At the BMW factory in Landshut, Germany, an on-site testing facility using digital cameras from Allied Vision Technologies tests instrument panel carrier frames for the MINI Countryman. This component, unnoticed by the average person, plays an important role in the safety and quality look and feel of the vehicle: the dashboard is mounted to this structural element. Important components such as the steering column as well as knee airbag modules belong to this assembly. Furthermore, the panel’s geometry must fit perfectly with the car’s interior, allowing for custom-fit assembly and delivering an high perceived quality.
The Challenge: 3D measurement from different angles
Ziemann & Urban, a testing and automation specialist located in Moosinning (Bavaria), was contracted by the BMW Group to develop and build a self-contained test chamber for inspection of the carrier’s geometry. Over the past 20 years, Ziemann & Urban has evolved into an established purveyor of turnkey solutions for automation testing and quality assurance. Their 35-member team designs test and measurement systems for many application fields, most notably for the automotive, plastics, semiconductor and medical technology industries.
The testing system for BMW must provide a 100% measurement of the frame’s dimensional accuracy during production and within the production environment. The position of attachment points – such as those for airbags and steering columns – must be measured three-dimensionally. Moreover, the presence and absence of burr on bore holes and openings for the assembly of interiors and other components must be monitored. The entire measurement should occur automatically within 5 seconds, and the installation must be able to inspect two product varieties – left- und right-hand drive models.
One additional technical challenge is that, due to the element’s angular form, not all test points can be detected from the same angle of view. For this reason, a mechanical rotation of the support frame was considered, to allow the cameras to visualize hidden points requiring inspection.
"Mirror, mirror, on the wall..."
Utilizing a particularly shrewd adjustment of both the test object and the cameras as well as the clever use of background illumination and tilted mirrors, Ziemann & Urban was able to forgo such an elaborate manipulation of the support frame. "Mechanically moving the object, no matter how well technically executed, brings about tolerance challenges and has a negative effect on the precision of the three-dimensional measurement," explained Markus Urban, Project Manager at Ziemann & Urban. "Doing away with it makes the measurement process simpler and faster."
The instrument panel carrier is positioned and clamped in a customized apparatus. Doing so excludes the possibility that the measurement will be corrupted by an incorrect position in the cabinet. For 3D measurements, 2 sets of 8 FireWire cameras with VGA resolution are used — 8 each for the left- and right-hand drive versions. With the aid of several laser line projectors, the object’s geometry is highlighted in the image. Measurement points are illuminated with LED spotlights as well as by surface background lighting.
For presence detection of bore holes and openings, 6 FireWire cameras with 1.25 to 5 megapixel resolution are installed that then inspect the component from top to bottom – with tilted mirrors where applicable. Bore holes are highlighted by the light box integrated into the fixture, that illuminates the frame from below. Four more cameras with VGA resolution inspect angular test points that are undetectable from above.
In that all cameras, laser emitters and lighting units are permanently mounted and the test object is not moved, Ziemann & Urban fulfills all of the BMW Group’s precision requirements. Even deviations caused by temperature are compensated for by special decoupling devices on the cameras as well as continuous monitoring of the temperatures of the component, the measurement device, and the environment. The result is a measurement precision of approximately 25 µm, which normally is only achieved in air-conditioned inspection rooms. Measurement precision is ensured by cyclical analyses, performed in regular intervals, of the measurement equipment’s capability.
Measurement results are stored in a database and can be assigned to each individual part. For this function, the system reads out the data matrix code of each component and stores it with the measured values.
Mini-cameras for MINI
In all, 26 digital cameras are used in the measurement cabinet. In the camera selection, Ziemann und Urban chose FireWire cameras from Allied Vision Technologies.
16 AVT Guppy F-033 monochrome cameras with VGA resolution take care of 3D measurement with line lasers. The Guppy is an especially small and economical industrial camera with FireWire (IEEE 1394a) interface. At 42 x 30 x 30 mm, Guppy is one of the smallest machine vision cameras on the market.
For presence detection of the bore holes, 6 AVT Stingray monochrome cameras with 1.25 to 5 megapixel resolution are used (Stingray F-125, F-201 and F-504). The Stingray is an especially high-performance, modular camera for industrial imaging.
4 AVT Guppy F-036 monochrome cameras take over conventional presence detection. Like the Guppy F-033, the Guppy F-036 is a VGA camera, but differs from its sister model in its use of a CMOS sensor that supports High Dynamic Range mode and is especially suited for applications with high contrast.
"We chose AVT cameras for a number of reasons," Urban explained. "Due to its very compact dimensions, the Guppy integrates wonderfully into an installation where 26 cameras have to be housed. Its FireWire interface is especially well suited for such a multi-camera system; with one cable, the camera can be supplied with power and controlled via Software Trigger, and its image data can be transmitted as well. Beyond that, we’ve been working together with Allied Vision Technologies for many years. The quality and reliability of their cameras have proven themselves."
The camera images are evaluated on a system PC with the aid of the ZVIS 4300 imaging system from Ziemann & Urban. The ZVIS 4300 distinguishes itself with its comfortable, Windows-based interface and its comprehensive functions. Using its global scaling function, it is especially well suited for 3D positioning. Ziemann & Urban’s ZMESS software assumes complete control of the measurement process. With this software, the other sensors (DMC, temperature sensor) are evaluated and the results are displayed on a 3D model of the component. Finally, those results are entered into BMW’s data bank.
100 percent quality control
The installation has been in successful use at BMW’s Landshut location since early 2010. Several hundreds of instrument panel carriers are checked every day. "We are proud to contribute to BMW’s quality assurance with our inspection system," says Markus Urban.