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Mobile 3D inspection with optical measuring systems.

Posted: April 28, 2008


One cannot imagine quality assurance in today’s industrial production without portable 3D measuring systems such as articulated arms. These systems are extremely flexible and hardly influence running production. However, many users realize that articulated arms reach their limit of capacity when very high accuracies are required, and when the components to be measured are big or difficult to access.

In these cases, more and more companies count on optical measuring systems such as laser trackers or photogrammetry systems that are not based on complex articulated mechanics but rather measure with light. Both systems feature high mobility and precision also for large objects.


Photogrammetry systems are portable 3D measuring systems with a digital camera for data collection. Using the digital camera, the object to be measured is photographed from a number of directions so that all relevant parts of the object are captured.

The digital measuring images are processed by the analysis computer, and the 3D coordinates of all relevant points are calculated. This calculation is based on the principle of spatial triangulation of images, and is conducted fully automatically by the measuring software. An elaborate pre-calibration of the system is not necessary as the calibration is done automatically on the job.

3D Inspection Of Bogies With Photogrammetry

Deutsche Bahn’s vehicle maintenance is among others responsible for restoring the bogies and wheel sets of the vehicles. For the quality check of the bogies, the plant in Bremen (Germany) applies DPAInspect, a photogrammetry system from AICON 3D Systems (Braunschweig, Germany).

In this case, 20 to 50 points and geometric features (depending on the bogie type) are captured. They are checked for compliance with the tolerances determined by manufacturers and Deutsche Bahn itself. Based on the measurement results, which are put into a standardized report, it is easy to choose between two options: either rework or scrap the bogie.

AICON programmed special macros for Deutsche Bahn to realize the desired automation of the measurement. Andreas Kahle, responsible project engineer at AICON, reports, “Not only does Deutsche Bahn profit from the macros, but also the individual workman who must operate the system. Since the user is guided through the whole measurement process by our macros, it is a lot easier for him. Long periods of training are not necessary. The operator is quickly able to conduct a complete measurement, including setup and teardown of the bogie, image acquisition, analysis, and report generation, within only 90 minutes.”

The measurement with this photogrammetry system not only saves time, it also helps to decrease measurement costs significantly ? an important factor for the economic success of the plant. Jens Makat, head of Locomotive Manufacturing in Bremen, explains: “We had to bear enormous acquisition, fixed and maintenance costs for the old surface plates and dial gauges. Additionally, bogie measurements that we had to subcontract because of those costs can now be done by our own workmen. By using this system, we have reduced the measurement cost per bogie dramatically. We have already been able to write off the investment, after just a few months.”

A welcome side effect is the repeatable and reproducible measuring results. “Deciding if parts are in or out of tolerance no longer depends on the user. With this new photogrammetry system, it’s impossible to measure the wrong positions, as may happen when you use a probing system,” remarks Jens Makat.

The Measurement In Detail

In order to measure the relevant points, the bogie is furnished with two steel scale bars and a reference cross, to give the measuring system dimensional scale and orientation. Then the geometric features to be analyzed, such as bore holes, edges or planes, are targeted with “dice” ? cube shaped adapters. For each bogie, between 60 and 80 adapters are necessary.

To start a measurement, the user opens the system macro on a notebook computer, starting the DPA measurement with a single mouse click. This same click starts the analysis software, PolyWorks/Inspector and the DPAInspect plug-in, in the background. Although transparent to the operator, this is the first step in an analysis process based on CAD data.

The acquisition of the 150 images necessary for the measurement takes about 30 minutes. Since the images are transferred directly to the analysis notebook via wireless LAN, the plug-in can begin image processing and calculation of 3D coordinates during the image acquisition phase. The points being measured are shown in different colors, which indicate the quality of each measurement. Hence the operator immediately notices points for which there may be a lack of images, and can easily correct by taking additional pictures. This functionality ensures a highly accurate measurement.

After image acquisition is complete, the user presses the “Analysis” button in the macro, which activates the comparison of the actual and target data in PolyWorks/Inspector. Finally, a measurement report is created. This process is also fully automatic, and creates a report formatted to Deutsche Bahn standards. In the report, data fields are colored green or red to help visualize the (non-)compliance with tolerance values.

Before the plant in Bremen employed this photogrammetry system for the first time, the system was subjected to extensive testing by the center responsible for quality assurance and test equipment at Deutsche Bahn. The acceptance test included the check of the absolute accuracy of DPAInspect by means of different measurement standards, which were determined by the quality assurance and test equipment center.

Moreover, its process capability was tested with a sequence of 25 repeated measurements of a bogie. The new photogrammetry system immediately met all requirements of Deutsche Bahn, and could easily win the race against conventional measuring systems without any problems.

The Transparent Factory

The laser tracker is an accurate portable measuring system based on the priciple of a tracking interferometer. The user directs a hand-held reflector automatically tracked by a laser beam to the object positions to the measured. The laser tracker calculates in real time the spatial 3D positions of the reflector using the distance to the reflector and the direction. The manually-operated reflector can be upgraded by a probe or a hand-held scanner. They allow for the measurement of hidden points, and the scan of a surface respectively. In doing so, a measuring volume of up to 262 ft is covered at an accuracy of 10 ppm.

The Transparent Factory in Dresden, Germany is the birthplace of each VW Phaeton. The futuristic plant is responsible for the final assembly of the vehicle and carries out the so called “wedding,” the bringing-together of the bodies with the pre-assembled undercarriages of the Phaeton.

Volkswagen’s dedication to perfection with respect to its premium segment vehicle manifests itself in many ways, such as in the attention paid to the width of the gap between individual sheet metal segments, to the way headlamp elements seamlessly hug the fender or the “thump” a door makes when it shuts. All of these are subtle yet perceptive indicators of the car’s quality even to non-technical people, and even an untrained eye will immediately notice if something does not sit perfectly.

Volkswagen chose to get two complete Leica T-Probe laser tracker systems that both measure in the same coordinate system. One system inspects the front end and the left side of the car while the second system inspects the car’s read and the right-hand side. This combination produces optimal time saving results.

AICON 3D Systems, which distributes these laser tracker systems and implements installations and customer support in certain parts of Germany, brought this idea to life. AICON’s Thomas Nickel explains, “We’ve created the first-ever laser tracker ‘Doublet.’ Because both systems are controlled by the Metrolog XG for Leica software and use the same coordinate system, data sets can be simply exchanged between the two. This way, the laser tracker duo can fully replace a mobile double-column CMM. This solution is one-of-a-kind.”

The advantages of the “double tracker” method became instantly visible. Frank Herold, head of the Metrology Department, states, “Applying the two systems, we could reduce the time for the check of a vehicle by four or five hours. At the same time, the accuracy of the measurement is tremendous. We’ve remarked an improvement of at least two tenths. All in all, the time necessary for the final check could be cut by half while we are a lot more flexible than ever before. We solve various measuring tasks, and we can even lend our tracker to other departments. The system is simply ideal.”

AICON 3D Systems GmbH, Biberweg 30 C, 38114 Braunschweig, Germany, +49 (0) 5 31 58 000 58, Fax: +49 (0) 5 31 58 000 60,

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