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Photoelectric Sensors
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391
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Photoelectric Sensors
Introduction
2. Diffuse Mode Sensors
In principle, a diffuse mode sensor is constructed
similarly to a retroreflective sensor. However, it does
not use a reflector and the receiver evaluates the
light reflected from the detected object instead.
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Due to the diffuse reflection from the object, the detection range
of the diffuse sensor is reduced compared with that of the
retroreflective sensor. This sensing range is the maximum distance
from the emitter at which an object can be reliably detected.
Sensitivity adjuster
Special versions of diffuse mode sensors
The standard principle of the diffuse mode sensors—
that they react to light reflected from an object’s
surface—is put to use by contrast and color sensors.
Contrast sensors
This sensor evaluates the difference in brightness between an
object and a marking on it. The color of the emitted light or the
color of the marking must be selected so that as large a contrast
as possible is achieved. Our contrast sensors are equipped with a
three-color LED light source. The optimum light emission color is
selected automatically during the sensor teach-in process. Many
sensors are based on the auto-collimation principle, i.e., emitter
and receiver are on a common optical axis (one-eyed system). This
permits greater flexibility in the installation position of the sensor.
Luminescence sensors
A variation of the contrast sensor, the luminescence sensor, works
with modulated ultra-violet light. Suitable materials (luminophores),
either inherent to the material or intentionally added, are stimulated
by ultra-violet light and emit visible light at a determined wavelength.
These materials include optical brighteners in white paper, but they
are also components in certain kinds of chalk, paint, oils, greases,
and printed marks.
The light emitted by the luminophores to the sensor’s receiver has
a longer wavelength than the light emitted from the emitter; it is
within the visible part of the spectrum. Because of the modulation
and the precisely defined frequency offset between the transmitted
and received light, luminescence sensors have high immunity to
ambient light and are highly reliable.
Color sensors
While a standard contrast senor evaluates only differences in
brightness within a specific range of the spectrum determined by
the color of the transmitted light of the sensor, the color sensor
breaks up the light reflected from the target into several portions
of the spectrum and returns an intensity value for each of these
sub-spectra. The distribution of this value determines the spectral
properties—and thus the color—of the target.
Pepperl+Fuchs color sensors work according to the three-range
procedure. Below are descriptions of the active and passive three-
range procedure.
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Active three-range procedure:
The sensor emits three emitter colors (red, green, and blue)
sequentially onto the sensed object. The magnitude of light
reflected is individually measured for each emitted color. The
three colors that are obtained are used to determine the color of
the sensor object.
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Passive three-range procedure:
The sensor emits white emitted light onto the sensed object.
After this, the red, green, and blue portions of the reflected light
are filtered out and the light magnitude is determined for each
one. Three receivers are used for this process. The color of the
target is determined in this manner.
