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5. APPLICATIONS IN JIGLESS ASSEMBLY
5.1 Jigless manufacture
The 3D-NET system is expected to find uses in jigless
assembly. In the past the Aerospace industry has relied heavily
upon jigs to produce assemblies of components to tight
specifications. All of the key information is tied up in the jig
and as a result these have to be measured to ensure that they are
within specification (figure 5 illustrates the current methods of
assembly of Airbus wings). It is currently perceived that large
static jigs constrain the manufacturing process in an undesirable
way. They delay the time to market of new products, fix the
method of manufacture for the lifetime of the product (up to 30
years for a civil aeroplane wing), and are expensive to make
and periodically certify. Civil Aviation Authorities can require
the aerospace industry to certify its jigs. Some jigs have never
been measured and in some cases no CAD design exists. In the
past errors in the manufacturing processes have resulted in large
qualities of shims which add the weight of an aircraft and
ultimately to the cost of flying that craft via the extra fuel
required. For instance, a new version of the Hercules aircraft
has resulted in a 40 percent greater range, cruising ceiling and
decrease in take-off distance together with a 21 percent increase
in maximum speed and a 50 percent decrease in time to climb
(Brown & Sharpe, 1998). Measurement can play a vital role in
this process by reducing unwanted shimming and product
material.
The move towards jigless assembly or what may also be called
minimal tooling assembly (as some tooling will always be
required) will take place using a mixture of humans, actuators,
and robots (figure 6). The objectives of the "jigless" method are
the: reduction of manufacturing costs; provision of greater
manufacturing flexibility; avoidance of expensive certification
of jigs; faster product to market times; and improvements in the
speed of manufacture. Measurement systems provide essential
information to ensure correct manufacture. In the future the
jigging function will be provided directly from CAD via the
measurement function rather than indirectly as at present
Figure 5. Airbus Wing
(Picture courtesy of British Aerospace)
A photogrammetric systems best advantage in this
environments is the ability to simultaneously measure many
33
points at one instant the disadvantage being the necessessity for
retro-reflective target to be used at each point to obtain the
highest accuracy.
Figure 6. Advanced assembly cell visualisation
(Arrow, 1998)
5.2 Part positioning task
To start the process of embedding the 3D-NET system in a
manufacturing cell a demonstrator was produced to illustrate
the capabilities of the system and allow development of
techniques. The demonstrator (figure 7) consisted of two parts
each with a five degrees of freedom movement. One part had a
laser mounted on it and the other had two small pinholes. The
task of the measurement system was to measure the relative
position of the two parts such that the laser would pass through
the two pinholes. One of the two objects could then be moved
to an unknown position and the transformations necessary were
calculated to move the other object back into the same
orientation. This process required each object to be recognised
and tracked.
Figure 7. Image of demonstration system.
5.3 Software
The 3-D co-ordinates of the object points obtained from the
separate adjustment or the bundle adjustment are in an arbitrary
datum if control points are not involved. It is necessary to
transform these object points from one co-ordinate system (the
arbitrary co-ordinate system) to another co-ordinate system (a
given or common co-ordinate system) for the purpose of
relative positioning. The relative position X,Y,Z and rotation
