First-ever robotic welding of pressure vessels to E.U. regulations

Citation

(2002), "First-ever robotic welding of pressure vessels to E.U. regulations", Industrial Robot, Vol. 29 No. 2. https://doi.org/10.1108/ir.2002.04929baf.002

First-ever robotic welding of pressure vessels to E.U. regulations

Keywords: Robots, Welding, Medical

At the Eynsham factory of Oxford Magnet Technology, 750 liquid helium-filled vessels are produced each year for housing the superconducting magnet that forms the heart of magnetic resonance imaging systems for body and brain scanners. It takes an experienced operator 14 hours to weld each stainless steel vessel by hand, but following the installation this summer (2001) of a turnkey robotic welding cell configured by Bauromat, the cycle has been reduced to just two hours (Plate 3).

Said Mark Tullett, production engineer in charge of the project at OMT, "The system has been coded by TuV in Germany as meeting its Pressure Vessel Regulations. It is the first time that any vessel that has been robotically welded has met a relevant European Union standard.

"It has been a painstaking process whereby the first 50 vessels off the robot cell underwent exhaustive tests including cross sectional examination of the full penetration circular welds. From now on, 1 in 100 vessels will be taken for quality assurance testing."

The automated welding system has eight degrees of freedom, six on a Motoman articulated-arm UP-series robot and a further two CNC axes on a bespoke, 10 tonne manipulator designed and built by Bauromat which orientates the two metre diameter vessels vertically, horizontally or upside down. Orchestrating the computer controlled movements of the entire cell is a Motoman XRC controller. In addition to welding, carried out at an average speed of 60 cm/min, the cycle includes a torch cleaning operation between runs and an automatic tool change to pick up a motorised brush for dressing the weld, all within the two hour cycle.

Plate 3 Bauromat selected a Motoman robot to automate welding of scanner magnet housings at OMT to EU Pressure Vessel regulations, reducing the cycle time from 14 to two hours

A Meta Vision weld seam tracking system has been fully integrated with the system. It uses laser light to check the position of the torch and adjust it automatically to within ± 0.5mm every time, a level of repeatability that would be impossible to achieve by hand. It comprises the company's standard MTR tracking system linked to a PC. The system compensates continuously for any deviation between the programmed weld path and the detected position of the butt joint. A real-time graphical representation of the joint is shown together with the wire position, and the deviation of the wire is given in two dimensions in millimetres as well as the joint gap to be filled (Plate 4).

Plate 4 Meta weld seam tracking is fully integrated with the robotic welding system at OMT. It uses laser light to adjust the position of the torchautomatically to within ± 0.5 mm.

Of particular interest is the novel expanding mandrel driven by a special six-arm gearbox for clamping each three tonne electromagnet in place while the tacked jacket is being welded. Each of six output shafts, comprising ballscrews driving linear slides from the central gearbox, rotate in unison to an accuracy of 1/120 of a degree of arc to secure the electromagnet very precisely. The shafts have internal pinion gears mounted onto the ends and driven by bevel gears at the top and bottom. The decision was taken to power the mandrel mechanically to avoid the risk of hydraulic leaks or the need for a separate compressor to prvide pneumatic thrust. In addition, the system pulses the drive to give a constant output pressure on the vessel to compensate for movement as the temperature changes.

Not only is the robotic welding process much faster than by hand, but also the weld is more consistent. Each new welder hired to operate the three manual TIG cells at Eynsham, which are now devoted mainly to special vessel production, has to be qualified to meet the TuV code whereas the robotic MIG cell only needs to be qualified once.

One of the problems associated with welding the product by hand is that the code stipulates full penetration of the 6 mm thick 304LN stainless steel, an OMT specification that combines austenitic (non-magnetic) properties with impact resistance at 4 degrees Kelvin. As there is a relatively wide gap – typically 6 mm – to fill with metal, six or seven TIG passes are needed to complete the butt weld. These take a long time and introduce variation in the weld seams. In contrast, the robot equipped with the laser sensor is able to MIG weld the joints quickly and consistently in just one pass.

Another consideration with manual welding of large components is the arduous nature of the work. There is a risk of neck injury if the head is held for too long in one position and additionally, continuous feeding of wire into a joint can cause repetitive strain injury. Such health and safety issues are eliminated if a robot is used.

When it came to invest in the automated welding system, Mark Tullett and his team drew on experience gained with a successful robotic cell installed at the factory in 1999 for high definition plasma cutting of aluminium and stainless steel parts for body scanner magnets. Bauromat had been the integrator but not the main contractor. For the new installation, it was felt that the integrator should be given turnkey responsibility for the entire project, to the extent that the cell was assembled and trialled at Bauromat's Pershore factory prior to delivery.

Continued Mr Tullett, "Bauromat is one of the few companies in the country undertaking this type of work. Motoman and Meta were selected for the project as, unlike the alternative robot and sensor suppliers, they too were prepared to put the effort into jointly developing this one-off application."

The success of the installation has prompted OMT to investigate a further cell to automate welding of the outer vacuum vessel for MRI scanners and feasibility studies are under way.

Contact: Paul Tilley, Bauromat (UK) Ltd, Unit 27, Aintree Road, Keytec 7 Business Park, Wyre Piddle, Pershore, Worcestershire, WS10 2JN. Tel: 01386 555542; Fax: 01386 555328.