Can computed tomography speed up automobile development?

Each year, auto makers tear apart hundreds of brand-new car bodies using crowbars to check the workmanship of welded and bonded joints, bolts, and rivets. Holub hopes to supplant this time-consuming procedure with a smart X-ray technique called RoboCT, in which a CT scan is performed by synchronously operating movable robotic arms.

In industrial computed tomography, the X-ray source and detector are usually fixed, while the object to be examined is slowly rotated on a plate. During this process, X-ray images are captured from different angles and subsequently used to generate a 3D model. This static setup allows resolutions in the range of a few micrometers. Performing X-ray scans of larger objects such as cars requires very big and extremely expensive systems.

How do the X-ray source and detector revolve around the object?

“Often, carmakers just need a CT scan of a particular vehicle component,” explains Holub. “RoboCT is excellently suited to this kind of situation.” The idea for the system arose at EZRT back in 2005, when researchers decided to perform CT scans of airplane wings by rotating the X-ray source and detector around the object with the help of robotic arms. This ambitious plan came to fruition in collaboration with an aeronautical supplier in 2015, when the concept was used to image the outer wing of a business jet.   

Although the procedure sounds simple enough, practical implementation is difficult. RoboCT requires the X-ray source and detector to always be located precisely opposite each other while rotating around the object. “The best industrial robots are only accurate to within around one millimeter,” Holub observes. A CT scan, however, requires an accuracy of at least ten micrometers. To achieve clear images in spite of this obstacle, Holub's team uses a method known as geometric calibration, in which the robots scan a calibration object from different directions. This data can be used to calculate the extent of the misalignment, so as to compensate for the error in the final scan.

What is BMW using RoboCT for?

BMW is already so impressed by RoboCT that it now has two of the systems in operation. One is located at the company’s Research and Innovation Center (FIZ) in Munich, where it is used to examine hand-assembled prototypes of forthcoming models at the pilot plant. The other is located at the test center in Eching, where BMW’s camouflaged prototypes undergo extremely rigorous testing. Besides bonded and welded joints, the prototypes are also tested for corrosion, proper seating of seals, and the positioning of cables in the vehicle’s roof liner. “As CT yields much faster results than the traditional dismantling process, using RoboCT enables carmakers to significantly shorten development times,” Holub explains.

For now, the systems still have to be operated by specially trained experts. However, the RoboCT team is already hard at work on a simplified control system using a digital twin of the systems, into which the test vehicle’s CAD data can be entered. This will open the door to widespread use in manufacturing, making the crowbar approach to vehicle dismantling a thing of the past.