Laser Triangulation Sensors
Laser triangulation sensors determine the position of a target by
measuring reflected from the target surface. A 'transmitter' (laser diode)
projects a spot of light to the target, and its reflection is focused via
an optical lens on a light sensitive device or 'receiver'. If the target
changes its position from the reference point the position of the
reflected spot of light on the detector changes as well. The signal
conditioning electronics of the laser detects the spot position on
the receiving element and, following linearisation and additional
digital or analogue signal conditioning, provides an output signal
proportional to target position.
The most critical element in this arrangement is the receiver, which
can take one of two forms:
a position sensitive device (PSD) or charge coupled device (CCD).
PSD triangulation has been around for about 25 years and thus
tends to dominate the field. Under 'ideal' conditions, PSD sensors
perform to expectation. However, the reliability and repeatability
of the PSD receiver is compromised by a number of 'real world'
variations in the nature of the target.
Should surface conditions, target texture or tilt change, for example,
this will change the shape of the light spot, altering the centre of
light distribution, and inducing a change in output of the PSD element,
even though the true 'Z' position of the target has not changed.
PSD systems are also very sensitive to light intensity, and if this
changes while the spot position remains the same, it will result in
an output change - the same net effect of a change in target colour.
CCD lasers first surfaced in Europe around ten years ago, and helped
overcome many of the limitations of PSD technology. However, the
speed of response of the laser to changing surface conditions was
still limited by the controlling microprocessor. If surface conditions
changed rapidly, the device simply couldn't react fast enough,
resulting in a measurement error. But technology moves on. Today,
the latest CCD elements and DSP devices have virtually eliminated
these early shortcomings of the CCD based receiver.
Smart CCD lasers can now react spontaneously to changing surface
conditions, to achieve accurate results regardless of surface texture
or colour. Target alignment is now a non-issue, and the effects of
stray and secondary reflections are eliminated as the CCD element
works as a function of light intensity only, and not light quantity.
For a stable measurement, as little as 1% diffuse reflectivity is all a
CCD detector requires, so black or shiny targets no longer present
the problems they posed in the past.
The CCD element is a digital pixelised array detector, with 1,024
discrete voltages representing the amount of light falling on each
pixel of the detector. A CCD element detector can carry 1,024 x 1,024
pieces of light intensity information. The intensity distribution of the
imaged spot is completely 'viewed' with the help of a powerful DSP
device, and image processing is then incorporated for the linear
triangulation measurement. The post data processing of the intensity
distribution enables almost all of the problems posed by non-ideal
targets to be overcome.
The DSP finds the one single pixel with the highest light intensity
and uses an algorithm to perform sub-pixel resolution by interpreting
the light intensity of adjacent pixels. The technology of thresholding
is used to discard unwanted information pertaining to stray and
secondary reflections, which would cause a PSD receiver to change
its output. Smart CCD sensors also use closed loop control to adjust
the power of the transmitting laser, according to the amount of
reflected light received from the target. An optimum light intensity
for the sensing element is achieved, regardless of the target colour
or its surface texture.
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