Auto Manufacturer Uses Machine Vision to Error-Proof Closure Panels

For automotive manufacturers, customer satisfaction is the key to increased sales and long-term growth. Among the hundreds of sub-assemblies that go into each automobile or truck, the quality of the body panels plays an especially important role in getting and keeping a satisfied customer. Specifically, closure panels such as doors, hoods, lift gates and tailgates must be properly manufactured to meet crash safety requirements. Equally important is the need to keep dirt, water and road salts out to prevent premature rust and corrosion. In addition to customer satisfaction, the direct costs of containment, scrap, rework, safety recalls and rust-through warranty expenses provide compelling reasons for error-proofing body panels through 100% inspection using automated machine vision inspection. This is why DaimlerChrysler's (DCX) Warren Stamping Plant (Warren, MI) has dozens of machine vision systems with plans to employ more in the future.

 

 

Production of closure panels (hinged body panels) at the DCX Warren Stamping Plant is a highly automated process, and automated processes demand automated inspection. The closure panel assembly process involves mating the inner and outer panels together. Inspection of hem adhesive, anti-flutter beads, structural reinforcements, sound deadener patches, hole dimensions, weld studs, and glass channels just before the two panels are permanently joined is a crucial requirement to producing a high-quality finished product.

 

After being stamped out in the pressroom and delivered to the assembly line, the outer closure panel is placed into a "nest" where the hem adhesive can be applied. A robot applies a three-millimeter wide bead of adhesive to the outside edge of the outer body panel, just inside a flange that will later be hemmed on the edge of the inner panel. The hemming process spreads the hem adhesive to form the "hem seal." Marrying the two panels with the hem seal gives rigidity and strength to the finished panel and seals out moisture and dirt to prevent leaks and rust. During this process, gaps in, or misplacement of the adhesive bead can occur due to the robot nozzle being out of position or a panel moving in the nest. A small gap near the top of a door panel is acceptable, but there can be no gaps in the bead at the bottom due to corrosion and rust-through warranty concerns.

 

When the robot returns to its pounce position, the line PLC triggers a Passport DSL™ Vision System, supplied by PPT VISION, Inc. (www.pptvision.com), which snaps images of the panel using multiple cameras and specialized LED strobe lighting. From 4 to 12 DSL digital cameras are used depending on the type of panel being inspected. The lighting was designed for this application by PPT VISION, and is immune to ambient light, excessive oil or other changes in panel reflectivity. Each image is processed to inspect for adhesive bead presence, location and gaps. Anti-flutter beads, sound deadener (NVH) patches, structural patches, weld studs and glass channels can be inspected at the same time or at another station. In addition, the vision system can determine the presence, location and size of window, door handle, and other openings.

 

The PLC waits for a pass/fail response from the vision system, which takes less than 1/2 second, and, if passed, moves the panel to the next assembly station. A new panel is then placed in the nest and the process repeats itself. If the PPT "BEAD" (Bead Evaluation And Detection) System finds a problem it alerts the operator through an alarm and tower light and locks the image of the failed area on its control panel. The BEAD System sends a signal to the line PLC that stops the line automatically and will not allow the panel to move to the next station. The operator confirms the failure and hits a reject switch that automatically removes the panel from the nest to a reject bin. Automated, closed-loop reject of failed parts will be implemented in the future. At this point, a non-conforming panel costs about $125 to scrap. Left uninspected, the same panel could cost many hundreds of dollars to replace when it fails in the field, not including the costs of labor, warranty administration and damaged customer relations.

 

Before implementing the Passport™ DSL Systems, operators would pull a panel off the line for a manual inspection. This was done on a schedule that varied from once an hour to once a shift. A destructive test where the inner and outer panels are cut apart for inspection is still performed once per shift to check for adhesive wet-out. At production speeds of up to 240 panels per hour, use of this "sampling" technique, however, led to many (in one case 3,000) faulty panels making it through to final assembly where the odds of catching the problem are low and the cost of scrapping is high. "The cost savings have been enormous, one catch usually gives us our return on investment" says Dale Frendburg, Adhesive Process Specialist at the Warren Stamping Plant. At first, operators were resistant to using the vision systems, but, as Dale comments, "Along with improvements in robustness and ease-of-use, the operators also now realize the system's value in helping them be more efficient, producing only high-quality panels."

 

The vision system is set up to automatically save failed images and other data at regular intervals for root-cause-analysis and process improvement. This helps to prevent recurring faults due to equipment wear or other variables and aids in long-term process understanding. The images and data are saved on the vision system's hard drive now, but in the future its network-ready hardware and software will be hooked up to the factory-wide F.I.S., Factory Information System.

 

Other future uses of the Passport™ DSL System include, among other things, having the robot reapply adhesive where there are gaps in the bead, press line inspection and performing comprehensive dimensional checks of each panel in a fraction of a second where it now takes a CMM an hour to complete. As Cindy Renault, Quality Manager for DCX Warren Stamping states, "There are endless possibilities for the use of machine vision."

 

This application note was provided by PPT Vision.  PPT VISION, Inc. is the world leader in the design and manufacture of completely digital 2D and 3D machine vision systems. Since our founding in 1982, we have specialized in industrial applications where accuracy, repeatability, high speed and flexibility are important requirements. For more information on PPT Vision, please visit their website at: www.pptvision.com.