Understanding the differences in
feature inspection with tactile probing
versus laser scanning
Traditionally, laser line scanners are being used for
creating digital copies of (generally freeform) objects. The high data rate of optical systems, compared to traditional touch probing, makes this
technology extremely suitable for collecting many
measurement points required to sample complex
(non-prismatic) geometrical shapes.
Laser line scanners have evolved over the last few
years to a point where they become a valid alternative for tactile inspection of geometrical primitives.
Improvements in resolution, quality of the optics,
image processing and data analysis have turned
laser line scanning into a sufficiently accurate but
much more productive substitute for tactile measurements, even for feature inspection. There are
several real-life situations where the higher measurement density of optical systems leads to more
representative (closer to reality) results than tactile
measurements.
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In the current situation where the technology of laser line scanners has
evolved to measurement accuracies that are very close to the accuracy
of tactile probes, the greater data density of laser scanning provides
a major benefit for feature fitting. The accuracy of tactile probing
technology relies heavily on the assumption of perfect geometry for
good feature fitting. Due to the low sampling rate, tactile data is very
sensitive to imperfections of the geometrical feature.
The out-of-roundness of a stamped hole in a sheet metal part is
typically much larger than the measurement accuracy of a tactile
CMM. When identifying the hole with just three to five measurement
points, the roundness error may result in an uncertainty that is many
times larger than the accuracy of the CMM.
Optical measurement systems collect much more data in the same
or less time, resulting in more correct handling of geometrical
imperfections. The individual measurement points in an optical point
cloud may be noisier, but the higher point density results in improved
representation of the real geometrical feature.
Limitations of tactile inspection
This chapter explains a number of issues that occur during tactile
feature inspection. They typically occur on imperfect parts that have
flatness and roundness errors, edge rollover, burrs, etc… which
could be generally termed as “near” geometrical features. The
‘undersampling’ of tactile inspection makes the results very sensitive
to such influences. Laser scanners on the other hand collect much
more data, allowing elimination or reduction of such influences.
1. Flatness errors around the feature - Material deformation
around a punched hole results in a flatness error in the surrounding
plane. In such cases, the locations where the few tactile points are
taken (to identify the relative plane of the hole) may influence the
result strongly.
2. Non-orthogonally between feature and the plane of the
feature: When the measured relative plane is not orthogonal to
the side wall of the hole, the points measured on the side wall are
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projected along a direction that is not parallel to the centre of the hole.
This results in a position error of the hole centre. This problem typically
occurs when the hole is located on a (slightly) curved surface.
In case of laser scanning, enough points are measured on the inner
side wall of the hole in order to identify its position and orientation
accurately. Also, the relative plane is measured with much more points,
resulting in much better conditioning of the fitting of the plane.
3. Hole roundness errors: The roundness error of a hole has a large
influence on the identified centre of the hole. There is a significant
difference between the location of the yellow and blue circle in the
sketches above, as a result of different touch points in a hole with
out-of-roundness issues.
Flatness errors around the feature
Non-orthogonality between feature and relative plane (sometimes as a result of part deformation)
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Hole roundness errors
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4. Rolled-over, burnished and fractured edges causing
uncertainty on where the tactile probe is making contact
(probing problem): When measuring a hole with a curved edge, the
position of the probe on this edge determines the position of the edge.
In the above right picture, the black dashed lines show the identified
edge, depending on how deep the tactile probe takes measurement
points. Especially on thin material, it is very difficult to assure that the
tactile probe makes contact with the material in the most extreme
point of the curved profile. Measuring chamfered features requires very
dense sampling in order to determine in what direction to calculate
radius compensation for tactile probes.
5. Radius compensation error (calculation problem because
of uncertainty of the surface normal). When a tactile probe
makes contact with material as indicated in the above picture, the
radius compensation may result in unexpected measurement points if
a neighbouring surface is touched first. For example, the green point
may be considered for determining the horizontal plane instead of the
blue point.
Measuring results on realistic sheet metal parts
A real sheet metal part (A-pillar) is was measured with a tactile probe
taking three points on the surrounding plane and five points on the
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inner mantle of all holes. The same holes were also scanned with a
Nikon Metrology LC15Dx scanner. The measurement uncertainty of
the LC15Dx laser scanner for feature inspection is not very different
from the tactile measurements, while there is a significant (35%)
reduction in measurement time. Diameter and position errors for both
measurement techniques fall well within the required tolerances for
sheet metal applications. Results of these measurement are presented
at various conferences and can be obtain on request.
Conclusion
Analysis and practical tests reveal that laser line probes achieve
a feature inspection accuracy that cannot be distinguished from
tactile accuracy. The higher data density of optical systems enables
elimination of the local imperfections on real parts that mislead tactile
inspection. Comparable accuracy but a much higher productivity
makes optical laser line scanners an attractive way to significantly
increase the productivity of traditional CMMs.
Acknowledgements
The work reported in this paper is supported by the IWT Project
120839 IDiFIX
No certainty on where on rounded edges the tactile probe is making contact (probing problem)
Radius compensation error