Mercedes reap benefit of arc welded cab structures

Citation

(1999), "Mercedes reap benefit of arc welded cab structures", Industrial Robot, Vol. 26 No. 6. https://doi.org/10.1108/ir.1999.04926faf.006

Mercedes reap benefit of arc welded cab structures

Mercedes reap benefit of arc welded cab structures

Keyword: Arc welding, Automotive

In many instances in the automotive industry the choice between using MAG welding or spot welding is not dictated by technical requirements, rather by the fact that spot welding is the low cost option.

For good quality MAG welding the seam should be positioned within 0.5mm; this is often exceeded by cumulative component and positioning tolerances so that the user is forced towards spot welding where a deviation of 5mm can often be tolerated.

For larger structures such as lorry cabs, maintaining 0.5mm is difficult, the components such as the single piece roof are heavy and not rigid and extensive fixturing would be required.

This was the dilemma faced by Mercedes Heavy Truck Division in Worth. The company set themselves the task of creating an automated and flexible production facility that could handle four basically different model truck cabs including the Actros and Atego, the "1998 Truck of the Year".

In addition to the four types the line also had to cope with variants such as increased sleeper cabs height, sun roof and increased cab width so the total number of different bodies that could be specified quickly mounted into the hundreds.

To provide this level of flexibility, a robot line was the only realistic option and the line which was eventually installed involves eight production cells and a total of ten robots. The first two cells with two robots processes the floor pan. Another cell uses two robots to finish the floor with the cab body in position. To maintain production levels, two identical cells of three robots assemble the main structure.

To accommodate all the variants, the robot programmes are selected by the carrier that holds the cab assembly. These are coded so that the appropriate weld programmes are down-loaded from control system memory.

A more intractable problem was ensuring that all seams on all variants were within the 0.5mm window dictated by the arc welding process without incurring prohibitive component and fixture costs. This was achieved by fitting each robot with a laser based seam tracking system supplied by Meta Technology of Didcot, UK (see Plate 6).

The system used was a Meta MTR, specifically developed for robot applications. This operates by laying down a laser stripe in front of the weld and at 90o to the seam. A CCD camera views this stripe and the picture is interpreted by sophisticated software to provide correction signals to the robot control system.

Plate 6One of the welding cells at Mercedes with robots equipped with Meta Laser Guidance System

Both laser diode and CCD camera are contained in a single sensor unit. For the Mercedes project, a field of view of 14mm was selected. As long as the seam falls anywhere in the field, the welding torch can be precisely aligned with it. A filter, located in front of the camera, excludes the weld arc and therefore the sensor can be mounted close to the weld torch.

An essential element of the Mercedes system was, once the assembly is identified, the robots select the appropriate program and complete the assembly without any operator intervention.

Another important consideration was that all sensors are interchangeable. This is achieved by calibrating each sensor, then permanently storing the calibration in the sensor.

Since installation the sensing systems have fulfilled expectations, maintaining the quality standards that are the hallmark of any Mercedes vehicles.

For Meta, lessons learned on the project suggest that the system is capable of handling the next major breakthrough in automotive body assembly - laser welding where a positional accuracy of 0.1mm is required. But that's another story.

Contact Mike Wilson - Managing Director, Meta Technology Limited, 10 Harrier Park, Southmead Industrial Estate, Didcot, Oxon OX11 7PL. Tel: +44 (0)1235 512215; Fax: +44 (0)1235 512115.