Laser System Welds Rocket Nozzles

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Europe's Ariane 5 rocket made its 50th flight from the Kourou spaceport in French Guiana on May 21, 2010, carrying two telecommunications satellites with a combined mass of almost 8 tons. One of Britain’s contributions to the successful launch was the vision equipment that enabled robotic welding of the rocket’s upper-stage nozzle.

Supplied by Oxfordshire manufacturer Meta Vision Systems, the laser-based tracking system is able to detect and follow a joint even when there is no discernible gap or step. This attribute was instrumental in the company's winning a prestigious repeat order to assist in automating the production process.

The Kuka robot, guided by a Meta-Scout laser-based seam sensor, welding an Ariane 5 upper-stage nozzle at the Ottobrunn factory of Astrium, near Munich. (Image: Kuka)

Called Meta-Scout, the equipment was specified by Astrium GmbH, which is a major contractor to EADS for the Ariane 5. The company selected the sensor due to the faultless operation of an earlier version of the laser vision system, which is still operational today despite having been supplied over 20 years ago.

Kuka was the robot supplier chosen to undertake the latest turnkey project to take over automated TIG (tungsten inert gas) welding of Ariane 5 nozzles. Each one is fabricated from 242 nickel alloy tubes approximately 4 mm square and with a 0.32-mm wall thickness, so that the nozzle can be regeneratively dump-cooled in flight by liquid hydrogen.

Before welding, the nozzle profile is created by bundling the tubes together and spiral winding them around a copper-coated aluminum mandrel. The tubes are held together with binding wire, which is unwound step by step as welding proceeds. Total welded seam length in each nozzle is approximately 730 meters.

The width of each tube varies slightly along its length to optimize coolant flow and enable uniform heat distribution. During a majority of the welding process, which does not use any filler material and therefore does not add weight to the nozzle, the Meta-Scout is able to resolve the seam between adjacent tubes and follow it conventionally. However, the square profile of the tubes near the engine end is flattened to achieve the correct fit, with the result that there is no visible feature to track.

It is here that the sensor uses its novel "zero gap" functionality to direct the robotically deployed torch to weld the joints. Five laser lines projected onto the workpiece surface allow measurements in six degrees of freedom, three orthogonal and three rotational.

A structured light technique together with gray-level vision analysis determines seam position, height and orientation with respect to the tool, despite the absence of a physical feature. The only provisos are that the material on either side of the gap must have similar machined finishes and that the sensor must be positioned at close to 90° to the surface.

Advanced planning software allows the nonlinear path to be predicted and followed to an accuracy of 0.1 mm. The robot is guided in real time, without the need to carry out a laborious teach-in process. Only the start of the joint and the search direction need to be entered.
Weld parameters are automatically selected to suit the measured gap dimensions. A calibration system aligns the sensor with the welding torch and calculates any deviation of the TCP (tool center point), which is automatically corrected by the robot controller. The automated welding process can be monitored on a screen located outside the enclosed cell in which the equipment is installed.

Other Meta-Scout sensors in use, where the butt joints to be welded have no gap or height difference, are to be found in France for the manufacture of ducts for the Airbus A380 and in the reconditioning of land-based turbine rings by a customer in the US.

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The automated TIG welding process in action:

Published: July 2010
1. A generic term for detector. 2. A complete optical/mechanical/electronic system that contains some form of radiation detector.
242 nickel alloy tubesAriane 5Astriumautomated TIGCommunicationscoolant flowcopper-coated aluminum mandrelEADSEnglandEuropeGermanyImagingindustrialKourou spaceportlaser vision systemlaser-based tracking systemMeta ScoutMeta Vision SystemsResearch & Technologyrobotseam positionsensorSensors & Detectorstelecommunication satellitesTest & Measurementweldingzero gapLasers

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