If you buy a Mini Countryman you expect quality both inside and outside. To ensure that everything inside fits to the millimetre, FireWire cameras with VGA resolution measure the support frame to which the dashboard as well as the steering column and the knee air bag will later be mounted.

Products of the BMW Group light metals foundry in Landshut include cast lightweight metal motor and suspension components for BMWs and Minis. There, the car manufacturer invested in a testing system which checks the dashboard support frame for the Mini Countryman. Although this support frame is inconspicuous to a layman, it plays an important role in the safety and perceived quality of the vehicle, because the dashboard is mounted on this structural element. This includes important components such as the steering column and the knee air bag module. In addition, the geometry of the support must be perfectly matched to the cladding, so that this can be precisely fitted and give an impression of high quality.
The BMW Group commissioned the testing and automation specialist Ziemann & Urban with the development and construction of an independent testing compartment to check the geometry of the support frame. With this testing compartment, the dimensions of the support was to be fully measured in the production environment in the course of manufacture. The positioning and attachment points, for example for the air bags and steering column had to be measured in three dimensions. In addition, the presence and freedom from burrs of holes and cut-outs for mounting the cladding and other components had to be checked. The entire measurement was to be carried out automatically within five seconds and the system had to be able to evaluate two versions of the product - for left and right-hand steering.
An additional technical challenge was the fact that due to the angular shape of the component, not all of the inspection points could be recorded from the same angle of view. Because of this, mechanical turning of the support frame as in a "Rhöhnrad" was considered, to enable the cameras to detect the concealed testing points.

3D measurement with FireWire cameras

By means of a special arrangement of the test object and cameras as well as the use of background lighting and deflecting mirrors, Ziemann & Urban were able to dispense with the complicated rotation of the support.
"No matter how well the mechanical movement of an object is technically performed, it involves tolerances and has a negative effect on the precision of the three-dimensional measurement" explains Markus Urban, Project Manager at Ziemann & Urban. "Eliminating this makes the measurement process simpler and quicker.
The support frame is placed in a custom-made jig and locked in position. This ensures that the measurement is not affected by an incorrect position of the object in the compartment. 16 FireWire cameras with VGA resolution are used for the 3D measurement - eight for the right-hand and eight for the left-hand steering version. With the aid of several laser line projectors, the geometry of the object is highlighted in the image. The measurement points are illuminated with LED spotlights and with background surface lighting.
Six FireWire cameras with 1.25 to 5 megapixels are used to detect the presence of holes and cut-outs. These cameras view the component from top to bottom - if necessary with deflecting mirrors. The holes are highlighted by a light table which is integrated into the mount and which illuminates the dashboard support from below. Angled testing points which cannot be viewed from above are checked by four other digital cameras with VGA resolution.
Because all of the cameras, laser beams and lighting units are fixed and the object which is to be tested does not move, Ziemann & Urban fulfilled all of the precision requirements of the BMW Group. Even temperature-dependent deviations are compensated for with special decoupling devices on the cameras as well as continuous monitoring of the component, the measuring system and the environment. The result: a measurement precision of approximately 25 µm, which is normally only achieved in air-conditioned measurement compartments. At regular intervals, the precision of the measurements is ensured by means of cyclical evidence of the performance of the measuring equipment. The measurement results are saved in a database and can be assigned to each individual component. For this, the system reads the data matrix code of each component and saves this together with the measurement values.

Supply, control, communication

A total of 26 digital cameras are used in the measurement compartment. For the cameras, Ziemann & Urban decided on FireWire cameras from Allied Vision Technologies. 16 Guppy-F-003 monochrome cameras with VGA resolution from AVT are used for the 3D measurements with the line laser. The Guppy is an especially small and cheap industrial camera with a FireWire interface (IEEE 1394a). In addition, with dimensions of 42 x 30 x 30 mm, according to the camera manufacturer, it is one of the smallest machine-vision cameras on the market.
To check for the presence of the holes, six AVT Stingray monochrome cameras with resolutions of 1.25 to 5 megapixels are used (Stingray F-125, F-201 and F-504) - a specially high performance and modular camera for industrial image processing.
The other checks for presence are performed by four Guppy-F-036 monochrome cameras - also from AVT. Just like the Guppy-F-033, the Guppy-F-036 is also a VGA camera, however unlike its sister model it is equipped with a CMOS sensor. This supports High Dynamic Range mode and is primarily suitable for high-contrast applications.
"We decided to use AVT cameras for various reasons", explains Urban. "Thanks to its small size, the Guppy was easy to integrate into a system which had to accommodate a total of 26 cameras. The FireWire interface is particularly suitable for such a multi-camera system: the camera can be supplied with power, controlled via the software trigger and transfer the image data with a single cable."
The images from the cameras are evaluated with a ZVIS-4300 image processing system from Ziemann & Urban on the system PC. The ZVIS-4300 features a convenient XP-based interface and a comprehensive range of functions. Thanks to its global scaling function it is especially suitable for the 3D determination of positions. Overall control of the measurement procedure is performed with Ziemann & Urban ZMESS software. This evaluates the other sensors (DMC, temperature sensors) and displays the results on a 3D model of the component and then communicates them to the BMW database.