Cut Sheet

Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com V8-T12-27
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Fiber Optics
Applying fiber optic
technology to photoelectric
sensors means applications
with space restrictions are
not a problem. A fiber optic
cable can detect objects in
locations too jammed for a
standard sensor. Fiber optic
cable is available in sizes as
small as 0.002 inches in
diameter.
Glass Fiber Optic Cable
A glass fiber optic cable is
made up of a large number of
individual glass fibers,
sheathed for protection
against damage and excess
flexing. Plastic fiber optic
cables include a single plastic
fiber in a protective coating.
Neither type of cable contains
electronics.
Because light—rather than
current—travels down these
cables, the signal is
unaffected by
electromagnetic
interference (EMI) and
vibration. The design also
has built-in immunity to
electrical noise and the
inaccurate readings regular
sensors can get.
Fiber optics can withstand
high temperatures; plastic
up to 158°F (70°C), standard
glass up to 480°F (249°C),
and specialized high
temperature versions up to
900°F (482°C). Glass fibers
can stand up to the harsh
wash-down chemicals used
in many food, beverage and
pharmaceutical applications.
However, fiber optics have
their disadvantages. They
have a limited sensing
distance, so they can be
used only in tight areas. The
maximum distance for the
thru-beam design is just 15
inches. Also, these sensors
have a small sensing area. A
small drop of water or piece
of dirt can easily fool fiber
optics.
In the Workplace
In this cookie kitchen, fiber optic photoelectric sensors are
placed in a hot oven. As long as the sensors detect motion as
the trays of cookies move by, the oven stays on.
A Photoelectric Sensor Prevents Cookies from Being Burned
If the conveyor stops, the sensors will detect light or dark for
too long, and the output device will shut the oven down.
Glass Fibers
Embedded in
Epoxy
Stainless
Steel Ferrule
Modes of Detection
In most applications, photoelectric sensors generate an output
any time an object is detected.
Light Operate vs. Dark Operate
If this occurs when the photodetector sees light, the sensor is
said to be working in the light operate mode.
Light Operate and Dark Operate—Reflex Mode Example
If the control generates an output when the photodetector
does not see light, the control is said to be working in the dark
operate mode.
Earlier, we briefly described
the four basic operating
modes that photoelectric
sensors offer. These are:
●
Thru-beam
●
Reflex (retro-reflective)
●
Diffuse reflective
●
Background rejection
(Perfect Prox)
Let’s now take some time to
understand how each
method works.
Thru-Beam
Separate light source and
detector units face one
another across an area. The
column of light traveling in
a straight line between the
two lenses is the effective
sensing beam. An object
crossing the path has to
completely block the beam to
be detected.
Thru-Beam Attributes
Thru-Beam Operation
Source
Photo Sensor Sees Light:
Load Is Operated
Light Operate Mode
Detector
Source
Detector
Photo Sensor Sees Dark:
Load Is Not Operated
Source
Photo Sensor Sees Light:
Load Is Not Operated
Dark Operate Mode
Detector
Source
Detector
Photo Sensor Sees Dark:
Load Is Operated
Reector
Attributes
Strengths
Long sensing distance (up to 800 ft)
Highly reliable
Can see through opaque objects
Weaknesses
Two components to mount and wire
Alignment could be difficult with a longer
distance detection zone
Source
Detector
Source
Object Blocks
Beam
Detector
Field of
View
Normal State
Effective
Beam
Field of
View
Target Detected