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The triangulation is a well known
technique of classical optics, especially used in geodesy. The
combination of the laser with the triangulation created a new
field of application for laser distance measurement. In
principle, the laser distance measurement can be classified to
three major sections where each one uses a particular property
of the laser. For distance or length measurement with
interferometers, the low divergence and the coherence of the
laser is used. In laser range finding applications, the low
divergence of the laser beam and the defined speed of light is
used. In the case of the laser triangulation, only the low
divergence of the laser beam is exploited. Each technique has a
particular measuring range and accuracy:
1. Triangulation 1 to 50 cm with an accuracy of 10-3
2. Interferometer up to 50 m / 10-8
3. Range Finder 1 m up to several km /10-3.
Therefore the laser triangulation is qualified for the absolute
distance measurement in industrial applications. It should be
mentioned that the triangulation technique cannot cover the
entire range from 1 to 50 cm because the principle of the
measurement is based on optical imaging.
However, the costs of a triangulation head compared to laser
interferometer are so small that one can use a number of units
to cover the desired range. In the automobile industry for
example it is common to equip a car body measuring cabinet with
nearly one hundred triangulation heads in defined positions.
The car is driven into the cabinet and in a very short time the
dimensions of interest can be measured. Since the laser
triangulation works without contact, a lot of other applications
in automated production processes exist.
One of them is shown in the figure at the top left, where the
presence of a drill is monitored. Meanwhile, such units can be
manufactured very small in size due to the existence of laser
diodes. Within this workshop, the principle of the laser
triangulation is studied and applied at different objects
mounted on a manual driven translations stage with micrometer
read out.
Principle of operation
The principle set-up consists of:
1. Laserdiode
2. Object (A)
3. Imaging lens (L1)
4. Position sensitive photo detector (DP)
The beam of the laser hits the object A and is scattered or
reflected, depending on the surface properties of the object.
The scattered light enters the lens L1 and the laser spot of the
object is imaged onto the position sensitive device (DP). The
laser exit, the centre of the detector and the object are
arranged in such a way that they form a triangle. The distance H
of the centre of the detector to the laser exit is known and
stays constant. If the object A moves by dx, the imaged spot
moves by dS which is measured. By using the known parameters H,
the measured value of dS and the focal length of the lens the
value for x+dx, the distance of the object from the unit, can be
calculated.
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Required Equipment
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