Cut Sheet

Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com V8-T12-5
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Photoelectric Sensor
This type of sensor uses
light to detect the presence
or absence of an object.
A Thru-Beam photoelectric
sensor uses two devices (a
light source and a detector)
facing each other. Detection
occurs when an object blocks
or breaks the beam of light
passing between them.
Thru-Beam—
Beam Complete
Thru-Beam—
Object Detected
A Diffuse Reflective sensor
emits a light beam that must
be reflected back to it by
the target object itself for
detection to occur.
Diffuse Reflective—
Beam Not Complete
Diffuse Reflective—
Object Detected
A Retroflective Sensing
sensor emits a light beam
that is reflected back to the
sensor from a retroreflector.
When an object blocks the
beam between the sensor
and the retroreflector,
detection occurs. We’ll cover
more on these types of
photoelectric sensors later in
this module.
Retro-Reflective/Reflex
Mode—Beam Complete
Retro-Reflective/Reflex
Mode—Object Detected
Most electric garage door
openers include a
photoelectric sensor for
safety reasons. If the
photoelectric sensor’s beam
is broken (by a child for
example) as the door is going
down, the sensor signals the
door opener to reverse the
direction of the door.
Although environmental
factors can affect
photoelectric sensors, these
devices have a long sensing
range. The objects they
detect can be of any material.
Sensor Comparison
Each of the three sensor
categories has its strengths
and weaknesses.
The table below provides
you with a comparison.
Sensor Category Comparison
Source Detector
Source Detector
Source
Detector
Source
Detector
Source
Detector
Source
Detector
Limit Switches Proximity Sensors Photoelectric Sensors
Method of Detection Physical contact Electromagnetic field Light beam
Sensing Range Physical contact Close: within 1 in (25.44 mm) Far: can be 800 ft (243.8m)
Target Material Target must be able to withstand
physical contact
Inductive: metallic only
Capacitive: metallic and non-metallic
Can be affected by target surface, for
example, if the target is shiny or
transparent
Object Markings Not able to detect Not able to detect Able to detect
Cost Low Low Low to high depending upon sensing
method
Sensor Size Tend to be large Small to large Very small (fiber optic) to large
Environmental Sensitivity Affected by debris Inductive: electrical interference
Capacitive: humidity
Light interference
Response Time Milliseconds Milliseconds Microseconds