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Automation Technologies 2/2016

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Automation Technologies 2/2016

Human eye vs. electronic

Human eye vs. electronic eye in sheet metal inspection Erischsen, the specialist for universal sheet metal testing devices, in collaboration with Stemmer Imaging, has developed an imaging module for routine sheet metal inspections. The module was developed according to ISO 16630 in order to further automate the inspection process, to eliminate the human error. These measures are the foundation for Industry 4.0 concepts and cloud applications. The cupping test, the first ever test method for sheet metal, was patented in 1913 by the Norwegian A.M. Erichsen. This test method was used all over the world and formed the foundation for the success of Erichsen GmbH’s universal sheet metal testing devices. Why sheet metal testing? Focusing on passenger safety, cars are developed in a way that, maximum possible kinetic energy will be absorbed by deformation of car body parts outside the passenger compartment in case of a crash. Various profiles such as bumpers, door sills, lateral and transversal supports etc. are made from sheet metal with optimum thickness in order to balance the safety and weight of the car. For deepdrawn parts such as soda cans, cartridges, nozzles, etc. the sheet metal must retain a uniform wall thickness and must not crack during the manufacturing process. In all these cases, the raw sheet metal is tested prior to production to prevent rejections. Relevant data for this cannot be obtained using simple tensile tests for determining the yield strength. Specimen preparation and test procedure The ‘hole expansion’ procedure according to ISO 16630 is an increasingly important test in the area of sheet metal testing. The specimens have to be manufactured before the actual test can be conducted. A hole with a diameter of 10 mm is punched into the sheet metal specimen. After inserting the sheet metal specimen, a conical steel spike with an electro-hydraulic drive is forced into the centre of the previously punched hole from below in the direction it was punched and expands it until cracks start to form on the inner edge of the punched hole that was created with the steel spike. How imaging application makes a difference? MACHINE VISION The formation of cracks is conventionally monitored by the operator who sees the specimen from above, stops the machine manually as soon as cracks start to form and subsequently measures the diameter with a calliper gauge. However, there is a better option - using machine vision: no involvement of personnel, no dependency on mood and physical condition, uniform visibility. The test procedure with its precisely defined parameters is optimized with a reproducible mode of observation using machine vision, which leads to increased reliability of results, increased accuracy and reproducibility, to narrower tolerances in the evaluation and a testing procedure that is fully documented in images and measured values available in digital format. The electronic eye Ludger Wahlers, general manager of Erichsen, an experienced user of vision systems, came up with the idea of integrating machine vision in their sheet metal testing devices during a customer meeting. An initial approach to a solution was outlined quickly. The concrete implementation, however, was a bigger challenge. “Having a good idea is one thing; the implementation using conventional components is an entirely different story. You need a supplier with whom you can discuss different variants in detail. I was looking to procure 30 AUTOMATION TECHNOLOGIES 1/2016

02 Sheet metal specimen with a deep crack after hole expansion testing: with machine vision technology, cracks can be detected at a stage where they are hardly visible to the human eye while the results are fully reproducible 01Ludger Wahlers, Managing Director of Erichsen, explains the advantages of industrial machine vision in sheet metal tests using the hole expansion procedure 03 The camera image of this specimen shows cracks in the upper right and lower left area the entire imaging solution from a single source”, Wahlers describes his situation back then. At Stemmer Imaging, Wahlers found the single source supplier for his application. Florian Mayr, an experienced sales engineer with Stemmer Imaging, was able to quickly select components from the company’s extensive product portfolio that were suitable and within the planned budget. Concrete implementation The final installation can be seen in the lead photo. The metallic surface of the specimen to be observed is illuminated with a diffuse ring light with low reflection properties. The test mandrel moving vertically upwards by up to 40 mm during the test procedure was a challenge during the design phase. The ring-shaped hole edge in the sheet metal must be observed in this entire relatively wide range without any distortion in order to get images that can be used for precise diameter measurement. These circumstances made it clear that an optimal solution was possible using a telecentric lens, in this case supplied by Opto Engineering. The camera takes over the work that was previously done by the human operator. A monochrome area scan camera model with a GigE Vision interface from the Manta series by Allied Vision was selected. At 30 images per second, the capture speed of this camera is slightly superior to the human eye. While the resolution of approx 30 µm/Pixel is significantly greater than the human eye and hence ensures reliable detection of cracks from 150 µm. The selected camera has the additional advantage of two integrated and freely programmable I/Os. These are used to control the PLC in the test device. As soon as the image processing software detects the beginning of a crack formation in the hole edge at a typical draw speed of 10 mm/min, a signal to stop the machine is sent immediately. Subsequently, the inner hole diameter is determined using circle fitting. Crack detection and measurement are executed using image processing tools of the software from Teledyne Dalsa, which is used for inspection tasks in various industries. Outlook Erichsen and Stemmer Imaging have jointly developed an approach to imaging in routine sheet metal testing according to ISO 16630 in order to improve the repetitive accuracy of the test procedure and the reproducibility of the test results. “This significantly improves the reproducibility of a test conforming to standard. Thanks to digital records of the test procedures in the form of measurement data and image series, users can evaluate the test data and use them for improving their own product quality. I am certain that this data will open up entirely new possibilities for sheet metal testing with regard to Industry 4.0 concepts and cloud applications.” says Wahlers with great satisfaction. Photographs: Stemmer Imaging GmbH/Weissler About Stemmer Imaging The company is Europe’s largest independent supplier of vision technology and services to industrial and scientific OEMs and system Integrators. The product line-up encompasses all that is needed to design and build a computer vision solution including optics, illumination, cameras, interfaces and software. Stemmer Imaging can supply several thousand different products many of which can be delivered within 24-hours. A high percentage of the company’s 250 staff are engineers enabling them to deliver true competitive advantage to our customers. AUTOMATION TECHNOLOGIES 2/2016


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