Case Studies: Virtual Verification
December 1, 2009
The Metris (Leuven, Belgium) cross scanner is used at Volvo Cars Gent (Gent, Belgium) to accelerate the design through manufacturing process. For the Volvo XC60, for example, Volvo’s engineers were able to digitize sheet metal and plastic body parts, virtually assembling vehicle bodies in software, completing pre-production geometry verification nearly twice as fast. Three-dimensional laser scanning technology, point cloud processing and virtual assembly shorten physical evaluation of prototypes and eliminate the need for specialized verification tooling.
The design and manufacture for the Volvo XC60 body required the coordination of many different groups. Sheet metal stamping and welding in combination with the use of new materials and joining technologies set ever-tougher geometric challenges. When shifting from the computer-aided design (CAD) model to the physical nominal model and finally to the serial-produced car, process and product tolerances, as well as material and equipment behavior, can influence body geometry. The position of edges, holes and other geometric features plays an essential role in correctly assembling the different body parts of a passenger vehicle.
In 2005, Volvo and Metris partnered to streamline the pre-production phases of vehicle body production by simplifying the geometric body verification process. The companies aimed to develop a new geometric verification method that would build on a digital inspection process using 3-D scanning and virtual assembly. This method would provide better insight and effectiveness compared to traditional body tuning, which involves extensive tactile inspection, physical part conflict analysis and complex verification tooling.
Ground-Breaking 3-D TechnologyIn close collaboration with Volvo, Metris optimized its existing cross scanner to match the performance level required to drive the new geometric verification method.
The laser scanner was modified for use on horizontal-arm coordinate measuring machines (CMMs) and its field of view was increased. The increased scanning standoff distance range offers higher measuring flexibility and better access to clamped body components. The cross scanner incorporates three laser beam/digital camera sets, each shifted 120 degrees in position.
This allows the laser scanner to capture slots, sleeves, holes and other features in a single scan. Although inherently designed for scanning geometric features, the cross scanner also is suitable for digitizing 3-D surfaces and edges. The positions of features and edges are imperative to correctly mate parts and assemble car bodies.
The development work also impacted laser optics technology and digital data processing. The cross scanner design has been enhanced to flexibly deal with all material types and colors without the use of spray. Now, reflective sheet metal as well as painted surfaces can be captured quickly and reliably.
Faster and Better Body VerificationIn the pre-production stage at Volvo, metrology engineers scan sheet metal and castings made of steel and aluminum as well as composite and plastic body parts. After acquiring data at an accuracy of approximately 20 microns, engineers filter the resulting point cloud and analyze geometry against nominal CAD data. Volvo relies on digital graphic reports to evaluate the parts and streamline supplier interaction with regard to adjusting molding and stamping equipment. Digital component verification only requires standard holding fixtures, whereas traditional inspection methods demand costly dedicated positioning and fixation tooling.
After digitizing individual parts, engineers align and virtually assemble sheet metal, interior, exterior and chassis components in software, building a complete virtual vehicle body. Even before body parts are physically assembled, the geometric verification approach gives information about potential part fitting issues. To run specialized investigations, virtual body assembly models are loaded into dedicated software for reverse engineering, variation analysis and spring-back prediction, for example. Analysis between scanned and numerical vehicle body models enables Volvo to efficiently tune component geometry to fall within the assembly processing window.
Handheld DevelopmentThe collaboration project with Volvo also contributed to the development of K-Scan, a handheld laser scanner with a single laser stripe for insitu inspection. An optical CMM continuously tracks the scanner, affording the operator the mobility needed to scan the entire vehicle. Volvo engineers use K-Scan to verify flush and gap, body deformation and static/dynamic geometry on prototype or early production vehicles. Color-coded visual inspection reports illustrate how flush and gap evolves along complete spines in between hood and front fender, for example. Optical handheld verification also includes special cases in which manual methods fall short, such as zero gaps, or in case an urgent issue comes up that needs fast troubleshooting.
Reduced Iteration LoopsThe new process reduces the time needed for matching loops and realizes an important cost reduction for test materials and screwed body, nominal blue bucks and dedicated fixtures. When ramping up Volvo XC60 production in 2008, geometry iteration loops and the lead time of individual loops were reduced. Having fewer physical evaluation prototypes also reduced material scrap and decreased expenditure on complex verification tooling, such as body-in-white cubing.
“Three-dimensional scanning has been used extensively when ramping up production of the successful Volvo XC60 crossover vehicle,” says Alfons Van den Bergh, head of the geometry division at Volvo Cars Gent. “For more than two years now, 3-D scanning has been serving as the standard method for virtual body geometry verification.”
Three-dimensional scanning technologies are already well accepted at Volvo, where they are used in different stages of the manufacturing process. Noncontact metrology is systematically applied in the early design stages in which clay models are digitized. In pre-production, engineering intensively digitizes body parts and body-in-white structures to optimize part manufacture and assembly. After kicking off serial production, specific aspects of car components or full cars are scanned to serve as statistical process control samples for quality monitoring and product audit purposes. For the future, laser scanning is seen as a key enabler of in-line quality control.