New high speed imaging CMM system aids car production

New high speed imaging CMM system aids car production

New high speed imaging CMM system aids car production

Keywords: Image processing, Inspection, Robotics

Traditionally, vehicle manufacturers have used mechanical probe-based co-ordinate measuring machines (CMMs) to inspect and validate the shape of an assembled car body but this involves measuring some 2,000 elements and takes around 10 h. As most manufacturers inspect each of the vehicles leaving their production lines, a more rapid technique would clearly be highly beneficial. Now, a novel CMM offering high speed dimensional sensing is set to revolutionise this process. Developed by ActiCM, a French company set up by two former French Atomic Energy Commission (CEA) engineers in 2000, this new CMM is based on vision sensing rather than mechanical probes.

About two years ago, Renault conducted a benchmarking programme with the major CMM manufacturers in an attempt to identify faster ways of validating body shape. One of the key requirements was that the system should achieve an accuracy of 300 μm over the whole car body on the production line. Although the large CMMs can achieve accuracies of around 50 μm, in a factory environment they are generally no better than 300 μm. However, no solutions were found and although ActiCM's product was then still under development, the company approached Renault who subsequently placed an order for a pilot system for trials at a production plant. This was delivered in late 2004. It is based on sensor units dubbed intelligent vision devices (IVDs) which are mounted on a robot arm. Each unit has two high definition (multi-megapixel) CCD cameras, a visible LED illumination source and a white light projector. The system can also operate under ambient lighting conditions if they provide sufficient contrast and is termed “intelligent” as it knows what light source to use and switches between different sources depending on what it has to measure. Up to eight sensors can work simultaneously on the same part and the IVDs' onboard cameras are able to image this from two different and known positions. These two views (i.e. 2D images) are then geometrically manipulated by the device software using photogrammetry to extract 3D data. Photogrammetry dates back to the 1850s but has only recently been a practicable technique, made possible by advances in computing power. To validate the part, these co-ordinates are referred back to a computer-aided design file and compared with the original specifications.

Plate 1 The robot arm-mounted automated Advent system (ActiCM)

This pilot system has reduced the time taken to perform a 2,000-element validation from 10 to 2 h, with multiple sensors offering prospects for further time reductions. According to ActiCM, “With four sensors the car makers can get down to 30 min for the whole car body measurement, which is 20 times faster than conventional CMMs”. Although the IVD is robot-mounted (Plate 1), the mechanical arm is simply there to guide the sensors around the car body and enable an initial camera calibration. Further, the sensor's CCD images provide all of the necessary distance and depth information. This is a major advantage, as it means that the accuracy of the device, unlike many conventional CMMs, is not related to the positional accuracy of the robot. Imaging technology offers further advantages over conventional CMMs when there is a need to re-examine data. With a CMM, you have to make a sequence tree and tell the system what elements you wish to measure. In effect, once the car body measurement has been performed, co-ordinate data will exist only for elements that have been specified within the tree. This problem is overcome by the ActiCM device.

Plate 2 The portable Acturis system (ActiCM)

Although working closely with Renault, ActiCM has also established relationships with Hyundai, Toyota and Nissan. Ultimately, every car manufacturing plant has the potential to use the system. The company is now manufacturing two different products: a fully portable unit (“Acturis”, see Plate 2) and the high speed, non-contacting CMM system for production line control and inspection, the “Advent”. Depending on the nature of the application and the measurement speed required, the price of the Advent system will be somewhere within the €200,000-500,000 range. Whilst the first application was for car body measurements, the company now has enquiries from manufacturers of vehicle doors and bumpers. Outside of the automotive sector they have also received interest from the aerospace industry.

Contact: ActiCM, 122, Rue du Rocher-de-Lorzier, F-38430 Moirans, France. Tel: +33 (0)4 7691 3766; web site: www.acticm.com

Rob BogueAssociate Editor, Sensor Review