusive marking techniques, especially laser etching, generally aren’t used in safety-critical applications without appropriate metallurgical testing.”
A non-intrusive, ink-jet marker, which is the result of ink globules sprayed from the printing head onto the part surface, is appropriate as long as permanence is validated.
“The permanence of the mark depends on the chemical interaction between the ink, the part surface, and other materials the part might be exposed to,” says Mike Lewis, co-author of Purdue’s technical evaluation. “When applied to metal parts, ink-jet markings should be coated with a clear lacquer before operations such as oiling.” The report says ink-jet is unlikely to be suitable for parts subjected to immersion in liquids, prolonged exposure to liquid spray or splash, high temperatures, and abrasions.
Laser etching, one of the newer intrusive technologies, applies heat to the part surface to cause substrate melting. The Purdue evaluation indicates excellent results can routinely be obtained at penetration depths of less than 0.001 in. Laser etching is not recommended for parts thinner than 0.05 in., adds the report.