Vision-based inspection of sheet metal parts by comparing CAD data
Eight Teledyne DALSA Spyder3 line scan cameras form the core of an inspection system developed by the Bavaria-based Sturm-Gruppe. The system checks sheet metal parts for compliance with required quality standards by using their CAD data for direct comparison. When selecting the appropriate machine vision components the company strongly relied on STEMMER IMAGING’s profound expertise.
Sheet metal processing is one of Sturm’s core competences. “No matter whether steel, stainless steel, aluminium or special materials – almost everything from 0.5 mm to 20 mm is feasible. We manufacture from the small-scale washer up to welded components with sizes of 10 metres and weights of several tons”, explains Alexander Luft, Production Manager at Sturm.
Processing methods such as laser cutting, stamping, forming, welding, grinding and various other surface treatments as well as the manufacture and assembly of complete components are ordinary tasks for Sturm. “Our clients bring along their drawings and we deliver the finished products – from simple laser cutting to a complex assembly, from lot size 1 to serial production”, summarises Alexander Luft. State of the art equipment is available on almost 11,000 m2 of production space in Salching in Lower Bavaria.
Automated quality control of sheet metal parts
Renowned automobile manufacturers such as BMW, VW/Audi, Renault and Land Rover as well as engineering manufacturers such as Buderus and Siemens are among Sturm’s most important customers. This requires 100% quality assurance and seamless part traceability which almost inevitably needs an automated solution with thousands of parts produced every year.
Sturm’s in-house machine vision division based in Munich has developed a quality inspection system. The »IQS Parts Metrology« measures sheet metal parts within an accuracy of 200 µm, by comparing recorded images of components with their corresponding CAD drawings by means of a sophisticated machine vision application.
Enhanced productivity with machine vision
One part after the other passes through the inspection system on a conveyor belt. At the end of the line the operator is supplied with an image of the inspected part in which possible deviations from the CAD drawing are colour-marked. The part contours which are within the defined tolerances are visualised by green marking and displayed on the monitor. The inspected part is automatically labelled. If the part does not meet the tolerance specifications it is rejected. The total inspection process from feeding to destacking takes approximately 25 seconds. “Nobody can carry out manual measurements so fast”, Alexander Luft praises the enhanced productivity.
The inspection system was developed in-house within 10 months from scratch specifically for the requirements of sheet metal production, with the support of STEMMER IMAGING's profound experience and service regarding the choice and layout of both vision systems used.
“Eight in-line connected Teledyne DALSA Spyder3 line scan cameras form the core of the machine vision system. Each camera offers a frame rate of 18 kHz and a resolution of 4k”, explains Dr. Wolfgang Ullrich, Head of Sturm’s Vision Technologies Business Unit. “However, we only utilise the medium resolution 2k cameras which sums up to a total of 16k in our case and results in an optical resolution of 110 µm per pixel. This way, we avoid the enormous expense of telecentric lenses.”
The cameras communicate via GigE Vision interface. The lighting devices in the system are developed in-house and consist of LED-boards in a special setup. The camera mounts are adjustable in height in order to automatically adapt the focus to the respective sheet metal thickness. This ensures that one pixel of the image covers exactly 110 µm of the metal part.
Identification with the Cognex code reader
The inspection starts with a complete Cognex code reading system, including the In-Sight 7402 reader, a Smart Vision Lights illumination and a 5 megapixel lens, which reads the laser engraved OCR code or barcode on the sheet metal part. A positioning laser helps the code reading system center the parts on the conveyor belt. In this way, the system identifies which test part is on the conveyor belt and automatically searches the Sturm data base for the appropriate CAD drawing for comparison.
Sturm’s vision software experts were facing a major programming challenge when converting the CAD data from dxf format into tiff format which is mainly used in machine vision. Since there was not any appropriate software available on the market at the time Sturm’s in-house machine vision experts had to help themselves. They developed a dxf converter which provides the output data for subsequent computing operations.
Comparison with CAD data
“In principle, the software compares the pixels of the image capture with those of the converted CAD drawing”, explains Dr. Wolfgang Ullrich. “That’s why part positioning and alignment on the conveyor belt doesn’t really matter – the software can distinguish between conveyor belt and sheet metal part.” The result of the complex computing operation is then displayed in colour.
The new inspection system is part of a sophisticated, fully digitised production process: the customer electronically orders the parts via direct connection to the server at Sturm; purchase orders are automatically converted into production orders. After manufacturing the sheet metal parts they are checked as described above, followed by exact documentation, labelling and finally digital archiving of order and production data.
Hardware and consulting service from STEMMER IMAGING
“The task was all but trivial“, explains Dr. Wolfgang Ullrich and praises the many years of a very good partnership with STEMMER IMAGING: “We had used a similar ready-made system before which did not bring the desired success. That’s why we decided to develop our own system. A crucial factor for obtaining this successful internal solution is the optimal hardware, which was provided by our longstanding partner STEMMER IMAGING.”
According to Dr. Ullrich, “Trustful cooperation and competent service are just as important: When it comes to selecting the appropriate products we have always had the best experiences when following STEMMER IMAGING’s recommendations. Their experts have an excellent market overview and profound machine vision know-how and experience to rely on.”
Not only for internal use
After extensive testing the »IQS Parts Metrology« system is now integrated into Sturm’s production process and used for diverse sheet metal processing tasks. But that’s not all: Since Sturm sees itself as a mechanical engineering company and since there is no such inspection system on the market yet, the company now intends to offer IQS Parts Metrology to other sheet metal processing companies.
Sturm-Gruppe employs more than 1,000 people, including about 650 at its Bavarian headquarters in Salching. The company realises worldwide projects in the fields of surface technology, cleaning technology and conveyor technology for the industry and airports, as well as automation and vision technologies. The sheet metal processing unit processes sheet metal parts for both the company’s own mechanical engineering, as well as for external customers with all kinds of methods.