The quest for nuclear fusion is taking its next step within the JET (Joint European Torus) project. The new experiments have presented fresh challenges for the JET team in that refitting the Torus now requires complex robotics to remove, manipulate and fit new components within the plasma chamber.
In the past, refitting the Torus with various experimental devices has been performed manually by engineers wearing protective suits due to the toxic dust in the Torus. The most recent experiments used Deuterium and Tritium isotopes, and have rendered the Torus chamber temporarily too radioactive to allow people inside for longer than an hour, hence the need for robotics.
Fusion is achieved and energy released when light atoms are fused together. Fusion power uses Deuterium, an isotope of hydrogen extracted from water (mainly sea water), and Tritium, also an isotope of hydrogen, which does not occur naturally and has to be manufactured. Heated to high temperatures, the nuclei of these atoms fuse releasing helium and high speed neutrons. In a fusion power station, these neutrons will be slowed in a blanket of denser material and the heat generated by this process used to generate electricity. The plasma (a gas of atoms stripped of their electrons) formed in the process is contained by a magnetic field within a high vacuum. The plasma reaches temperatures of hundreds of millions of degrees centigrade – hotter than the surface of the sun.
The current phase of refitting is to replace the carbon tiles that form what is called the divertor. This extracts impurities from the plasma in the chamber which are then removed using a cryo pump. Each tile weighs about 35kg and there are 144 tiles to be replaced by 192 new ones around the floor of the Torus.
Entry to the Torus’ tubular ring shaped interior is via two relatively small hatches. To negotiate the continuous bend of the Torus’ chamber required an articulated boom that could be directed via a motion control system. Accurate positioning is essential to enable a robotic manipulator to perform the dismantling, removal, alignment and fixing procedures. A second boom is used to remove used tiles and to deliver new ones to the primary manipulator.
The manipulator boom is controlled using DC brushed servo motors on 18 axes, each controlled using a Delta Tau PMAC motion controller. The secondary handling boom which removes and delivers tiles uses 15 axes, also controlled using PMAC. JET’s engineering team chose the Delta Tau solution because it was cost effective and flexible. The numerous built-in safety features of PMAC enabled the project to be completed quickly and with far less addition in house software.
PMAC can also run in simulation mode which checks that no collisions occur during the actual work.