Cut Sheet

V8-T12-28 Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Reflex or Retro-Reflective
The source and detector are
positioned parallel to each
other on the same side of the
object to be detected.
Another element, called a
retroreflector, is placed
across from the source and
detector. The retroreflector is
similar to a reflector on the
back of a bicycle. The
retroreflector bounces the
light from the source back to
the detector.
When a target object
passes between the
source/detector unit and
the retroreflector, the beam
is no longer reflected, and
the target is sensed. The
target has to block the entire
beam.
In some cases, a reflex
sensor can be falsely
triggered by reflections from
a shiny target’s surface. To
avoid this, a polarized reflex
sensor can be used. The
polarizing filter on the sensor
ensures that only the light
reflected by a retroreflector is
recognized by the sensor.
Reflex Attributes
Reflex Operation
Attributes
Strengths
Medium range sensing distance
Low cost
Ease of installation
Alignment does not need to be exact
Polarizing filter allows detecting shiny
surfaced objects without false tripping
Weaknesses
Reflector must be mounted
Problems detecting clear objects
Dirt on retroreflector can hamper operation
Not suitable for detecting small objects
Source/
Detector
Retro-
Reector
Normal State
Target
Object
Target Detected
Retro-
Reector
Source/
Detector
Object Does Not
Reect Beam
Target
Object Is
Detected
Diffuse Reflection
The source and detector are
positioned on the same side
of the target. The two
components are aligned so
that their fields of view cross.
When the target moves
into the area, light from the
source is reflected off the
target surface back to the
detector.
Diffuse Reflective
Attributes
Diffuse Reflection
Operation
Background Rejection (Perfect Prox)
This detection scheme is
really a special type of
diffuse reflective sensor. It
combines extremely high
sensing power with a sharp
optical cut-off. This allows
the sensor to reliably detect
targets regardless of color,
reflectance, contrast or
surface shape, while ignoring
objects just outside of the
target range.
This method uses two
different photo-detectors.
For the Perfect Prox unit with
a six-inch range, the near
detector has a range of 0 to
24 inches. Its far detector has
a range of 6 to 24 inches.
Objects closer than six inches
are detected only by the near
detector. Objects between 6
and 24 inches are detected
by both detectors.
If the near signal is stronger
than the far signal, the
sensor output is ON. If the
far signal is stronger or
equal to the near signal, the
output is OFF. The result is a
sensor with high excess gain
for six inches, followed by a
sharp cut-off.
Perfect Prox Sensor
Attributes
Strengths
Application flexibility
Low cost
Easy installation
Easy alignment
Many varieties available for many
application types
Weaknesses
Short sensing distance (under 10 ft)
Sensing distance depends on target size,
surface and shape
Source/
Detector
Normal State
Source/
Detector
Target
Object
Object Reects
Beam
Target
Object Is
Detected
Target Detected
Far Detector
Field
Near Detector Field
Cut-Off
Distance
Sensor