Cut Sheet

Volume 8—Sensing Solutions CA08100010E—February 2015 www.eaton.com V8-T4-3
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Capacitive Proximity Sensors
Introduction
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.
Plates
Dielectric
Switch
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.
Oscillator Damping
The detector circuit
monitors the oscillator’s
output. When it detects
sufficient change in the
field, it switches on the
output circuit.
The output circuit remains
active until the target leaves
the sensing field. The
oscillator responds with a
decrease in amplitude, and
when it is no longer receiving
sufficient capacitance
feedback, the detector circuit
switches OFF.
There is a built-in difference
between the operate and
release amplitudes to
provide hysteresis.
100%
0
mm
Distance
Current
Inductive Capacitive
100%
0
mm
Distance
Current