Triangulation Approach to Scanning
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secondary reflections on concave parts of an object, such as mirror-like
surfaces on machined parts, get reflected back into the imager and are very
difficult to filter out. Here is an example of a poor quality scan from a laser
line scanner caused by spurious reflections from a machined surface. Source:
Hexagon Metrology |
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The
triangulation laser scanner is an active sensor that actively projects a light
pattern to probe the environment. The triangulation laser shines a collimated
laser beam on the subject and exploits a camera to look for the location of the
laser spot. Depending on how far away the laser strikes a surface, the laser
dot appears at different places in the camera’s field of view. This technique
is called triangulation because the laser dot, the camera and the laser emitter
form a triangle.
The length of one side of the triangle, the distance between the camera and the
laser emitter, is known. The angle of the laser emitter corner also is known.
The angle of the camera corner can be determined by looking at the location of
the laser dot in the camera’s field of view. These three pieces of information
fully determine the shape and size of the triangle and give the location of the
laser dot corner of the triangle.
The use of laser light is advantageous for three reasons:
1) its source, a laser diode, is compact and generates little heat;
2) the laser light is monochromatic, thus it is easy to bring to a focus; and
3) ambient light (the light appearing in the environment) is easy to filter out
from the image.
The single spot scanner can be extended to a laser line system by extending the
above system into the plane. This is achieved by extending the image sensor
from a linear 1-D image sensor to a 2-D array image sensor, and the laser from
a single point laser to a line laser. This duplicates the single point sensor
typically 1,000 times over resulting in 1,000 measured points in each measuring
cycle.
However, the basic extension from a single spot device to a line device has
three significant drawbacks. First, the secondary reflections on concave parts
of an object, such as mirror-like surfaces on machined parts, get reflected
back into the imager and are very difficult to filter out.
Also, objects with highly contrasting surface finish, for example, a casting
with some machined faces, are very hard to measure accurately.
And lastly, if the scanner has automatic signal level control, a complete
camera image must be analyzed before the system can feedback any changes to the
laser power to maintain an optimal level for the surface being measured.
Due to these issues, a basic laser line triangulation sensor needs to have
several parameters adjusted depending on the part being measuring. So, while
these basic devices can capture data quickly, it is often at the expense of
accuracy, reproducibility and reliability.
How Does the Technology Work?
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Figure
1
The single spot scanner can be extended to a laser line system by extending the
above system into the plane. This is achieved by extending the image sensor
from linear 1-D image sensor to a 2-D array image sensor, and the laser from a
single point laser to a line laser. This duplicates the single point sensor
typically 1,000 times over resulting in 1,000 measured points in each measuring
cycle. Source: Hexagon Metrology
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Today’s
sophisticated laser scanners employ a flying dot approach to solve the surface
variation and reflectivity problems. Measurements are performed by using the
same basic system show in Figure 1, but forming a laser line by sweeping the
laser spot over an angle using a rotating polygon mirror. (See Figure 2).
Since it is still a single spot device, the intensity of the laser light can
still be continuously adapted for each point on the line, in real time, by
feedback to the optimal intensity for the surface of the part. And because the image sensor is only 1-D,
the camera speed is much faster—at 20,000 Hz, almost 500 times than of a 2-D
image sensor. The result is a more accurate measuring system that eliminates
the fine tuning of the scanner to the object being measured, removes the need
for surface treatment such as powdering and does not require photogrammetric
targets.
When a flying dot scanner is linked to a laser tracker, the measuring process
can be conducted within large distances to microns of accuracy. This system
works through the use of extremely accurate distance measurements using the
tracker’s laser and a digital camera system that tracks infrared light-
emitting diodes (LEDs) mounted
on the scanner’s housing.
This integration of long distance location and orientation measurement together
with a laser line scanner enables an operator to scan objects, such as
aircrafts, in a very large volume up to 98 feet, a distance nearly 10 times
larger compared to a standard hand scanner coupled with a portable arm.
New Applications
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Figure
2
A polygon mirror makes a laser “fly” across the surface to form a line. Source: Hexagon Metrology
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Presently,
flying dot scanners with laser trackers are available as a manual device or
robot-mounted for fully automated, repeatable path scanning. Because flying dot
technology can automatically scan and reliably handle a diverse range of
surfaces, there is now demand from the field for a highly flexible scanning
probe to be mounted on portable arms and DCC CMMs. This will require more
research and development from industry players.
While it is anticipated that the basic laser line triangulation scanners will
proliferate at the lower end of the market, flying dot technology will dominate
in the areas where there is a customer need for reproducible, high accuracy
solutions that work reliably without the need for time-
consuming operator input during the setup and/or measurement process. Q
