How The Technology Works
Most 3D digitizing systems are either
a non-contact device or one that uses a touch probe. Touch probe systems
are the least expensive, but they are manually operated. The operator must
touch the object for every necessary 3D point, in the data base, to be
produced.
This makes it challenging, if not
impossible, to produce computer models at a high resolution. For instance,
if the tolerance is +/- 0.020-inch of the part to be reverse engineered
and manufactured, the operator must digitize points no greater than 0.020-inch
apart.
For high resolution scanning of
mechanical parts, non-contact digitizing systems are the most practical
choice. The non-contact type of digitizers collect from 20 to more than
25,000 points per second and provide resolution of better than 0.001-inch
to 0.020-inch, depending on the particular technology. Laser digitizers
have become the most popular non-contact systems for most applications.
The laser digitizers use triangulation to determine position in space.
For example, if light arrives on a surface from one direction, and if the
light is seen from another direction, the location of the point can be
inferred. Multiple degrees of freedom can be accomplished by relative motion
between the specimen, the sensor, and the laser source. Thus capturing
the size and shape of odd-shaped objects.
The speed of digitization results
from the laser moving quickly across the surface as the sensors detect
the location of the light. The resolution results from the distance between
the points, which can be restricted by the size of the illuminated area
on the surface. Accuracy of the process results from a number of factors,
including resolution, the system's optics, and the precision of the mechanical
parts that comprise the system. The object itself can also impact the accuracy
of the data. If the surface quality of the part reflects or absorbs the
specific wavelength of the laser, the data will not be as accurate as if
it where of a more neutral shade or dull.
Fine edges, sharp corners, and grooves
can cause laser scatter, reducing data accuracy, and resulting in a jagged
edge in the data. Most systems produce triangles from the points. Deep
openings and concavities can also be areas prone to error. If the opening
to the surface is smaller than the angle created by the triangulation process,
the laser and sensors cannot project and see the light. The result is missing
data in the polygonal mesh surface that appear as holes in the model.