Feature Article

Marking and Vision Technologies: Forming a More Perfect Union

Posted in Radio-Frequency Identification by Brian Buntz on June 1, 2010

Regulators and end users alike demand product traceability. The right combination of marking techniques and machine vision systems can help to achieve that.

The rise of traceability
Handheld code readers provide a high degree of flexibility.
As consumer demands continue to spiral upwards and stringent legislation and regulations blanket all aspects of manufacturing, the importance of tracing products from “cradle to grave” is paramount. Traceability not only appeases regulators, but satisfies end users who receive products to the agreed specifications and are able to track any part’s history, should issues arise in the future.
With this in mind, both marking and vision technologies have continued to evolve at a considerable pace and the use of 1D and 2D codes is now commonplace. A substantial increase in 2D usage has been seen within the medical device sector, where the benefits of code marking, reading and traceability are being realised.
It starts with the code
To ensure traceability, the first essential ingredient is a code. Typically, 1D bar codes encode only numerics, whereas 2D can encrypt alpha-numerics (up to 150 alpha-numerics are standard in a 48 × 48 cell, 10 × 10 mm) for a printed label. Usually 1D bar codes operate against a “look-up” table, or database: a unique serial number is encrypted within the code and referenced against a database. By comparison, all 2D code data can be encrypted within it, allowing for full traceability with or without access to the database.
The Data Matrix star
The 2D Data Matrix code ECC200 emerged as the industry standard, as it allowed essential information to be included on a product, thus ensuring full traceability, whilst dealing successfully with space issues. Significantly smaller than a standard bar code, the versatile nature of the Data Matrix code propelled it to the forefront of product traceability. This code uniquely identifies each product or part manufactured. A digital imprint marked directly on a part surface ensures internal traceability on the production line and external traceability during the entire life cycle of the product. Primary tasks of the Data Matrix code are to ensure error-proofing, part traceability, part authenticity, and supply-chain management. The technology is used across a range of medical devices including surgical instruments, pacemakers
and medicine bottles.
Marking options
The primary methods used to produce Data Matrix codes for direct part mark identification include dot peening, laser marking, electrochemical etching, ink-jet printing and key dot marking. Important factors influencing the marking process decision include part life expectancy, material composition, environmental wear and tear, and production volume. Other considerations include surface texture, the amount of data to be encoded on each part, as well as the available space and the location of the mark on the part.
Dot peening is achieved by pneumatically or electromechanically striking a carbide or diamond tipped stylus against the surface of the material being marked. The technique is widely used in the automotive and aerospace industries because of the demanding life-cycle requirements.
Laser marking applies heat to the surface of a part, causing the surface of the part to melt, vaporise or change in some way in order to produce a mark. The resulting quality depends upon the interaction of the laser with the material it is marking. A laser can produce both round and square modules and offers high speed, consistency and a high level of precision. Laser marking is widely used in the semiconductor, electronics and medical device industries.
Electrochemical etching (ECE) is a process whereby the mark is produced as a result of the oxidation of metal from the surface being marked through a stencil impression. ECE is recommended for round surfaces and for stress-sensitive parts, and is often used to mark medical devices.
Ink-jet printers precisely propel ink drops to the part surface, after which the fluid that makes up the ink dot evaporates, leaving a coloured dye on the surface of the part creating the pattern of modules that make up the mark. Ink-jet marking provides fast marking of moving parts and offers very good contrast.
Key Dots are small sticky labels with a Data Matrix code printed on the surface. They come in a variety of sizes and materials, and are affixed to the product.
Code reading
Once the code is marked on a part or product, it is of little use unless it can be accurately read by a device. This is
where machine vision takes up the reins and ensures that full product traceability is achieved. In a typical manufacturing application, the marked part passes in front of a vision sensor and an image of the Data Matrix code is captured and processed using specialised image preprocessing and identification algorithms. Using this technology, code reading performance is unaffected by low contrast or poorly formed codes, which can result from marking issues or general wear and tear of the product.
In addition to reading the data stored on the code, the sensors can also provide production process feedback on the quality of the specific marking to ensure products are marked with the highest quality 2D codes. Perfecting the quality of the codes to eliminate waste will lead to improved overall production efficiency and reduced operating costs.
Code reader options
Most machine vision systems are integrated into the production line in the form of fixed-mount sensors, which identify parts that are handled and moved automatically by a conveyor, indexer or robot. In operation, this type of reader is mounted in a fixed position where the mark can be repeatedly placed in front of it in continuous or indexed motion. Readers often can be configured with an integrated or external light source, as required.
To allow maximum flexibility, advanced ID code readers are also available as hand-held devices. These types of readers are particularly useful in hospitals where, for example, surgical instruments must be scanned before and after surgery.
Although both 1D and 2D readers are used in medical device manufacturing environments, 2D area-based imagers are the most popular. The main reason for this is that 2D readers are future proofed because they can read both 1D and 2D codes, whereas 1D laser scanners can only read 1D bar codes. In addition, 1D laser scanners (the preferred method for bar code scanning) suffer from the need to have 80% contrast between the foreground and background. By comparison, 2D code readers can read at 20% contrast levels (and sometimes less) and they are unaffected by code rotation. All of these factors should be considered when investing in code reading technology.
Optimised combinations of marking and vision systems are a significant tool manufacturers can use to overcome everyday production challenges whilst reaping the substantial benefits of product traceability.
Leigh Jordan
is Senior Sales Engineer at Cognex UK Ltd,
43 Caldecotte Lake Drive, Sunningdale House, Caldecotte Lake Business Park
Caldecotte, Milton Keynes, Bucks MK7 8LF, UK
tel: +44 1908 206 000
e-mail: leigh.jordan@cognex.com


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