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COMET 250 White Light Scanner
Since August 2000, QC Inspection Services has
offered scanning services using the COMET 250 white light
scanner. Much of the following information pertaining to the
capabilities of the COMET 250 was obtained from the Steinbichler
website.
COMET Measuring System
Based on white light triangulation, the systems are specialized for applications ranging
from small, precision components to hand-sculpted models as well as large tools, dies and
vehicles. Using patented fringe projection techniques, COMET produces very dense, accurate
point cloud data which permits rapid surface and tool path generation.
COMET digitizes surface geometry patch by patch with ‘point-and-shoot’ ease of
use. The various patches of high-density data are aligned using various techniques, making
it a flexible system.
Software (Win NT) functionality includes high-speed point cloud processing, polygon model
generation, surface reconstruction, alignment to CAD and color-map inspection reports.
Import data to any CAD system.
COMET Optical 3D Digitizing System
The Comet 250 has a Measuring Volume Per View of 230 x 180 x 250 mm, and accuracy of
+/- 0.06mm.
Each view or "scan" measures 420,000 X-Y-Z points in 30 seconds. With large
objects or objects with complex surface geometry it is necessary to take several measurement
views to ensure that all surfaces are recorded. There is no limit to the number of views, or
"patches" that can be recorded per object.
Global Alignment of Digitized Point Cloud "Patches"
COMET digitizes surface geometry patch by patch with ‘point-and-shoot’ ease of use.
The various patches of high-density data are aligned using various techniques, making it an
extremely flexible system.
After the scan, the patches are globally aligned, or transformed together in the COMET
software to form one 3-dimensional point cloud. The 3-D point cloud can vary in size, up to
millions of measured points. The coordinates of each of these points are identified by the
system and can be output as a dense point cloud, poly-line cross-sections or polygonal model.
Formats include AC, ASCII, TXT, DXF, VDA, IGES, OBJ, and STL. Software also allows
alignment of point cloud data to CAD models and computation of graphic color-map
inspection reports.
Three Levels of Processing
There are three distinct levels of processing that can be performed on scan data once it has
been aligned into a single point cloud.
The first stage is direct application of the point cloud in a CAD comparison based 3-D
inspection. Because this type of comparison does not require any manual editing of the 3-D
data (with the possible exception of a quick removal of small marker added to aid alignment),
this application offers the quickest turnaround time. 3-D CAD comparison is also possible
with more fully processed data.
The second stage is creation of a watertight polygon model, typically exported in STL format.
STL files are suitable for N/C programming for many milling machines and are often used for
rapid prototyping. Processing applied at this stage ranges from simple filling of gaps in
the surface not captured by the COMET scanner, usually due to an obstructed view, to
sophisticated edge sharpening operations which restore feature qualities in the part beneath
the precision level of the COMET.
The third and most advanced stage of scan data processing is the creation of a complete
NURBS surface representation. NURBS surface are the best suited format for machining
purposes and allow addition of features through import of industry standard formats such as
IGES and STEP into 3-D CAD packages.
Optical Setup and Principle of Triangulation
Optical scanning, whether using a laser stripe or white light fringe pattern is passed on the
principle of triangulation. In the diagram above the light source at the bottom projects a
point seen by the camera at the top. Because the angles and distances between the light
source and camera are fixed, and the direction of the light ray is known, the depth of the
surface where the light hits it can be calculated. In the image above, if the car door were
moved closer the highlighted spot would be seen further down by the camera, and a closer
depth would be calculated.
A white light scanner like the COMET 250 which captures nearly half a million points in one
shot is a little more complex, but the basic idea is the same.
FAQ's Regarding the White Light Scanner:
Q) WHAT IS THE DIFFERENCE BETWEEN WHITE LIGHT AND LASER?
A) Obviously there is a vast difference in the properties of the two light sources, but as
they relate to measurement results there is very little difference. Mathematically they both
utilize triangulation algorithms which inherently have the same characteristics for accuracy
and resolution - they are both light projection techniques. The user's choice in utilizing
one of the light projection techniques is determined by the application. Laser light is
focused in a single beam or sheet illuminating a limited area at one time and therefore can
only measure a certain number of points that fall inside of the laser point or
"stripe".
White light, as utilized by the Comet system, is able to illuminate a "region" at
a time. Each scan within this region produces 420,000 XYZ data points! By projecting a coded
pattern of shadows within that region, many more points can be measured than can be done
with laser light. This means a far faster scanning system.
Q) IS WHITE LIGHT BETTER THAN LASER?
A) A laser system can be constructed at a far lower cost due to lower component cost and
simple design. They are however substantially slower that white light systems.
Q) WHICH IS MORE ACCURATE?
A) We have developed systems utilizing both light sources and have not experienced any
accuracy benefit with either. Laser system are more susceptible to noisy data with highly
reflective surfaces but there are techniques for overcoming this problem.
Q) WHAT IS THE MEASURING ENVELOPE FOR THE COMET 250?
A) Each "patch" is approximately 8 inches squared. If the part is larger than 8
inches, multiple scans will need to be taken. Each scan would need to "overlap"
the previous scan for overall part alignment.
Q) HOW MUCH ACCURACY IS LOST WHEN SCANNING LARGER PARTS?
A) This is almost impossible to quantify in a general fashion. There are virtually dozens of
techniques for scanning large parts and each could result in different variations. Again,
the optimum scanning technique to be applied is determined by the applications requirements.
We consistently achieve accuracies of 0.1mm bumper to bumper on a full size car model and
0.25mm on the wing of an aircraft at Boeing.
Q) WHAT TYPE OF PARTS IS THIS SYSTEM BEST SUITED FOR?
A) The Comet 250 is ideal for parts from tightly toleranced motorcycle parts and injection
molding tools, to low accuracy applications like foam seats and plastic toys.
For more information, please call:
(800) 959-0632 or (952) 895-1150
QC Inspection Services, Inc. (HQ):
11975 Portland Ave. S., Suite 102, Burnsville, MN 55337
Phone: (952) 895-1150 Fax: (952) 895-1152
eng@qcinspect.com
www.qcinspect.com
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