Precision robot welding for world's largest particle physics apparatus

Citation

Rooks, B. (2005), "Precision robot welding for world's largest particle physics apparatus", Industrial Robot, Vol. 32 No. 4. https://doi.org/10.1108/ir.2005.04932daf.001

Precision robot welding for world's largest particle physics apparatus

Precision robot welding for world's largest particle physics apparatus

Keywords: Robotics, Welding, Precision

The Large Hadron Collider (LHC) currently under build for CERN, the European Organisation for Nuclear Research will be the world's largest particle physics apparatus when it is completed in 2007. Located 100 m underground close to CERN's headquarters near Geneva, the LHC will accelerate two beams of particles in opposite directions within a 27 km ring of super conducting magnets held at temperatures approaching absolute zero. The resultant collision between the two beams will help provide research information at the very edge of scientific knowledge.

Critical to the functioning of the LHC are dipole and quadrapole tube assemblies being manufactured by Birmingham-based Accles & Pollock. Dipole tubes are used to bend the path of accelerating particle beams and to keep them on course, while quadrapole tubes are used to focus the particle beams for collision at detector points.

The special, non-magnetic, stainless steel alloy tubing assemblies are being manufactured in a laser welding cell designed and manufactured by Garrandale Systems and developed in collaboration with Ferranti Photonics. The cell is built around two ABB IRB 140 six-axis robots, and is to quote Garrandale's Managing Director, Chris Moore, “probably the most precise and demanding application of a standard robot ever devised”.

Garrandale's task was to design a manufacturing system and jig that could handle both “dipole” and “quadrapole” assemblies and in a range of variants. Each is an extremely accurate and complex welded assembly, comprising a beam screen tube, cooling tube, cooling tube feedthroughs, beam screen fixed points, sliding rings, contact rings and cooling tube supports. The tubes, in lengths of 15- 18 m, have to be straight and accurate to extremely tight tolerances in all planes. Welding specifications are extremely demanding. On one detail alone, the tube assembly requires 0.3 mm diameter spot-welds every 1mm of axial length. For the two particle accelerator rings that works out at 54,000,000 welds alone!

The Garrandale system is contained within a temperature and humidity controlled environment and operated for safety reasons through remote CCTV. Each of the two ABB IRB 140 six-axis robots, which travel independently along linear tracks, is equipped with a Luxstar 100 W Nd: YAG laser welding head. The IRB140 is ABB's smallest robot and provides an extremely compact automation solution while boasting one of the fastest cycle times of any articulated robot and an industry-leading payload for its size, of 5 kg.

Commenting on the selection of the standard IRB140 robot for these tasks Chris Moore said: “When surveying the market we could have gone for a specialist robot, however we knew the ABB IRB 140 was inherently precise by the nature of its design. ABB allowed us to check out the IRB 140's positional accuracy and repeatability during a demonstration at the company's Milton Keynes facility, and the robot did indeed achieve 10 microns. In service, it has performed every bit as good with no faults whatsoever” (see Plate 1).

Laser welding was chosen because it is a high speed, precise process that is ideal for the application due to its ability to produce very controlled weld penetrations. Other benefits of the process include low distortion, excellent aesthetics and a reduced heat-affected zone. The Garrandale system takes advantage of the Nd: YAG laser's ability to focus through standard optics. Thus, the laser's lens is linked to a CCTV camera and used as a highly accurate positioning tool by the system operator, providing an extra check for the accuracy of weld positioning.

Plate 1 Cell incorporating two ABB IRB140 robots for welding tube assemblies for the CERN Large Hadron Collider, designed by Garrandale Systems for Accles & Pollock

Fixed tooling in the welding cell is used to locate the screen tube assembly from below while an integrated sliding carriage moves along the tooling to position the cooling tubes as they are welded. On either side of the tooling, linear tracks provide a seventh axis for the robots. The robot-mounted lasers weld most of the tube assembly components, the exceptions being the cooling tube and feed through. For cooling tube welding, the laser delivery heads are removed from the robots and located on a special tooling carriage, which is towed along one of the robot track slides.

The ABB robots work in close synchronisation moving along the linear track in a primary robot/secondary robot configuration. In some instances, the robots even work together on a single weld, such as the circumferential welding of the contact ring.

Both robot controllers are linked through standard ABB software to effectively give a single robot control system. To assist this and primary robot/ secondary robot synchronisation, extensive use is made of ABB's RobotWare advanced programming options, including “World Zone” software. World Zone software is especially useful when two robots are working in close proximity to prevent collision and establish working protocols.

Chris Moore adds, “The combination of precision tooling and a standard, though high performance robot, coupled to a high-level of robot programming, has produced incredible levels of accuracy and repeatability within the application for which we are all justly proud”.

Garrandale's Operations Director and senior manager on the project, Tony Hart sums-up, “The project uncovered some challenging technical problems, probably the most difficult of which was achieving weld positional accuracy and repeatability on the relatively small and complex area of the tube assemblies. Garrandale Systems is extremely proud of the innovative way in which these challenges were overcome through our own research and development, and the technical discourse and collaboration between the customer, our project partners, suppliers and CERN engineers. To this end, the help and advice from ABB plus the exceptional positional accuracy and repeatability of the ABB robots, were invaluable in achieving these goals”.

Brian RooksAssociate Editor