quality
The complete system
set-up is mounted on a
robot arm; it is possible to
move it around the object to
be inspected, for example, a
car body.
A stationary installation is
also possible, in which the
parts are placed on a
measuring table.
BMW is to use an automatic surface defect inspection system
for the quality control of sprayed car parts
Making sense of inspection
SHINY or reflecting surfaces occur frequently in the
automotive industry, and guaranteeing their quality is
critical. The quality control of such surfaces still uses
mainly manual techniques. Typically, personnel will
observe reflected images using fluorescent lamps on the
surface to detect defects. It’s expensive and time consuming
and provides results that are often difficult to reproduce
and are not quantifiable. To detect dents, bulges and other
surface defects on a reflecting object - for example, the
bonnet of a car - normally the reflected image of the
surroundings on the surface are observed. The difference
between the surroundings and the reflected image is a
measure of the quality of the surface. This process occurs
naturally in the human brain and eye; to apply it to an
automatic quality control solution demands considerable
expertise in sensor systems and image processing.
BMW is working closely with sensor system specialist
Micro-Epsilon, developing and testing complete surface
defect detection systems. These can operate as stationary test
rigs or by using robot arms, which scan multiple inspection
points on car body parts, automatically identifying and
marking any surface defects; they will enable complete car
bodies to be inspected in typically less than 90 secs typically.
By introducing this procedure at various stopping points in
the assembly line, production delays can be avoided.
Franz Hochwimmer, Sales Engineer at Micro-Epsilon’s
Systems Division comments: ‘Since 2005, we’ve been
working with BMW, initially developing systems to detect
surface defects on small, plastic and rubber components on
the car. This involved the inspection of components such as
car bumpers, mudguards and cylinder heads - but not the
complete body of the car. At the end of 2005, Micro-
Epsilon supplied a system for BMW’s Dingolfing plant, to
automatically detect surface defects on the sides of the car,
including front and rear mudguard areas and the doors.’
Micro-Epsilon’s reflectCONTROL ‘deflectometry’
system can now be used in industrial production. To
ensure reproducible, well-structured surroundings - the
reflected image of which is to be recorded and evaluated - a
striped pattern is shown on a display.
Cameras record the reflected image of
the display and pass the data to an
industrial PC for evaluation. The
complete system set-up is mounted
on a robot arm; it is possible to move
it around the object to be inspected, for
example, a car body. A stationary installation
is also possible, in which the parts are placed
on a measuring table.
Hochwimmer continues: ‘In 2006, we
installed reflectCONTROL on a robot arm,
which inspected around 36% of the car body for
defects. BMW Digolfing is evaluating these
results now with a view to installing a complete,
88 MWP march 2008
four-robot arm detection system shortly, which will inspect
complete car bodies across two vehicle assembly lines.’
In Micro-Epsilon’s reflectCONTROL system, an LCD
with a screen diagonal of up to 40 inches is used as the
display. The cameras used are high-resolution CCD
cameras. It is therefore possible to measure an area of 60cm
x 80cm in one recording. If a larger area has to be
measured, the measurement system can simply be
positioned at various places along the surface by the robot.
Here, the control of the robot is integrated into the system.
With this set-up, even very small defects down to 0.5mm
can be detected reliably.
In QA a decision has to be made whether a part is
acceptable or defective. In a manual procedure, the worker
must reliably recognise different faults in order to be able
to make this decision. With a sprayed surface, for example,
these may be large dents, inclusions in the spray coating,
porosity, grinding and polishing marks, and so on.
With deflectometry, the reflected image recorded by the
camera is processed in a computer. During the analysis of
the image data, the various faults must be reliably detected
and unambiguous decisions made. Whereas a dent causes a
slight change in the surface curvature over a comparatively
large region, the change of curvature for an inclusion in
the spray coating is very large, but is restricted to a very
small area. To enable the system to accurately and reliably
measure to such a high resolution, the system constantly
compares the inspected data with the exact 3D CAD model.
Otherwise, for example, openings in the body parts may be
erroneously detected as faults.
Complete bodies inspected in under 90 secs
At BMW’s Dingolfing plant in Germany, in-line stations
are planned, which will evaluate the complete car body. To
do this, four robots in a measurement station, will acquire
approximately 65 to 80 inspection points on the body. The
inspection of the complete body will last for between 60
and 80 seconds. If the test rig is positioned at a stopping
point in the production line, the process can take place inline
and without introducing delays.
There are many advantages with this fully automated
quality control system. The primary factor here are the
dependability and absolutely reproducible decisions as to
whether a part is acceptable or defective. For further
inspection and possible processing, two marking robots,
which mark the defective places, follow the inspection unit.
Also, the defects that occur are saved and documented, so
that they are available for more thorough analysis later.
Consequently, it is possible to generate statistics
documenting which types of defect occur and how often.
Statistics of this nature can then be used for optimising the
processes, leading to improved quality, less scrap and
rework costs.
www.micro-epsilon.co.uk
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