In 2012, Mini drivers quickly embraced the auto icon’s new ad campaign that had launched that year in late September. The campaign’s motto, Not Normal, embodied the tight bond that Mini owners have with their cars. And in short order, proud Mini drivers began using the hashtag #mininotnormal when posting photos of themselves with their unique vehicles.
That same year, those overseeing the production of the Mini must have inadvertently embraced the campaign, as well. At BMW Mini, the “normal” way of manufacturing some parts, like doors, was changing. A new, improved manufacturing method was in the works.
In terms of the established approach to producing hang-on parts, like doors and seats, scanner-based remote welding was – and is – known as an efficient technique. But, without appropriate seam tracking, current scanner systems are constrained to only welding overlap joints. While overlap joints are insensitive to position errors, fillet seams are highly position-critical. Fillet seams, however, allow smaller flanges and other process advantages, such as zero gap welding of galvanized material.
Therefore, to extend the application scope of remote welding to fillet seams, a fully coaxial approach for seam tracking through scan heads was developed. And that system was qualified for industrial use and implemented in the serial production of BMW Mini doors.
Adapting for automotive
With today’s high-brightness, multi-kW laser sources, remote laser welding with a stand-off up to several hundred millimeters became widely feasible in industrial welding applications. For this, scanning optics steer the beam through a pair of high-speed galvo-driven mirrors (x and y) and, in the case of 3-D applications, a fast focusing mechanism quickly adapts the focus distance (z).
These systems are capable of guiding a laser spot with high precision and can relocate the spot to any position in their work envelope within a few milliseconds. However, the work envelope of scanner optics is limited in size. Using an industrial articulated arm robot to guide the scan head, the working range can be extended to cover large work pieces – like a vehicle – and to access them under well-defined inclination angles.
With conventional joining methods, like resistance spot welding, riveting and fixed optic laser welding, the robot needs to navigate to each weld seam, which imposes a large share of repositioning time on the overall cycle time. These non-productive repositioning times can be dramatically reduced when a remote welding head is controlled synchronously with the robot that is guiding it.
In this so-called “on-the-fly” operation, the robot is guiding the scan head on an even and smooth trajectory while all high-speed movements are executed by the much faster scan axes, shown in Figure 1. With this technique, laser-on times in the order of 90 percent or more can be realized. Experience has shown that a single weld using remote laser welding can be made in as little as 20 percent of the time required to make an equivalent weld using conventional welding techniques, like resistance spot welding, riveting or fixed optic laser welding [1].
Advantages of fillet seams
When considering scanner systems implemented in the industry today, the majority of applications have been developed for overlap joint configuration, which is less sensitive to positioning errors. However, as automakers are striving for weight and material savings, shorter flanges on body components and welding of fillet seams are more desirable, as seen in Figure 2.
Additionally, fillet seams offer laser process advantages: For zinc-coated materials, a fillet seam configuration allows welding with zero-gap, which eliminates time and cost-intensive measures to create a controlled gap as required for degassing in overlap geometry.
Furthermore, quality assessments can be achieved in a more direct manner for fillet seams as their surface topography can be directly measured. However, to process fillet seams with remote laser systems, a robust, accurate and non-tactile detection and tracking of the edge on the top sheet, is required.
To fulfill that requirement, BMW initiated the novel development of a dynamic seam tracking solution, combining the remote welding systems and controls of Blackbird and Scanlab with the camera solutions of Lessmueller Lasertechnik. The result was an enhanced scan system with a co-axial illumination and online seam tracking functionality.
Dynamic seam tracking
The system is based on a model of the Scanlab intelliWeld scan head shown in Figure 3, which implements the principle of post-objective scanning. This design projects the beam focus within the x-y-working plane by a highly dynamic z-axis, eliminating the need for an F-Theta lens package. The outcome is an undistorted camera image for the coaxial computer vision over the entire scanner operating field.
Lessmueller’s Weldeye camera system is adapted to the scan head to provide coaxial observation. The area around the processing zone is illuminated by an 850-nm laser source. The reflection propagates backward along the optical path to the camera where it’s analyzed. The fillet seam geometry reflects the illumination light, such that a dark/light contrast line is observed by the camera at the joint location shown again [[identify where the photo of Figure 3 can be found in the layout]].
Based on the Blackbird ScanControlUnit, an integrated control scheme is implemented to ensure robust and stable tracking of the actual edge position in real time during robotic on-the-fly welding. In every robot motion time increment, the controller sends the expected value for the position of the edge in the camera image and also a distinct search window (e.g. ± 3 mm from the programmed welding position) to the vision system, shown in the image referenced above.
This minimizes the amount of image data processing and enables both high speed and fidelity in the seam finding function. The control scheme corrects the programmed scanner path in each time increment based on the deviation between the programmed and actual weld seam position.
The seam tracking function of the resulting system ensures both accurate and robust processing of fillet seams. At the same time the advantages known from robotic scanner welding are preserved:
- Robust seam tracking with an accuracy of typically 0.1 mm with more than 100 Hz
- High flexibility through omnidirectional processing
- Maximum efficiency thanks to minimized repositioning times and robot movement
- Improved weld joint accessibility through long stand-off from the work piece (~0.5 m)
With this approach, the scope of remote welding in automotive components is greatly extended: For the first time, edge welds can be processed at the high speeds associated with on-the-fly welding.
Testing for success
An initial prototype of the remote welding system had proven viability of the concept and an intense industrialization phase followed at BMW to develop the process and enable the system for industrial production. Early in 2012, a first welding system was implemented at BMW’s Munich R&D center and both the system functionality regarding seam tracking and the hardware design regarding industrial use were intensively tested and optimized in iterative cycles.
By the end of 2012, the first serial production systems were installed in BMW’s Swindon, U.K., plant, shown in Figure 4 . The part output was ramped up from pre-production to the start of serial production in mid-2013. Due to the extensive evaluation phase in close cooperation with the suppliers, the system proved the expected performance in an industrial environment: robust seam tracking and thus stable remote welding of fillet seams at an unmatched efficiency, in terms of cycle time and throughput.
In conclusion, remote laser welding is a highly productive processing method, which is enabled by the seamless control integration of optical scan heads with industrial robots. Historically, it was used to replace resistance spot welding of overlap joints in automotive components, such as attachments and seats. With this newly developed approach, however, the application scope of remote welding has been extended to enable remote welding of fillet seams. And, the developed scan system with coaxial seam tracking provides highly robust edge tracking during on-the-fly welding and thus, an unmatched combination of accuracy and speed.
M. Pälmer: “Laser application in manufacturing aluminum doors for the new Mercedes Benz S-Class,” European Automotive Laser Applications (EALA) 2014, Bad Nauheim / Germany
