In-vessel photogrammetry survey

As a consequence of the extreme forces during plasma disruptions, high thermal loads, and new tile materials, the geometry of the JET plasma facing wall is constantly changing. The installation of the ITER-Like Wall requires the highest level of accuracy yet attempted at JET. This is the motivation for a survey of big parts of the in-vessel geometry of JET -  a process, which is comparable with the development of a 3-dimensional representation of a landscape. Accordingly, similar mathematical triangulation methods as used in geodesy are applied.

The first step after an extended theoretical preparation of the survey is the installation of 300 photogrammetry target tools (Fig. 1). These incorporate 1500 calibrated small circular retro-reflective dots, which are positioned precisely by remote handling. After this, a special camera with very well defined optical characteristics and resolution to a tenth of one pixel is mounted on the remote handling boom. The boom, which is a robotic arm of more than 5 meters length, brings the cameras very close to precalculated positions in order to produce pictures. This is done from 90 different positions. When this procedure is performed, every target has been photographed from a large number of positions giving high redundancy within the subsequent calculations for greater accuracy (Fig. 2).

In the next step, the centres of the target dots are automatically identified within the pictures. Once this information is available an iterative mathematical process localises the real position of the centres of the target in the 3-dimensional space. In order to do this, some targets on the inner very stable part of the torus are taken as a reference, to which the virtual coordinate system is attached.

The fully remote photogrammetry cameras used at JET are also used by NASA and ESA. With these cameras it is possible to measure a target with accuracy in the order of 0.010mm within the torus volume.

Fig 1 - In-vessel picture of JET taken by a photogrammetry camera. The dots are the photogrammetry targets.

Fig 2 - 3D view of all photogrammetry targets and all camera positions. The lines indicate all camera positions, from which one single target can be seen.

21st August, 2007