Significant advances in 3-D scanning technology have been driven by
customer demand and the increased processing power of today's
computers. Entire airplanes and automobiles can now be scanned with
more precision in less time than previously thought possible.
When
compared to only two or three years ago, portable scanning solutions
are very much a reality today. Using mobile scanners, operators can
capture surface data from a part or tool on the shop floor, and get
immediate visual feedback about the area of interest. The resulting
point cloud can be processed by many specialized software packages to
create inspection data, generate reverse engineering data, and much
more.
Today's scanning sensor technology can be broken down into three
main categories: Structured light scanners project an array of
different lines onto the surface in a grid-like pattern, and use
cameras to determine each 3-D point location in a large patch of data.
Line scanners project a laser line onto the surface and measure the
profile of that line on an image sensor to determine the specific 3-D
coordinates for each point on the surface. Laser-dot scanners project
single laser points in rapid succession onto the surface, and calculate
each point's location as its reflection returns to the sensor.
Each type of scanner has its strengths and weaknesses. For instance,
some devices can measure more points in a shorter period of time. Other
systems can measure many different surface types without any special
surface preparation. But from a user's perspective, the most important
difference between all of these technologies is how all of the
collected data is held together.
Structured light scanners measure points in patches, line scanners
measure points on a line, and dot scanners measure individual points.
So how do all of these patches, lines, and points combine into a single
coordinate system?
Looking at structured light scanners first, these systems require a
series of reflective targets to be placed on the surface of the object
to be measured. Enough dots need to be used so that the scanner can see
multiple common dots between scan patches. These dots are then surveyed
with a photogrammetry system to locate their position relative to the
part's coordinate system. Not until all of the previous steps have been
completed (and possible problem surfaces have been sprayed with a white
powder to allow a homogeneous light return) can the scanning begin.
On the other hand, line scanners can only determine each point on
each line, but cannot associate each line to the next. There must be
another system involved that can associate each line to the other, then
relate all of them to the part's own coordinate system. An articulated
arm is typically used for this procedure, but the scan volume is
limited to the reach of the arm. The arm can be moved to locations
around the measured object using common points, but this procedure
drastically reduces the system's achievable accuracy.
Dot scanners have a similar limitation. All measured dots are
relative to the sensor, but the sensor still needs to be located in a
common coordinate system as it is moved around the part.
Technology has advanced to solve these challenges. Major laser
tracker manufacturers have entered the scanning world with new
solutions that use the laser tracker to do what it does best: locate an
object with excellent accuracy in a very large volume. Some
manufacturers have started to use their laser trackers in combination
with an articulated arm. The tracker locates the arm, the arm locates
the scanner, and the operator can reposition the arm without losing
accuracy.
Another innovation is the 6DoF (Six Degrees of Freedom) laser
tracker that can track a scanner body directly.This capability allows
the operator to scan an entire part without the need to reposition an
intermediate device such as an articulated arm. In addition, this laser
tracker allows a measurement probe to be used in conjunction with a
scanner. The combination enables the tactile probing data and
noncontact scanning data to be combined in a single coordinate
system-all measured from the same machine. Merging 6DoF laser-tracking
and portable-scanning technologies can now make it straightforward for
an operator to precisely scan very large parts.
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