MACHINE VISION SYSTEMS SUPPLEMENT FEATURE LET WITH A GOLDEN HAMMER
poor aspect ratio codes and can compensate for non-existent items on the code itself such as missing quiet zones and unprinted pixels. Matrix cameras are very similar to those used in smartphones, with a lens which focuses the image onto a receiver comprising many pixels. For consistent reading, the smallest bar of the barcode should cover a minimum of two or three pixels in the receiver. So if the width - the X dimension - of the smallest bar in the barcode is too small, the code will need to be read closer to the camera than if the X dimension is larger. As the barcode gets further away from the camera, the light needed on the code increases according to an inverse square law. A lot more light is needed at larger distances - too bright and it could be disturbing to production operatives or disrupt other sensors. Matrix cameras capture multiple
images of the code as it passes by. The ‘Frame Rate’ needs to be balanced with the speed; too fast and the camera processor will have insufficient time to decode the data before the next barcode arrives. Too few frames read and the scanner will not ‘see’ the code properly; the ‘just right’ zone is usually between one and three frames per label. Decoding time can also be affected by
the print quality and contrast ratio parameters set. For a typical black on white code, the contrast and print quality
would be set at ‘good’ or 50 per cent as a starter; below this level of contrast, print marks would be ignored and not decoded, reducing the amount of time taken to decode the label.
LINE SCAN CAMERAS Instead of focusing an image onto a matrix of pixels, in a line scanner, the configuration of lens and pixels results in a 1mm line scanning across the light- sensitive read area. A much higher frame rate is possible with the image data building up a picture as it is scanned. The reading width, length and range of line scan cameras are generally greater than laser scanners. With its sensitivity and the ability to
cope with a range of light levels, a line scan camera is at the top of the range in terms of performance and cost, with a price differential of four times or more over other devices. The range of applications can include telecodes and Optical Character Recognition as well as 2D and 1D code reading. Even ‘no read’ occurrences are also recorded as an image for print and maintenance checking.
LASER READERS
Laser scanners use a laser beam directed onto a rotating polygon and reflected onto the code where it is read. Some devices use Time of Flight technology to measure the distance between the laser and the code, to achieve precise focus.
There are a few things to take into consideration when choosing the best scanning technology for your application
Although eye safe, the laser spot is very
bright and the polygon scanning action over the code is a rapid 1,200Hz. The high power of the laser and its tightly focused spot enables reading over long range without disturbing operatives. To achieve omnidirectional code reading,
two or more laser scanning heads are required. They offer a wider and better depth of field than matrix cameras of the equivalent price, but perform less well where they is a very low contrast, poor aspect ratios or badly-printed codes. For basic 1D code reading function laser
RFID being used for a bakery application
cameras provide a low cost option with an excellent depth of field and large reading field width. They are not sensitive to external light pollution and don’t need additional illumination. Codes can be read easily whether the object is still or moving. Having identified the right auto-ident
technologies, achieving optimum communication between devices is the next step. To begin with, choosing common interface hardware such as Ethernet, Fieldbus or Serial is advisable and will save commissioning time. Using a common configuration software tool like SOPAS from SICK, allows quick and consistent commissioning of all types of auto-ident devices. To achieve faster communication and
eliminate errors in data transfer between devices, the unique ID-Pro platform from SICK is the last piece in the jigsaw. The ID-Pro platform achieves seamless connectivity between laser barcode scanners, 2D/3D vision cameras and RFID interrogators. It can ensure that all product identification equipment in a manufacturing or logistics environment works smoothly together without interruption.
IF I HAD A HAMMER… As good as a hammer is, you need all the tools in a toolbox to achieve the right solution. No one device suits all applications. With the most up-to-date software solutions, it is now much easier to integrate a wide range of auto-ident solutions in a single network. The most impartial advice is likely to
come from a vendor offering all three technologies, especially where they can become part of a seamless communications network.
SICK (UK) T: 01727 831121
www.sick.co.uk
/AUTOMATION MACHINE VISION SYSTEMS | OCTOBER 2015 S19
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