01 October 2008
At the Littelfuse manufacturing plant in China, each production line for 5x20mm cartridge fuses manufactures products in any of 500 variants which, except for their markings, are visually identical.To be sure that every fuse meets exacting quality standards, and that no stray products with an incorrect rating are ever included in a batch, it is essential to check the data embossed on both fuse end caps. One of these contains information about the fuse rating, the other about the national and international standards with which the fuse complies.At the production rates involved, manually checking the data would be both costly and unreliable, so Littelfuse decided to adopt an automated solution based on the use of vision systems. There were, however, a number of significant challenges which had to be addressed.DIFFICULT TO READ SURFACESThe first and most important of these was that the data on the end caps is embossed on a cylindrical surface, rather than the flat surface which vision systems typically expect when used in character recognition applications. Another challenge was that the fuses are presented in random orientation, so the vision system had to provide 360 degrree coverage in order to be able to read data at any point around the circumference of the end cap. A further, albeit more minor, complication was that the data to be read is in the form of embossed characters on a shiny metal surface, which are inherently more difficult to recognise reliably than, for example, well defined printed characters on a non-reflective background.For assistance in developing a dependable and cost effective solution, Littelfuse approached vision system expert Peter Hage of Phasor Developments who, in conjunction with Marc Bell, Project Manager at Littelfuse, decided on the use of OMRON vision equipment.EQUIPMENT SELECTIONUltimately, the fuse end cap inspection system was designed around an OMRON F270 vision system which supports simultaneous input from four cameras. This allows the cameras to be positioned to provide the full 360 degree coverage needed by this application. The F270 also incorporates dual high-speed processors, which enable it to achieve the fast throughput required.“The powerful macro programming functions offered by the F270, which, for example, allow advanced computational decisions to be made on the basis of results from specific regions, were crucial to the success of the project,” said Phasor’s Peter Hage, “Without these functions, it would have been difficult if not impossible for us to develop the special—and now patented—algorithms needed to read characters from a cylindrical surface.”“It’s worth mentioning, however, that end users don’t need to get involved with this type of programming,” he continued, “since the vision products offer a second level of programming which is very easy to use, yet powerful enough for making the on-site alterations which may be needed for fine-tuning or to cater for changes in the products which are being inspected.”INSPECTION AND CHARACTER RECOGNITIONIn the Littelfuse application, the character recognition installation is complemented by another inspection station which uses an F210 vision system. This checks the overall length of each fuse cartridge, and inspects the flat faces of the fuse end caps to ensure that they are correctly plated and that they are free from scratches and other blemishes.A final inspection station, this time with an F250 vision system, checks the body of the fuse for marks and cracks.The transport system which moves the fuses through the inspection stages is implemented using OMRON motion controllers. For the end-cap inspection station, where high levels of accuracy and repeatability in positioning the fuses are needed, a high-performance Sigma II servo drive is used. For the fuse body inspection stage, where accuracy is less critical but high operating speed is essential, a cost-effective SmartStep drive was chosen.MODULAR CONTROL SYSTEMOverall control of all vision stations and also the associated fuse packaging machine is provided by a single CJ1 programmable controller. These compact controllers offer fast program execution, and support for up to 2,560 I/O. In addition, their functionality can be accurately matched to the requirements of the application by selecting from a wide range of plug-in option cards.“It was important to us that the design for this project should be modular,” said Marc Bell, “as we intend to use the same design on future production lines, some of which may not need the full range of features. The CJ1, with its Compobus networking facilities, allowed us to design each section of the installation as a separate unit. These could then be linked by Compobus.”Multiple inspection and packing systems of the type described have now been in use at Littelfuse’s manufacturing plant in China for over a year, and a further system is currently being installed. All of the equipment has performed faultlessly and, out of the millions of fuses handled by the lines already in use, not one single defect or error which could have been spotted by the inspection system has been reported by the company’s customers.
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