Best-fit is the best practice! Quality down to the last detail

German automobiles stand worldwide for high quality down to the smallest detail. "There is no second chance for a first impression" - that is the motto of the engineers and designers - and in fact, perceived quality is a big factor in the decision to purchase a vehicle. The precise fit of body parts, such as doors, with the smallest and above all most uniform gaps possible, plays a decisive role. That makes the requirements of automotive manufacturers for their production processes all the higher. In automobile construction, attached components are assembled predominantly using conventional methods with gauges, which keep the attached components in a constant, rigid position for installation. This means that sometimes substantial rework has to be invested during production for manual adjustment. Uniform distribution of gap dimensions, as required today by both designers and quality engineers is hardly possible using these conventional methods except through correspondingly high levels of expensive labor. This was recognized by the engineers at EDAG, the world's largest independent development partner of the mobility industry. The company worked with a leading Vision Technology company to develop an intelligent sensor for use with their image processing systems and other applications. In the first step, a pilot system was implemented in Fulda,Germany, in collaboration with GM/Opel.

Best-Fit: Sensor-controlled assembly:

The system known as "Best-Fit" is a highly intelligent sensor -based assembly process, which makes custom-tailored, highly precise assembly of attached parts in automotive production possible. For example, for door installation, sensors first determine the dimensions of the gap and transition for each manufactured door along with the cut-out in the sidewall of each corresponding vehicle; then calculate the optimal position of installation of that door with that car body; and finally use robots to target this position precisely.

The joining robot then installs the door so that it meets the requirements for a high-quality vehicle, despite the unavoidable geometric variations due to production tolerances. A particular feature of the Best-Fit system is that it learns and thus independently controls placement behavior in the joining process. Beyond that, a final quality inspection step is carried out and documented, so that downstream measurement stations can be eliminated. Depending on the design of the attached part, 6 to 11 sensors are installed on the gripper of the robot. In addition to electronic control and monitoring systems, the intelligent sensor consists of a laser source and a digital camera. With the aid of these sensors, the components, e.g. door and side wall, are measured with each other. The Best-Fit system detects the current status quo of the alignment along with the width and distribution of the gap dimensions and calculates the optimal position for installation. The software ensures not only the most parallel possible gap dimensions, but also uniform distribution of the gap dimensions at all measurement positions.

Optimal choice for multi-camera operation:

For the Best-Fit process, EDAG depended on digital cameras with a resolution of 1.4 Megapixels, that permit precise gap measurements. For better integration in the intelligent sensor, the housing of the standard camera was removed and the board-level version was selected.. In addition, the cameras are equipped with a digital interface (FireWire IEEE 1394a). This interface can be used to activate all the cameras of the EDAG Best-Fit system via a broadcast trigger. Thanks to the internal memory and the deferred mode, the Best-Fit system can flexibly call up the images of each camera for analysis.The deferred mode of the cameras and the industrial suitability of the FireWire interface was important to the design team in selecting the best camera for the Best-Fit application.

Practical use: Milestone for assembly quality in automobile construction:

The Best-Fit process is a milestone for assembly quality in automotive production. For each manufactured component and each manufactured body, the system calculates the optimal position for assembly - for a consistently uniform and high-quality appearance of the overall vehicle. But in addition to the quality advantage of perfect distribution of the gap dimensions, the Best-Fit system also achieves economic advantages: The costs of manual re-adjustment are reduced drastically, as the implemented projects have shown clearly. Assembly is now being done at two European locations with the Best-Fit system, for instance, for all the attached parts of the Mercedes Benz S-Class and the doors of the Opel Corsa. In addition, the Best-Fit system is also being used for other applications, such as precise positioning of robot tools, e.g. for punching, stamping, or riveting in automobile production.

The above article is an extract from AVT Best Practice monograph, and published under authorization from Allied Vision Technologies.

- Anand P Chinnaswamy