Cognex In-Sight® sensors checking the quality of laser welds at Renault – Sandouville
The Renault LHA factory is located in the Haute Normandie region, in the town of Sandouville, and more precisely in the port and industrial estate of Le Havre (in the Seine-Maritime Department). This sizeable bodywork-assembly plant is specialized in the manufacture of top-of-the-range vehicles.
The vision application presented here was set up in the pressing workshop. This workshop produces sheet metal parts that make up the body shell of the vehicles. It mainly comprises cutting lines, assembly and sheet welding points, as well as sheet presses which make up the various bodywork components. The pressing workshop supplies the sheet metal workshop which carries out the assembly of the various parts of the body shell. This then passes on to the paint shop before final assembly.
Each side of the body shell is made up of two parts which have been assembled by laser welding. It is around the weld that defects can appear and need to be detected. While the various pressing operations – which give the part its shape – are being carried out, clear or partial breaks can be produced. As the parts are stored manually at the end of the line, those parts showing such faults are easy to spot and remove.
It is not so easy to spot when the faults are small holes, some of which can measure less than 3/10th of a millimeter, which form along the bead of the weld. If the presence of these holes is not detected at the end of the line, and if the part is not removed, breaks can be produced during shaping, causing numerous problems which can have serious effects on the productivity of the assembly line. Sometimes minute holes are enough to give the bodywork an unsightly appearance after the paintwork has been done.
If these defects are not detected before the vehicle is assembled, the consequences can be financially very expensive as it is very difficult to repair…and sometimes there is no other choice but to send the chassis to the scrapyard.
Checking the welding was previously done by several operators who had to struggle to handle parts of a considerable size and weight: the sides of the body shell measure around 3.4 meters in length, 1.6 meters wide and weigh nearly 30kg. To carry out the operation, they used to place a light source on one side of the part and check from the other side that the light did not pass through, showing breaks or holes in the part. This form of control, even when it was done in the optimum way, cannot highlight holes of a very small diameter.
Numerous defective parts passed through these checks, so a solution had to be found which was capable of putting a stop to this situation while carrying out a continual and reliable check of the lower body shells.
Four Cognex In-Sight 1000 cameras, each placed in protective casing, are positioned at the top, over the platform, on the cross support beam. A control screen is fixed on one side of the test bench, in a box where the control system is also installed. A marking system for defective parts is fitted at the other end.
The parts to be checked – body shell sides – measure around 3.4 meters long, 1.6 meters wide, and weigh nearly 30kg. They are put onto the supports provided for this purpose on the test bench by a robot. There are two areas to be checked. The target areas measure approximately 10 centimeters in length, by 5 centimeters wide, with a covering. Two of the cameras are inclined at 45 °, the part forming a U-shape over the welded area.
The cameras are able to detect very low light levels coming from the back-lighting system and passing through any holes, some of which are only a tenth of a millimeter in diameter.
If the part is declared sound, this is displayed on the control screen. If the part is defective, it is also displayed on the screen, a red light comes on at the end of the line for each fault, and finally, the part is marked by a jet of ink which stops it from being used.
The speed of the control carried out on the line was a determining factor of the project: all the operations – positioning the part, capture and analysis of the image, detection and marking could not reduce the production rate in any case. At this stage, the Cognex systems proved their worth: the production rate specified in the Terms and Conditions was 900 parts per hour; the current production rate is 420 parts per hour for the body shell sides and 850 parts per hour for the side frames.
The system is connected to a PC fitted with a hard drive for saving the photos of the defective parts detected over one year. This procedure allows the problems encountered to be analyzed at a later date; it contributes in ensuring the traceability of parts and for monitoring the various shifts (days, hours, cameras 1 and 2…).
Checking welds reliably for individual parts at full production rates, eliminated entire body shells being scrapped at the end of the line.
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