Reflection lines can be simulated on a computer screen much
easier than on the physical prototype, because usually the
prototype is made up of rather dull material making it
impossible to sufficiently visualize reflections. Even ıf the
prototype is given a paint coating there would still be the need
for highly specialized lighting equipment. The corrections
made to the CAD model to ensure well behaved reflection
lines are usually very small, a few tenths of a milimeter, which
ın tum are easier applied to the mathematical model rather
than to the physical model.
It is however often necessary to manufacture a new physical
model from the CAD data, because additional changes are to
be made, impossible to simulate or judge within the CAD
world. The modifications applied to this model in turn have to
be brought back into the CAD system.
This leads to the second application of photogrammetry in the
design process. There is already an exsisting design stored in
the CAD system which has to be corrected for the changes
applied to the physical model. The process of defining Bezier
patches is a rather time consuming task and it is not desirable
to repeat this process everytime the CAD model has to be
modified.
The idea is to change to CAD model on a global basis, i.e.
modifications to the prototype which may affect a number of
patches are to be applied such as to preserve the initial lines of
the design but on the other hand to adapt the design as close as
possible to the new data.
A typical example would be that the door of a car was enforced
side impact
few milimeters to give greater stability.
Fugure 4 - Differences (exaggerated) between original design
and new measurement data
The new data can easily be captured using the
photogrammetric system. The deviations between the original
design and the measurements can be used to compute a
difference surface which in turn can be applied to the original
data, preserving the original as much as possible.
If the original surface is composed of Bezier patches with
continous transitions, the addition may violate the conditions.
However provided the differences are small, the violations will
be nearly invisible. The addition of the difference surface will
480
also work with large differences, manual corrections to ensure
continous transitions will have to be made.
4 Conclusion
The combination of a photogrammetric measurement system
with a powerfull CAD system enhances the possibilities in
reverse engineering.
Let us raise a final question: reverse engineering quo vadis?
CAD systems are so far purely interactive requiring an
experienced operator to describe the surface of a digitized
object.
Attempts are made in the moment to automize the process of
patch definition in the initial phase (Eck, 1996, Hoschek,
1996). This promisses a further speed up of the design process.
An automatic process that analyses the measurements and
extracts possible locations for the Bezier patches could
significantly enhance productivity.
5 References
Bonitz, P., Krzystek, P.: Reverse Enginerring with ICEM
SURF in Combination With Digital Photogrammetry
ICEM PHOTO, Proceedings of Workshop Reverse
Engineering, Tauberbischofsheim, February 26-29,
1996
Eck, M.: Reverse Engineering using B-Splines, 1996
Gründig, L., Bühler, W.: Zur Náherungsbestimmung und
Bündelausgleichung von Konvergentaufnahmen.
Bildmessung und Luftbildwesen 53 (6) 1985
Hoschek, J.: Smooth B-Spline Surface Approximation To
Scattered Data. Proceedings of Workshop Reverse
Engineering, Tauberbischofsheim, February 26-29,
1996
Krzystek, P., Petran, F., Schewe, H: Automatic
Reconstruction of Concept Models By Using A Digital
Photogrammetric Measurement System, Proceedings
ISPRS Intercommision Workshop "From Pixels To
Sequences", Zürich March 22-24,1995
Schewe, H: Automatic Photogrammetric Car-Body
Measurement. Proceedings of the 41st
Photogrammetric Week, Stuttgart, pp. 47-56, 1987
Schultes, M.: Untersuchung der Abbildungseigenschaften
einer Still Video Kamera vom Typ Kodak DCS200,
Diplomarbeit, Universität Stuttgart 1995.
Sorgatz, U.: Das Geometriedatenerfassungssystem VWSCAN
im CAE-Konzept, VDI-Z 7/88, pp. 38-46, 1988
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
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