|
Laser scanning
technology
Laser scanning systems
work 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’s PC based controller 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.
Instead of collecting points one by one, the
laser scanner picks up tens of thousands of points every second. This allows
reverse engineering of the most complicated parts or tooling to be
accomplished in ¼ of the time required using conventional touch probe
digitizing of the part or tool. Laser scanners are able to quickly
measure large parts while generating millions of data points instead of
hundreds without the need for templates or fixtures.
Class II diode lasers are
used in laser scanners because they do not present a hazard to the human
eye. The lasers are about the same power as the bar code scanners used to
check out purchases in super markets. They typically have a wavelength of
670 nm which is within the visible spectrum so the eye will blink when
exposed to the laser, thus limiting the eye’s exposure to the laser light.
The newest generation of probes captures up to 50,000 points per second and
weighs less than a pound. The most recent development in laser scanning is
the availability of probes with dual detectors that view the laser line from
two different directions, reducing the number of scanning passes required to
capture steep sidewalls and deep geometries. This new laser probe design
further reduces scans times by 30% to 40%.
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. Off the shelf
software can be used to compare original design geometry to the actual
physical part, generating an overall graduated 3D color error plot that
shows in a glance where and by how much, surfaces deviate from the original
design. This makes it possible to easily compare the as-built die to the
original part geometry to see what changes were made during the debugging or
try out process.
Comparison to CMM
The conventional method for digitizing parts and
dies is done by using CMMs to capture their geometry point by point. But it
has become increasingly difficult to utilize CMMs for this purpose as the
complexity of dies tends to increase with each new product generation. The
operator must guide the probe to each individual point. Millions of points
are frequently required to fully characterize complex stamping dies. There
is never enough time to capture them by hand. This is means that a
considerable amount of time is required to program the CMM to check out the
part or die either off line or at the machine. Reverse engineering the
changes made to tooling can take days and is thus rarely done due to this
overwhelming time input. When a new tool is needed with the same changes
incorporated, the entire process of try out and tooling modification then is
reacted. Laser scanning can overcome this problem by capturing the millions
of coordinate points so that the CAD modeling changes can be quickly
incorporated into the original CAD model for use for repair or replication.
On the other hand, the CMM provides greater
accuracy in determining the position of individual tooling points to an
accuracy of 5 microns when needed so it is sometimes used to verify tight
tolerance features when a hole center location needs this high accuracy
input. Laser scanning heads are commonly mounted on CMM arms so that the
CMM’s motion system moves the probe while the probe’s computer monitors the
CMM’s encoders to track the probe’s position. The retrofitted CMM can easily
be switched between laser scanning and mechanical touch probe measuring
modes so that surfaces can be scanned by the laser and points can be
digitized with the full accuracy of the CMM device.
Applications in the automotive metal forming
industry
Some shifts in the
metal forming industry have increased the demand for laser scanning. Many of
the stamping dies used by Japanese automobile manufacturers to produce body
and chassis components in their United States plants are now built in Asia
under very tight deadlines. The dies are generally modified during the
proving out process but as soon as they are capable of building acceptable
parts it is usually necessary to rush them onto a boat for the trip to the
United States. So there often is not enough time to produce the dimensional
data that is needed later to produce replacement parts when the die becomes
worn or is damaged.
In the past, before
they began production, the stamping companies that used the dies sent them
out to engineering firms or tool and die shops for reverse engineering with
a CMM. This was expensive and time-consuming because of the need to manually
move the machine probe around the die in order to capture each of the
thousands of points needed to accurately characterize its geometry.
Several innovative companies have improved on the traditional approach by
using a laser probe to scan the die surface while viewing the reflected
light with a camera that captures millions of points in minutes. “The new
high definition laser scanning systems available today in conjunction with
high-end data processing software have given the metal stamping industry the
resources to provide very quick and accurate reverse engineering data for
the manufacturing of complex surfaced die components,” said the owner of one
such company.
“The ability to
accurately digitize the existing component to within 12 microns (0.0005
inches) and then, if necessary, virtually recondition the component using
the tools within the data processing software has dramatically improved this
process,” he added. “The higher scanning speed provided by the new method
is another important advantage. An average-sized die component can typically
be scanned on all surfaces in about 30 minutes and a large component
generally takes less than an hour. This compares to several hours or days
that are typically required to digitize similar parts on a CMM.”
Another important application is reverse
engineering and inspection of complex sheet metal assemblies. The sheet
metal housing shown here was scanned to obtain millions of coordinate points
which generated a CAD/CAM model that was used to update the manufacturer’s
database of components that have changed since the original CAD models were
developed. A color error map was generated to identify exactly how the
current product differs from the original design intent. New products can
now be developed from existing products by modifying the CAD model level
rather than starting from scratch. CAD modeling from laser scan data is
typically about eight times faster than the time required for the previous
process which involved manually creating the model from CMM touch probe
data. The millions of coordinates from 3D laser scanning define all
of the subtle freeform and surface transitional shape changes that were
difficult or impractical from a time standpoint to measure with touch probe
based CMM technology. The advantages of laser scanning in these applications
helps explain why the use of this technology is growing so rapidly in the
metal forming and fabricating industry. |
