Mobile Measuring System Assures Quality Of Vehicle Geometry

An enormous change took place on the automobile market in the past years – and thus also the work of the development departments of automobile manufacturers has changed. Reasons are the growing number of niche models that are offered besides volume models, as well as the increasing demands in quality of car customers – even in the lower automobile classes. Additionally, the faster change of car models is of high importance. This results in a significant reduction of development times. As during the development of a new automobile, the basis is provided for its later success on the market, numerous new technologies have been introduced in development and quality processes. Here, the laser tracker of Leica Geosystems has to be mentioned, which is used by the BMW Group in their research and innovation center (FIZ) in order to assure the quality in prototype design and model technology.

Prototype design plays an important role in the course of development of new vehicles. One part of it is for example the special field `car-body-prototype-components´. Its main duty is: Cubing measurements – interior and exterior measurements, component measurements, laser-releases, laser-offline-programming in Catia with Laser-Cut as well as the application of CAD-Bend, a Catia-Tool for edge-parts and measuring-plan-drawing. In order to manage the posed tasks, the Laser Tracker LTD 500 of Leica Geosystems has been used since autumn 1999 - in conjunction with the Leica-Software Axyz and the measuring software CAM2 Automotive of Faro Europe. At that time, prototype design searched for a high precision, mobile measuring system, which delivers alternatives to the given stationary measuring machines. In addition to the laser tracker of Leica, another laser tracker and two measuring arms - one mechanical and one tactile – were tested in the course of the selection process. The decision was in favour of the laser trackers of Leica Geosystems.

The Leica Laser Tracker LTD 500 is an easy to transport, mobile coordinate measuring sys-tem. Speedy and high precision measurements can be carried out due to the installed laser-interferometer. May there be single-point or surface measurements, the laser tracker can record objects in a measuring volume of up to 70 m diameter with an accuracy of +/- 10 ppm (µm/m) at a measuring rate of up to 1000 points/second in one single set up. The laser tracker is particularly used in high precision tool manufacture and in geometry tests in the automobile and aircraft industry as well as in other fields. Periodical inspections, checking tests and other jobs can be made completely automatically. The set-up of the laser trackers is flexible and can be adopted to the size of the object or to the confined room conditions.

"Furthermore, the laser tracker, which is used in prototype design, is fitted with a CCD-Camera – an observation camera and with several special adapters, which are produced especially for the BMW Group. Also the interface from Axyz to CAM2 Automotive was particularly made for the laser tracker. Meanwhile, the interface was installed also for other customers in Germany," explains Simon Moser - the sales manager of the Metrology Division of Leica Geosystems in central Europe.

There are various fields of application for the mobile measuring system. The laser tracker is mainly used in order to assure the quality of measurements in the cubing process. Furthermore, it can be applied in order to digitize and measure gaps and seams. Especially, in interior cubing, the laser tracker with its laser-light offers an excellent measurement acces-sibility. This was not or hardly possible with tactile measuring machines, which were used in the past. In all these fields the laser tracker can offer its advantages – high ac-curacy and easy, quick handling.

In the development process cubing models are the first three-dimensional objects on a scale of 1:1, which are based on CAD-data. They show the geometry and assure the possibility of carrying-out vehicle projects. In model technology the BMW Group works with part cubings as well as with 1:1 cubing models – for the interior and exterior process. Feas-Cubing-Models are different from Function-Cubing-Models. While Feas-Cubing-Models - for the
interior and exterior- are set-up separately in order to work in a highly flexible way, Function-Cubing-Models represent interior and exterior in just one model. Every new automobile passes these phases, first the Feas - Cubing is built, then the Function-Cubing. Centerfix mountings are used for fixing single-models on the base frame. Those were meas-ured before assembling the cubing model. Leica has produced special adapters for Centerfix measurements, which makes the laser tracker considerably more suitable than a tactile, mo-bile measuring machine.

As the laser tracker is an optical system, the preparation of measurements is different from those of the usual coordinate measuring machine. After setting-up the model, the measuring positions for the Laser Tracker are selected. As the laser tracker is mobile in contrast to a stationary measuring machine normally more than one location is chosen with regard to the measuring object. This means that the measuring sensor will be re-positioned from A to B to C. In this case it has to be made sure that work takes place always in the same coordinate system.

During the planning-phase, various questions have to be answered regarding the use of the laser tracker as a direct visual contact from the tracker-head to the measuring points is nec-essary. Where has the tracker to be positioned in the room? How often has the process to be repeated? Which aids have to be used? Which points have to be visible?

Normally, component parts are measured with traditional measuring machines. Here, only the manufacturing accuracy with comparison to the Centerfix-mountings is checked. If all parts are mounted, the laser tracker can measure the `total picture' yielding the add-on parts with regard to the vehicle coordinate system.

The laser tracker checks if the component has the defined position. The location of the point within the vehicle network is checked and the measured points are compared with the CAD-data, for example, that the lateral wall is at the desired place. If the result is positive, the model will be painted. It then looks like an original vehicle.

The laser tracker of Leica fulfilled all the defined expectations regarding accuracy, flexibility and speed. Thus, for example, speedy measurements of large areas are possible due to the bigger measuring-volume and the hand-guidance. This was clearly more lavish with stationary measuring machines.

Source: Leica Geosystems