Direct part identification

How can plant engineers ensure that a part is easily identified by machine vision systems? Justin Testa, senior vice president of ID Products for Cognex Corp., suggests these considerations:

• Code selection — The ANSI code standard is the most widely supported for DPMI applications involving metal, glass, ceramic or plastic materials. Data Matrix offers a number of advantages for DPMI applications, including small size, high data encoding capacity and error correction that allows a code to be successfully read even though as much as 60% of the code may be damaged.

• Available space — Limited space may require the Data Matrix code be used as a ‘license plate’ to simply identify the part, reducing the amount of data encoded. In this case, a centralized database containing manufacturing and historical data on the device is updated as it is identified during manufacturing and supply chain processes. When space isn’t an issue, users may encode much more information about the part, creating a ‘portable database.’

• Marking processes — The primary methods for producing machine-readable symbols for DPM ID include: dot peening, laser marking, electro-chemical etching and ink-jet printing. Your choice depends on part life expectancy, material composition, environmental wear and tear and production volume. Other considerations include: surface texture, amount of data to be encoded, and the available space and location of the mark.

• Mark placement — The location should be clearly visible throughout the manufacturing process; it’s best to mark on a flat site. Choose a location where the mark is in a prominent position easily viewed by the reader. Avoid locations where a surrounding surface relief could potentially affect the illumination of the code by the reader’s lighting source. Provide a ‘clear zone’ around the mark where no features, part edges, surface texture or other interferences come into contact with the code. If a mark must be placed on a cylindrical part, take care in selecting the size of the code. Curved surfaces can create distortions to the code and adversely affect illumination.

• Readability — Features that comprise the Data Matrix symbol are the quiet zone, the finder pattern, the clocking pattern and the data region. Each individual element is referred to as a module. Robust and reliable code reading requires a pattern with individual modules consistent in shape and size with each other, and distinctively different in shape and size from other features on the part’s surface.

• Verification — A verification system can immediately detect a mark-process problem, which could be due to poor part fixturing; damage to the machine, such as a broken tip on a dot peen machine; or incorrect settings during part changeover. Additionally, a code verification system can also provide a process feedback on the marking process that can be used in preventive maintenance. For example, the verifier can monitor tip wear on a dot peen machine by monitoring dot size and flag the operator on the floor when a pin should be changed.

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