An in-vessel datum system was designed for use with the CAT
svstem and to be compatible with future installations. Special
targets were developed and their optimum positions in the
vacuum vessel were optimised on JET's CAD system. This
was carried out in parallel with a major hands-on system
development .programme which included building and
surveying a simple full size mock-up of the vacuum vessel.
This was used to develop, prove and qualify the accuracy of the
optimum multiple set-up survey technique needed to combat
the difficult geometry of the JET vacuum vessel. After
optimisation, the mock-up was used for intensive operator
training in a successful bid to cut survey times. Trials were
also carried out to develop the adjusting and measuring
technique required for each set of components. After surveying
the in-vessel datum system, CAT software was used to
transform target coordinates into the machine datum system,
which has its origin at the machine centre, by referring to
master in-vessel targets which could also be sighted from ex-
vessel as shown in figure 3.
3.4 Preparations for In-vessel Use
Wall mounted vibration-free theodolite supports were
developed. The colour laptop computer was built into a special
aluminium case along with the necessary interface modules. In
addition, the computer was fitted with an integral modem
which allowed fast downloading of data at the end of every
shift to a dedicated computer ex-vessel. Transfer of data by
floppy disc was complicated by the possibility of beryllium and
tritium contamination. Use of optical instruments in-vessel
(figure 4) was hampered by the necessity to wear full
pressurised suits to protect against the hazard of air-borne
beryllium dust. Special suits with optially clear visors were
developed and used successfully during the MKII Divertor
installation. Fortunately full suits were not required during the
majority of the MKI Divertor installation.
Figure 4. Surveying in-vessel with the CAT system during the
MKII Divertor installation.
3.5 The MKII Divertor Support Structure Manufacture
and Installation
The main components of the support structure (figure 5) are an
inner and outer ring mounted on a baseplate. The outer
diameter of the assembly is 6.2m, the inner diameter 4.5m,
height 0.5m and weight 7 tonnes. The structure breaks down
into 24 sub-assemblies or modules to allow insertion and
assembly in the vessel (Celentano, 1995). The modules were
connected using 32 precision dowels and bolts at each joint. It
was clear from an early stage that the precise assembly of the
components of the structure was fundamental to the
achievement on installation of the conflicting tight tolerances
on position, roundness and concentricity to machine centre.
Consequently the use of the CAT was integrated into the
design, manufacture and assembly (Macklin, 1995) as well as
the installation of the structure.
Figure 5. The components of the MKII Divertor structure.
The CAT system was used to check the large fixture used for
the final machining and assembly sequences at the
manufacturers (figure 6). With 24 joints in the structure it had
to be assumed that the structure would build to a slightly
different shape each time it was dismantled and reassembled (a
total of 3 times). This led to a concept of ‘as-built’ co-ordinates
and a ‘best-fit’ approach where the actual dimensions are
allowed to float into their best average fit to theoretical
dimensions. The comparison between the three builds is
illustrated in figure 7.
Figure 6. Using the CAT system to survey the MKII Divertor
332
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
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