Cut Sheet

Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com V8-T12-21
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Capacitive Proximity Sensors
Let us now turn our attention
to another proximity sensor,
the capacitive proximity
sensor. This sensor
operates much like an
inductive proximity sensor,
but its means of sensing is
much different.
Capacitive Proximity
Sensors
Capacitive proximity
sensors are designed to
detect both metallic and
nonmetallic targets. They
are ideally suited for liquid
level control and for sensing
powdered or granulated
material.
Strengths and Weaknesses
Consider these strengths and
weaknesses of the capacitive
proximity sensor:
Capacitive Proximity
Sensor Attributes
Applications
Here are some examples
showing how the detection
power of capacitive proximity
sensors is used:
●
Liquid level detection
applications, such as
preventing overfilling or
underfilling, are common in
the packaging industry
●
Material level control
applications, such as
assuring that a sleeve of
labels on a labeling line is
not empty
●
Counting applications,
such as tracking units
passing a point on a
conveyor
●
Induction molding
process, detection of level
of plastic pellets in feed
hopper
Attributes
Strengths
Can detect both metallic and nonmetallic
objects at greater ranges than inductive
sensors
High switching rate for rapid response
applications (counting)
Can detect liquid targets through non-
metallic barriers (glass, plastic)
Long operation life, solid-state output for
“bounce free” signals
Weaknesses
Affected by varying temperature, humidity
and moisture conditions
Not as accurate as inductive proximity
sensors
Operation of the Capacitive Proximity Sensor
A capacitor consists of two
metal plates separated by a
insulator (called a dielectric).
The operation of this type
of sensor is based on
dielectric capacitance,
which is the ability of a
dielectric to store an electrical
charge.
The distance between the
plates determines the ability
of the capacitor to store a
charge.
Measuring the change in
capacitance as an object
enters the electrical field
can be used as an ON/OFF
switching function.
Capacitor Operation
When this principle is applied
to the capacitive proximity
sensor, one capacitive plate
is part of the switch, the
enclosure (the sensor face)
is the insulator. The target
is the other “plate.” Ground
is the common path.
Capacitive proximity sensors
can detect any target that has
a dielectric constant greater
than air. Liquids have high
dielectric constants. Metal
also makes a good target.
Capacitive Proximity Sensor Operation
The capacitive proximity
sensor has four basic
elements: a sensor (which is
a dielectric), an oscillator
circuit, a detector circuit and
an output circuit.
As an object approaches the
sensor, the dielectric
constant of the capacitor
changes. The oscillator
circuit’s oscillation begins
when feedback capacitance
is detected. This is just the
opposite in the inductive
proximity sensor, where the
oscillation is damped when
the target is present.
Plates
Dielectric
Switch