|
Laser Scanners Used to Generate Programs for Coordinate Measuring Machines
.jpg) Laser scanners are often seen as being competitive with coordinate measuring
machines (CMMs) and in some cases even providing a replacement for them.
Although ever more frequently, laser scanners are being used in conjunction with CMMs
in order to take advantage of the complementary capabilities of each type of
measurement system.
In the most common example, a laser probe is mounted on a digitizer arm,
making it possible to obtain laser scans of the surface of an object as well
as accurate point measurements with the digitizer probe.
Researchers at a leading Asia-Pacific university have pioneered a new method
in which laser scanning is first used to generate a surface model of the
object to be reverse engineered. Then this model is imported into CNC
programming software and used to create a program to drive a CMM probe and
produce highly accurate surface measurements.
Characteristics of
CMMs
Originally introduced in the late 1950’s by Ferranti, CMMs began to achieve
wide popularity during the 1970’s when they became the gold standard for
quality control and reverse engineering. CMM’s key advantages include the
ability to measure individual points to a high level of accuracy and to move
from sample location to location under computer control.
Trends of the past decade, however, have highlighted several weaknesses.
Part geometry has grown increasingly complicated and, in particular, 3D
contours are becoming more and more common. As geometric complexity grows,
the number of points required for accurate measurements increases at an
exponential rate. Frequently, tens of thousands and sometimes hundreds of
thousands or even millions of points are required to accurately model
geometrically complicated parts.
The result is that the time needed to capture points one by one has grown to
days or sometimes weeks for complicated parts.
Characteristics of
Laser Scanners
Laser scanning is a new technology that works by projecting a line of laser
light onto surfaces while cameras continuously triangulate the changing
distance and profile of the laser line as it sweeps along, enabling the
object to be accurately replicated.
The laser probe computer translates the video image of the line into 3D
coordinates, providing real-time data renderings that give the operator
immediate feedback on areas that might have been missed. Laser scanners are
able to quickly measure large parts while generating far greater numbers of
data points than probes without the need for templates or fixtures. Since
there is no contact tip on a laser scanner that must physically touch the
object, the problems of depressing soft objects, measuring small details,
and capturing complex free form surfaces are eliminated.
.jpg) Instead of collecting points one by one, the laser scanner picks up tens of
thousands of points every second. This means that reverse engineering of the
most complicated parts can often be accomplished in an hour or two.
Laser scanning can reverse engineer parts that are so complex that they
would be practically impossible to scan one point at a time. Finally, the software
provided with the scanner greatly simplifies the process of moving from
point cloud to computer aided design (CAD) model, making it possible in
minimal time to generate a CAD Model of the scanned part that faithfully
duplicates the original part.
Special, but readily available software can be used to compare original
design geometry to the actual physical part, generating an overall graduated
color error plot that shows in a glance where and by how much, surfaces
deviate from the original design.
Combining the two
approaches
The researchers are believed to be the first to have pursued the idea of
using the laser scanning model to program the CMM. This approach overcomes a
key weakness of the CMM, which is the need to manually move the laser probe
over the many points on the part’s surface required to generate an accurate
model.
This is not normally a problem with parts that have been designed in-house
because the design definition in the form of computer-aided design (CAD)
geometry can be used as the basis of the CMM program. But CAD geometry is
typically not available for parts that are to be reverse engineered.
Programming typically makes no sense in this situation because it would take
so long and because the program would normally only be used once.
The method developed by the university researchers works as follows. First
they use a Surveyor laser scanning system from Laser Design Inc. to scan the
part. They selected the Surveyor DS 3D laser scanner from Laser Design Inc.,
Minneapolis, Minnesota, because it offers an exceptionally high level of
accuracy based upon a highly automated, CMM machine base of granite and
steel composition that positions the laser probe with a very high level of
accuracy.
.jpg) The laser probe scans parts from all directions then rotates the data back
into a common coordinate system. The standard scale resolution is 0.00004
inches and 0.00002 inch resolution is available as an option. Linear
accuracy of the table is 0.000220 inches plus 0.000010 inches per inch and
repeatability is 0.00024 inches. Various models are available to handle
maximum part sizes ranging from 20” X 20” X 15” to 93” X 105” X 156”.
Surveyor scans parts from all orientations, and then easily rotates the data
back into a common coordinate system.
The researchers begin by digitizing the physical part on the laser scanner,
and then they use special software to convert the resulting point cloud to a
surface model. The next step is to import the surface model into Z-Master
computerized numerical control (CNC) software. Z-Master was originally
designed to generate programs for guiding CNC machine tools in producing
parts but it also can be used for the fairly similar task of producing
programs to guide a CMM measurement probe. The researchers use this software
to divide the measured data in segments. After this process, probe data can
be generated according to the feature type of each segmented surface. The
probe path topology is similar to CNC toolpath topology. Automatic feature
recognition eliminates the need to manually select individual features such
as pockets, islands, holes, bosses and grooves.
The next step is applying toolpaths to each feature.
High
accuracy achieved in measuring sample part
The researchers developed three modules based on the Z-Master
CAM (computer
aided manufacturing)
system to generate the CMM programs. The probe compensation module
compensates for center shift, the tilting angle that arises when a probe is
mounted in the spindle of a machine. It compensates for misalignment between
the probe internal coordinates system and the machine coordinate systems.
The pulse rate is also calibrated with respect to the rate of axis motion.
The measuring model determines the path topology and generates the CMM
program using the centerline data generation function in Z-Master. This
module also includes abnormal signal detection to prevent crashes. The
inspection module compares the digitized data with a reference source. The
inspection module was used to evaluate the accuracy of the new method by
comparing the results to the CAD model that was originally used to produce
the sample part.
A
typical sample part that was reverse engineered by the researchers is based
on an example model with a 30o draft angle, cavity shape with 30o
draft angle and concave hemispheric shape. The part dimensions are 50 by 50
by 25 mm and the part was made of mild steel. The features were classified
into floor, wall, volume and strip types. Each measuring path topology
is
classified into serial-pattern, radial pattern, contour-pattern and
strip-pattern. The diameter of the probe tip used in this example was 6 mm.
The probe moves along the part automatically following the program, scans
each surface, and generates real coordinate data. The researchers
demonstrated excellent accuracy using this approach, ranging from an average
error of 0.0002 inches (0.005 mm) to 0.004 inches (0.1 mm) in different
areas of the part. The error is calculated as the difference between the
measured data and the surface of the CAD model.
The new approach of using laser scanning to digitize the part and then using
the resulting surface model to drive a CMM probe has several important
advantages. First, the method provides the high accuracy of using CMM probe
to capture data point by point. Second, the method avoids the main weakness
of a CMM by eliminating the need to manually move the probe around a part
for which CAD geometry data is not available. Instead, surface geometry data
is quickly generated using the laser scanner and this surface geometry is
leveraged to produce a program that guides the CMM to capture the part
geometry without manual intervention.
For further information, contact Marty Schuster, by phone (952-884-9648,
ext. 202), fax (952-884-9653) or via email to
sales@laserdesign.com |
