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| The measurement system in use. The complete surface is checked on the production line for defects. (Source: BMW) | |
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The identification of surface structures and defects plays an important
role in process and quality controls. The projection of a stripe system
on a reflective surface shows up form deviations and faults in the
reflected light. The analysis of the reflections enables the safe
detection of form deviations and defects on the surface, right down
into the nanometer area.
Faults on the surface of reflective objects are noticed by the eye and
reduce the quality of the product. The analysis of errors is often
difficult to perform because it is partially manual and is not easy to
automate. Deflectometry is an automatic optical procedure for the
industrial error analysis of reflective surfaces, which allows
qualitative and quantitative conclusions on forms, surface structures
and defects to be made quickly and reliably.
The measuring principle generates a system of straight stripes with a
sine-shaped contrast distribution, which is projected onto the object
to be tested. The stripe system is, for example, generated on a screen
with white light diodes. Curvatures on the surface deform the linear
alignment of the stripes and provide a more or less deformed image. The
reflected stripes are recorded with a camera. The angle of the surface
normals to the local curvature of the reflected stripes is analyzed as
measured data. These data are processed in a computer with special
software.
With the analysis of the image data, different errors are definitively
identified and clearly differentiated. While a dent causes a small
change in the surface curvature over a comparably large area, the
curvature change with an inclusion is very big over a small area. The
algorithms used for the automatic error analysis and classification are
therefore very complex.
Depending on how much an object is reflected, various black-white
contrasts are required. With use on lenses with greater focal lengths
in combination with adjusted stripe patterns, it is possible to achieve
resolutions down into the area ranging from tenths of a micrometer to
the nanometer area. Depending on the display size and the resolution of
the camera, only a limited surface area of the object can be examined
in a measurement process. A complete surface check may therefore be
necessary with bigger objects.
The RC-Robotic system from Micro-Epsilon (picture) is an example of
objects as big as car bodies. The optical part of the measurement
system is used here as a sensor on a robot. A measurement process
covers an area of 80 x 60 cm2. The robot moves the measurement system
to various positions around the object to be inspected. The full
inspection of a car body is completed on an industrial production line
in cycle time with four RC-Robotics within 60 seconds. The defects
found are then shown on the object by a marking robot.
Deflectometry is generally a good solution for measuring all reflective
surfaces, e.g. in the automotive industry, the aviation industry, the
painting industry, and for solar cells and liquids. Several research
institutes, such as the Bremer Institut für angewandte
Strahltechnik - BIAS (Bremen Institute for Applied Laser Technology),
the Fraunhofer-Institut für Angewandte Optik und Feinmechanik
(Fraunhofer Institute for Applied Optics and Fine Mechanics) and the
Ravensburg-Weingarten University are currently working in this area.
BIAS was awarded the "Innovationspreis Lasertechnik 2007" (innovation
award for laser technology) for its "Zebra" stripe analysis software.
This very reasonably priced contact-less optical measurement technology
is also used in inspecting round and cylindrical objects.
