Vision system does it in a flash

Automotive companies are not known for forgiveness in production targets. Ford’s wheel production is no exception.

The production line at Ford’s Dagenham UK plant, where Fiesta wheels are made, has a target of 10000 wheels a day. But two factors were preventing this being reached – the time taken to locate the valve hole position for punching, and the need to prove the integrity of the flash weld.

Wheel rims are made from continuous steel strip which, after pre-stressing and flattening, is cut to length, formed into a circle and then joined by flash welding. Rims next undergo ‘fairly dramatic forming processes’ to create the right profile and are then passed on to have their valve holes punched in a particular position relative to the weld.

Two vision systems made by Cognex have streamlined the QA and production challenges of weld inspections. The vision systems help meet Ford’s stringent QA levels automatically and speed up production by 20%, unblocking what was considered a bottleneck.

Wheels are now conveyed horizontally through the valve hole punch machine. Previously, the position for the valve hole was determined by spinning the rim under a camera, looking for the weld using an optical system and then spinning the rim another 30%. This meant the wheel had to be spun for up to one complete revolution just to find the weld, though in practice the average figure was 210°.

Weld inspections are done immediately after the hole position is located because by then the weld position is also fixed. Previously, weld tests were performed electromagnetically by searching for the eddy current variations caused by a bad weld. But setting-up this equipment was difficult and its positional accuracy needed to be very precise, which meant faults were being missed and an additional inspection process was necessary.

Development work by Ford’s system integrators Cimac confirmed the requirements could be met, so orders were placed. The systems were built off-site and commissioned by Cimac at the next shutdown.

The weld location system uses an In-Sight vision system. A camera is located above the production line, looking down onto each wheel, with lighting arranged to illuminate the inside of the rim which is less likely to be marked by the forming machine. The In-Sight software uses the PatFind tool to locate the rim, then FindCurve to locate the centre of the wheel. FindMultiLine then locates the weld and GetAngle pinpoints its angle. After determining if this is a positive or negative angle the data is sent to the PLC, which rotates the wheel to the correct alignment.

The method allows accurate positioning within an average 90° degrees rotation. Even using the same motor as before this has had a major impact on production rates, reducing the time taken by 20%.

Once the weld position is fixed, the wheel moves to a second inspection point, where the weld is checked by inspecting for light escaping through any faults in the weld. ‘With the weld lined up accurately, it’s easy to do,’ says Chris Greenleaf, Cimac Project Engineer.

‘We use two cameras outside the rim to give us full coverage of the weld and we shine a powerful light through from the inside. The PC-based vision system uses the Cognex 8000 PC Frame Grabber card to capture an image and the PC software checks for pixels of light.’

‘Both inspection systems are fully automatic,’ says Chris Greenleaf. ‘Ford set us the target of 5 seconds overall process time to ensure that the line can be kept in step. This creates manpower savings because there is no need for a buffer line. We have been able to greatly improve the rigidity of the mechanics, increasing lifetimes and decreasing life cycle costs.’

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