Cut Sheet

Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com V8-T12-15
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Inductive Proximity Sensors
The inductive proximity
sensor can be used to
detect metal objects. It
does this by creating an
electromagnetic field.
With the ability to detect
at close range, inductive
proximity sensors are
very useful for precision
measurement and
inspection applications.
Strengths and Weaknesses
Inductive Proximity
Sensor Attributes
Applications
Proximity sensors are used in
a variety of applications.
Consider these:
Proximity sensors can be
used to detect the end of
travel on a positioning table,
to determine speed by
counting a gear’s teeth, or be
used to check if a valve is
fully opened or closed.
Proximity sensors can be
used to detect the presence
or absence of a metallic
workpiece or metallic pallets
on conveyor lines.
When a robot arm swings
around for a pick and place
operation, a proximity sensor
makes sure the arm actually
has a part in its grippers.
In metal machining, proximity
sensors can make sure the
workpiece is mounted in the
fixture, and that the drill bit
has not broken off.
How an Inductive Proximity Sensor Works
Inductive proximity sensors
produce an oscillating and
invisible radio frequency (RF)
field at the sensor face.
When metal objects are
brought into this field, this
oscillating field is affected.
Each type and size of sensor
has a specific sensing range
switch point so that metal
target detection is very
accurate and repeatable.
The presence of a metallic
target interrupts the field and
alters (by damping) the
current in the sensor coil
(eddy current kill) causing
the detector circuit to sense
the change. The sensor then
triggers an output to a
connected device.
Components
Let’s look at the components
and the process step-by-step:
Components
A metal object, or target,
enters the sensing field.
The sensor coil is a coil of
wire typically wound around a
ferrite core. If you could see
the electromagnetic field
created by it, it would be
cone shaped. The target will
pass through this field.
The ferrite core shapes the
field and the size of the coil
determines the sensing
range.
The oscillator circuit causes
the field to cycle at a specific
set radio frequency (100 kHz
to 1 MHz). The presence of
metal causes a change in the
oscillation, and an eddy
current forms on the target.
The metallic object induces a
change in the magnetic field.
This change creates a
damping effect on the
amount of signal that cycles
back to the sensor coil.
The detector circuit senses
the change and switches ON
at a particular set point
(amplitude). This ON signal
generates a signal to the
solid-state output.
The output circuit remains
active until the target leaves
the sensing field. The
oscillator responds with an
increase in amplitude, and
when it reaches the set point,
the detector circuit switches
OFF. The output returns to its
normal state.
Hysteresis
Hysteresis is an engineered-
in difference between the ON
and OFF points.
If they were exactly the same
point, there would be a
chattering—a very rapid on-
off-on-off cycle. That would
cause a lot of needless stress
on components activated by
the circuit.
Hysteresis
With hysteresis, the operate
point and the release point
are slightly different
distances from the
sensor face.
Attributes
Strengths
Immune to adverse environmental
conditions
High switching rate for rapid response
applications
Can detect metallic targets through non-
metallic barriers
Long operational life with virtually
unlimited operating cycles
Solid-state to provide a “bounce free” input
signal to PLCs and other solid-state logic
devices
Weaknesses
Limited sensing range
(4 in or 100 mm maximum)
Detects only metal objects
May be affected by metal chips
accumulating on sensor face
Motion
Hysterisis
Operate
Point
Release
Point