Page 392 - Sensors and Systems - North American Catalog
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Photoelectric Sensors
2
Photoelectric Sensors,
Standard Sensors,
Thru-beam sensors
2
.1
Photoelectric Sensors,
Standard Sensors,
Thru-beam sensors
2
.1
Photoelectric Sensors,
Standard Sensors,
Retroreflective sensors
2
.1
Photoelectric Sensors,
Standard Sensors,
Retroreflective sensors
2
.1
Photoelectric Sensors,
Standard Sensors,
Diffuse mode sensors
2
.1
Photoelectric Sensors,
Standard Sensors,
Diffuse mode sensors
2
.1
Photoelectric Sensors,
Standard Sensors,
Diffuse mode sensors with background suppression
2
.1
Photoelectric Sensors,
Standard Sensors,
Diffuse mode sensors with background suppression
2
.1
Photoelectric Sensors,
Standard Sensors,
Fiber optic sensors
2
.3
Photoelectric Sensors,
Standard Sensors,
Fiber optic sensors
2
.3
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Photoelectric Sensors
Introduction
The Functional Principles of Photoelectric
Sensors at a Glance
Type, size, shape, and surface characteristics of the objects to be
detected, the distance between the sensor and the object, and the
ambient conditions determine the optimal photoelectric sensing
mode.
1. Diffuse mode sensors with background
suppression
Diffuse mode sensors with background suppression
were developed to achieve a defined sensing range
for objects regardless of their brightness, color, and
other characteristics, as well as the presence of a
background. The following figure illustrates the
functional principle of a diffuse mode sensor with
background suppression. A focused light spot from the emitter and
directed by the lens reaches the object. If the object is within the
sensing range, then a portion of the reflected light, focused by the
receiver lens, is directed to the “near” element of the receiver, and
the sensor’s output switches on.
As the object’s distance increases, this light spot moves towards
the “far” element of the receiver. Near the sensing range limit, half of
the light spot is on the “near” and half on the “far” elements, and the
sensor’s output turns off. If the object moves farther away, the light
only falls onto the “far” element and the sensor’s output continues
to stay “off.”
Emitter
Object in
sampling range
Detection range
Background
Bright
background
Object in the
background
Scanning range
Near element
receives more light –
object is detected
Far element
receives more light –
object is ignored
Scanning range
Receiver
Far
Near
Emitter
Receiver
Far
Near
Features:
•
A nearly constant sensing range to materials with sharply
different reflectivity
•
Dark objects in front of a bright background are reliably detected
•
Insensitive to interfering reflections from objects outside of the
specified sensing range
•
High excess gain
•
Quick installation since the sensor consists of only one housing
and no reflector
The principle of
foreground suppression
is similar to background
suppression. Diffuse mode sensors of this type ignore all objects
that are located closer to the sensor than a preset minimum sensing
range.
Multipixel array photoelectric sensors
Multipixel array photoelectric sensors combine
the
advantages
of
background/foreground
suppression with the measuring functionality of a
distance sensor. At its core is an integral measuring
technology, otherwise found only in distance
sensors, that enables different sensing functions in
one device and provides outstanding performance data.
Diffuse mode sensors with convergent mode
A convergent mode sensor is a more efficient type of
diffuse mode sensing. The emitter lens is focused to
an exact point in front of the sensor and the receiver
lens is focused to the same point. The range for a
convergent mode sensor is fixed and defined around
this focus point. The sensor is able to detect an
object at the focus point, plus or minus some distance. Because all
the emitted energy is focused to a single point, a high amount of
excess gain is available. This excess gain enables the sensor to
easily detect narrow or low reflectivity targets.
Diffuse mode sensors with background evaluation
In addition to background suppression, the reverse
principle, background evaluation, is used for certain
applications. While the first process ignores the
background and only looks at objects within the
sensing range, the other evaluates only the light
reflected from the background. The background, not
the object, is the reference. (See the following figure) If an object
obstructs the light path to the background, the sensor switches
regardless of whether or not the reflected light reaches the receiver,
thus detecting or not detecting the object in front of the background
(even for very reflective objects).
Sensors with background evaluation have no minimum sensing
distance and are more suitable for detecting complex, highly
reflective objects. In addition, they can be tested and are designed
to be self-monitoring.
Receiver
Vehicle is only
detected in
background
evaluation mode
Receiver
Receiver
Back-
ground
is
detected
Vehicle is
detected
I
Near
< I
Far
= OFF
I
Near
> I
Far
= ON I
Near
= I
Far
= 0 = ON
Emitter
Emitter
Near Far
Near Far
Emitter
Near Far
Target
Sensing window
Receiving
optics
Emitting
optics
Maximum
sensing
distance
Maximum
sensing
distance
