QMT Features: June 2009
Digital acceleration
 Digital measurement and virtual assembly techniques have speeded up pre-production geometry verification of the new Volvo XC60 crossover vehicle by a factor of 2.

Volvo Cars has built up a solid reputation for vehicle safety, environmental and design innovation. Recently, the Swedish car maker released the brand new Volvo XC60, causing excitement in the new crossover vehicle segment. The vehicle body of this innovative car required the coordination of many different groups to design and manufacture.

Sheet metal stamping and welding, in combination with the use of new materials and joining technologies, set ever-tougher geometric challenges. Process and product tolerances, as well as material and equipment behaviour, can influence body geometry, as it shifts from vehicle body CAD model through to physical nominal model and then,  finally, to the serially-produced car. The position of edges, holes and other geometric features plays an essential role in correctly assembling the different body parts of the passenger vehicle.

In 2005, Belgium-based Volvo Cars Gent and Metris (Leuven) participated in a project that aimed to streamline the pre-production phases by simplifying the geometric body verification process.. Both companies joined forces to develop a new geometric verification method for vehicle bodies, which builds on a digital inspection process using 3D scanning and virtual assembly. This method provides better insight and effectiveness compared to traditional body tuning, which involves extensive tactile inspection, physical part conflict analysis and complex verification tooling.

In close collaboration with Volvo Cars, Metris optimized its existing cross scanner to match the performance level required to drive the new geometric verification method. The laser scanner was integrated for use on horizontal-arm CMMs, and its field-of-view depth was increased. The increased scanning standoff distance range gives higher measuring flexibility and better access to clamped body components. The cross scanner incorporates 3 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 is also suitable for digitizing 3D 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 associated digital data processing. Metris explain that the cross scanner design has been enhanced to handle all material types and colours without the use of spray. Now, reflective sheet metal as well as painted surfaces can be captured quickly and reliably. Laser scanning generates point cloud data at high scan rates, outperforming by far tactile point-by-point acquisition technology. In addition it is much easier to define the linear and polygon scanner travel paths than to specify individual touch sensor points for a tactile inspection task.

Design to pre-production
In the pre-production stage at Volvo Cars, metrology engineers scan sheet metal and castings (steel and aluminum) as well as composite and plastic body parts. After acquiring data at approximately 20 micron accuracy, they filter the resulting point cloud, and analyze geometry against nominal CAD data. Volvo Cars rely on digital graphic reports to evaluate the parts, and streamline supplier interaction with regards 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 in order to build a complete virtual vehicle body. Even before body parts are physically assembled, the new geometric verification approach is already  able to provide information about potential part fitting issues. To run specialized investigations,  such as reverse engineering, variation analysis, and spring-back prediction, virtual body assembly models are loaded into dedicated software. Analysis between scanned and numerical vehicle body models enables Volvo engineers to efficiently tune component geometry to fall within the assembly processing window.

The collaboration project with Volvo Cars also contributed to the development of the Metris K-Scan, a handheld laser scanner with a single laser stripe for in-situ inspection. An optical CMM continuously tracks the scanner so that the operator can freely walk around and take scans in an area that spans an entire vehicle. Volvo engineers use K-Scan to verify flush & gap, body deformation and static/dynamic geometry on prototype or early production vehicles. Colour-coded visual inspection reports illustrate how flush & gap evolves along complete spines in between hood and front fender, for example. Optical handheld verification also includes special cases where manual methods fall short, such as zero gaps, or in case an urgent issue comes up that needs fast troubleshooting.

 In 2006, Volvo applied  the virtual body geometry approach for the first time when preparing the production rollout of its C30 model.  In parallel, traditional tactile verification methods were performed to set benchmarks in terms of inspection precision and throughput.

When ramping up Volvo XC60 production in 2008, geometry iteration loops and the lead-time of individual loops were reduced. Fewer physical evaluation prototypes  were required, material scrap was reduced and decreases in expenditure of complex verification tooling, such as body-in-white cubing, were achieved.

Now 3D scanning technologies are well accepted at Volvo Cars where they are used in different stages of the car manufacturing process, from early design stages - where engineering styling by digitizing clay models - through to pre-production - where  engineering intensively digitizes body parts and body-in-white structures to optimize part manufacturing and assembly. After kicking off serial production, specific aspects of car components or full cars are scanned to serve as SPC samples for quality monitoring and product audit purposes. For the future, an important role for laser scanning is seen as a key enabler of in-line quality control. www.metris.com
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