Motion controller proves a boom for jet engineers

Citation

(1998), "Motion controller proves a boom for jet engineers", Industrial Robot, Vol. 25 No. 6. https://doi.org/10.1108/ir.1998.04925faf.004

Motion controller proves a boom for jet engineers

Motion controller proves a boom for jet engineers

Keywords Nuclear, Robots

The quest to achieve the endless energy supplies which will be unlocked through nuclear fusion is taking its next step within the Joint European Torus (JET) project funded by the European Union at Culham in Oxfordshire. The vital new experiments have presented fresh challenges for the JET team in that refitting the Torus itself now requires complicated robotics to remove, manipulate and fit new components within the plasma chamber. The demanding motion control requirements for the articulated booms that carry the robot manipulators have been met using Delta Tau PMAC motion control systems (see Plate 3).

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 about one hour, hence the need for robotics.

Fusion is achieved and energy is 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.

Plate 3 The Delta Tau motion control solution proved to be both cost-effective and flexible enough to enable the manipulator boom design to be implemented

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 deliver the new ones to the primary manipulator.

The manipulator boom is controlled using DC brushed servo motors on 18 axes, each of which is controlled using a Delta Tau PMAC motion controller. The controller has the capability of driving up to 22 axes if necessary. The secondary handling boom which removes and delivers tiles uses 15 axes, also controlled using PMAC. JET's engineering team investigated several options for the motion control system including one which was to be developed in house. The Delta Tau solution proved to be both cost effective and sufficiently flexible to enable the boom design to be implemented. The numerous built-in safety features of PMAC enabled the project to be completed quickly and with far less additional in-house software.

One of the factors in selecting PMAC was its ability to run in a simulation mode the numerous "teach" files that had been generated for the system. These teach files are lengthy to generate and their accuracy is critical in avoiding mechanical collisions when carrying out work within the Torus. In the event, the PMAC controllers presented no problems. Another feature of PMAC was the ability to read the resolver to digital converter outputs using Delta Tau's Accessory 14 universal interface cards.

The PMAC cards are all installed on a VME bus, and if joints within the articulated boom are changed, there is no need even to open the control cubicle since all changes are carried out in software. A program for the motor of the newly installed joint is merely downloaded and the parameters assigned to the new motor.

An impressive graphical operator interface has been created by the JET team's software engineers. This provides not only a clear and simple man machine interface, but also a full 3D graphical model of the boom and its environment. Together these systems can be used to "prove" PMAC programs in simulation mode prior to actually moving the boom mechanics.

The PMAC has to work first time and reliability is essential. The recent refitting was successfully completed inside a 17-week shutdown schedule, with the manipulator booms working 20 hours a day, six days a week to meet the deadline. Downtime on the equipment was really not an option.

According to JET's Mike Irving, "Using PMAC we have not come across a problem that could not be easily solved. Delta Tau gave good support and the chief European support engineer Brad Pedersen was extremely helpful".

For further information about Delta Tau contact Andy Joslin, Director, Delta Tau UK Limited, Holland Cottage, Kirby Road, Great Holland, Essex CO13 0HZ. Tel: +44 (0)1255 670196 Fax: +44 (0)1255 850768.