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  • Vision-enabled Barcode Readers

    Pixel grid showing PPM on a DataMatrix code using a vision-enabled reader

    Vision-enabled readers essentially consist of a digital camera that acquires a picture of the code. Then a microprocessor, running special image-processing software, locates and decodes the code before distributing the resulting data across a network.

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    Image Resolution

    One of the biggest differentiators for choosing an image sensor, or camera, is resolution. Resolution refers to how many individual pixels make up each image.

    When it comes to matching a vision-enabled reader’s resolution to an application, one of the most common criteria is pixels per module (PPM). PPM refers to how many pixels it will take to cover one cell or module of the code, and will confirm whether the camera has enough resolution to read the code. PPM is calculated by dividing the camera' s resolution in one direction (for example, 752 pixels for a standard resolution reader) by the Y-field of view in millimeters (78 mm), and then dividing the code size in millimeters by the modules (12 mm/22 modules). Finally, multiply these numbers together (5.26 PPM). It may sound complicated, but a configurator app or image-processing software running on industrial vision-enabled code readers can quickly calculate PPM. While the industry standard is a minimum of 1.5 to 2 PMM, Cognex's latest Hotbars IITM algorithm lowers that requirement to just 0.8 PPM.

    Liquid lens technology

    Lenses

    An vision-enabled barcode reader's optics are key for acquiring a good image of the code. Quality vision-enabled readers offer both S- and C-mount lens options, depending on the amount of resolution required at a given working distance to acquire an image of the code. The latest vision-enabled readers from Cognex also offer liquid lens technology, which allows the reader to adapt to changes in working distances between a fixed-mount reader and a product on a conveyor, for example. Liquid lenses use electrical charges to change the shape of the interface between two different liquids, water and oil. This bends the light and brings the image into focus. Unlike traditional zoom lenses, liquid lenses do not move or use motors and therefore are much more robust than mechanical or spinning optics.

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    Lighting

    Lighting also has an important part to play in acquiring a good code image. Industrial vision-enabled readers typically come with lighting options to accurately read any code from printed labels to recessed dot peen codes. Cognex vision-enabled handheld readers offer all three types of primary lighting: bright field, dark field, and diffuse dome lighting. Bright field highlights the marks that make up the code, while dark field refers to low-angle light that highlights the areas around the code marking, and is best for reading dot peen and recessed codes. Diffuse dome light is best used on reflective and curved parts, as it reduces hotspots and generates a high-contrast image. Cognex fixed-mount readers offer integrated lighting options as well as external lighting options.

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    Lighting technology

    Communications

    Finally, best-in-class vision-enabled readers offer a full range of industrial communication protocols including Ethernet, USB, RS-232, discrete I/O, Ethernet/IP, PROFINET and Modbus TCP/IP. This simplifies integration between reader and plant network, which is critical not only for reading and sending product tracking information, but also for storing archived images in the event of a rare no-read or misread. Best-in-class vision-enabled readers allow the failed image to be analyzed and stored for later review, and alerts to be issued to operations staff that a code printer isn't printing with sufficient contrast, or that a dot peen head needs to be replaced, for example. This ability to evaluate performance in real time and provide statistical process control (SPC) is another benefit of image readers over their laser scanner predecessors.

    Cost of ownership

    With the additional power and flexibility of vision-enabled readers, industrial customers might expect vision-enabled readers to cost considerably more than laser scanners. While that was true in the past, the latest vision-enabled readers cost about the same as industrial laser scanners that have far less functionality. New microprocessors and CMOS digital sensor chips also mean vision-enabled systems can be nearly as fast as the fastest laser scanner. And these developments come in addition to the traditional benefits of vision-enabled readers: no moving parts, resulting in a longer life than lasers; ability to read damaged and omnidirectional codes; and ability to store images for audits and tracking, and to monitor code marking systems.

    Communication protocols
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