Reading codes and labels
Published: 14 October, 2010Standfirst: Coding, marking and labelling have become an essential component of modern production technology. Correct packaging labelling is critical for consumer safety, especially in the food, beverage, pharmaceutical and medical industries and, of course, for ensuring that the correct spare parts are available for maintenance. In addition, “mark and read” applications are used for product identification and traceability, especially in safety-critical industries such as aerospace and automotive. Don Braggins, UK Industrial Vision Association reports. Tracking a component and all the processes it has gone through, from manufacturing, assembly right through to end-user requirements for spare parts replacement (from the cradle to the grave) is becoming an essential requirement in complying with industry guidelines and standards as ERP, MRP and quality assurance systems become more widespread throughout the manufacturing supply chain. Coding can facilitate inventory management. Coding and labelling usually takes the form of alphanumeric codes (e.g lot details and best-before information), barcodes and the more recently introduced 2D codes. Products and packaging can be tagged either by a stick-on label or by information printed directly onto them. Label-free direct marking methods are intended to survive the most unfavourable of production and operational processes, as well as environmental factors. Practical applications The complexity of the code reading solution required for date codes, lot codes, stock keeping units, serial numbers and other text based codes depends on a number of factors. These include: •\tRegularity of code or label orientation on the product or package •\tPosition of the code on the product (irrespective of orientation) •\tRegularity of the codes or characters (e.g. are the characters stretched or skewed?) •\tIs there damage to the code? •\tThe contrast between the code or characters and the background •\tThe code/background colour combinations •\tReflections from surrounding surfaces •\tThe space available for the code reader •\tAny interface required to a reject mechanism and/or the rest of the process •\tSpeed of measurement The sheer variety in coding types and labelling methods places high demands on industrial code readers. Marking Marking methods include lasers, dot-peening, electrochemical etching, and ink-jet printing. The appropriate technique is chosen according to the application, material composition, surface texture, the amount of data that the mark needs to contain, the available space for printing, and the mark's location. In practice, many codes are in poor condition when they are read. This may be due to normal wear and tear during the life of the product, or may even be due to poorly maintained marking equipment. Fortunately, vision based code readers are well-equipped to deal with code defects. Code reading The basic process in reading codes is to: illuminate the code, locate the code, and then extract the data. Checking a stick-on label on a packet or carton in a warehouse presents a significantly different challenge to checking a code dot-peened onto a small automotive component as it progresses along a production line. Industrial vision code-readers provide an alternative to the more conventional laser scanners. A 1D bar code has a 'check sum' built in so that a laser-scanning type of reader can tell if there is an error in reading it, but can do nothing about it except to say 'unreadable'. A vision system has a better chance of reading a 1D code because if any part of a bar is intact, it can use that information. Vision-based readers tend to be used in circumstances where it is simply not acceptable to receive a 'cannot read - check sum not correct' from a laser scanner. Vision-based code readers fall into two basic categories: dedicated code readers, both for 1D and 2D code reading, and systems based on industrial cameras. Both could vary from a hand-held code reader to a dedicated system fully integrated into a manufacturing control system. Vision technology Dedicated vision-based code readers generally feature an integral illumination source and lens and an image of the code is produced using a CCD sensor. They feature onboard software to process the image and read the code. 1D codes are less robust than 2D codes but vision-based readers can be effective in reading low contrast or damaged codes or those distorted by severe perspective. Since these readers generate an actual image of the code, it is possible to archive images of read failures and download them to a PC in order to diagnose the root cause of a problem. 2D codes allow much more information to be stored than is possible with a 1D code and in a much smaller space. The 2D Datamatrix code format includes built-in ECC220 error correction to allow the recognition of codes that are up to 60% damaged, making it particularly popular for direct part marking. Some code readers now support the ability to read multiple codes in the field of view, including different code types (1D/Stacked, Data Matrix, QR) at the same time. The ultimate in code reading flexibility is offered by using a fully functional industrial vision system equipped with code reading and optical character recognition (OCR) and optical character verification (OCV) software. These differ from the dedicated code readers in that they feature a full camera and lens, often with an external illumination source. Images of the code are recorded for subsequent processing and measurement. The camera may be connected to a PC where the processing is carried out, or smart cameras can be used which are self-contained, standalone vision systems with built-in image processing capabilities. A third option is the compact vision system, where all the processing power is contained in a rugged, DIN mountable enclosure, instead of the camera head. With a comprehensive choice of camera types and connectivity standards available, industrial vision systems can be chosen for a huge range of industrial environments. Code reading and OCR software is available from a number of different suppliers. OCR uses sophisticated pattern recognition algorithms. As barcodes have become mandatory in some industries, so their quality has needed to be analysed. There are a number of grading standards relating to different industries. 1D grading usually specifies a minimum contrast for a colour of illumination in addition to print quality. 2D codes have far more parameters such as the squareness and uniformity of the grid and it may be important to check that a particular software package supports the required grading standard. Producing an image of the code or alphanumeric with the best possible contrast can often be dependent on getting the optimum illumination conditions and the development of high intensity LED light sources has helped enormously. An added benefit of using more powerful vision systems is their flexibility, since they can carry out a host of inspection tasks as well as reading the code. For label applications, this can include checking that the proper label is on the package and that the label is correctly positioned, as well as looking for consistency in colours and logo positioning for brand image purposes.