Cut Sheet

V8-T12-22 Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
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.
Capacitive Proximity Sensor Influences
Many of the same factors
that influence the sensing
range of inductive proximity
sensors, also influence the
sensing range of capacitive
proximity sensors.
Typically, capacitive sensors
have a greater sensing range
than inductive sensors.
Sensing distance for
capacitive proximity
sensors is dependent on
plate diameter. With
inductive proximity sensors,
the size of the coil is the
determining factor.
Typical Proximity Sensing Ranges
100%
0
mm
Distance
Current
Inductive Capacitive
100%
0
mm
Distance
Current
Sensor with a
Tubular Diameter of:
Inductive
Unshielded Sensor
Capacitive
Unshielded Sensor
18 mm 8 mm 15 mm
30 mm 15 mm 25 mm
34 mm — 35 mm
In the Workplace
As oil pours into this storage tank, a capacitive proximity sensor
near the top signals the fill valve to close once the tank reaches
capacity.
Capacitive Proximity Sensors in a Liquid Level Detection Application
Another sensor near the bottom alerts the filling system if the
level of the tank becomes too low.
Sensitivity Adjustment
Most capacitive proximity
sensors are equipped with
sensitivity adjustment
potentiometers. Because the
sensor measures a dielectric
gap, it is important to be able
to compensate for target and
application conditions and
adjust the sensing range.
Target Material and Size
A capacitive sensor should
not be hand-held during set
up. Because your hand has a
dielectric constant greater
than air, the sensor may
detect your hand rather than
the intended target.
Capacitive sensors can detect
both ferrous and non- ferrous
materials equally well. There
is no derating factor to be
applied when sensing
metal targets. But, other
materials do affect the
sensing range.
Because they can be used to
detect liquid through a
nonmetallic material such as
glass or plastic, you need to
ensure that the sensor
detects just the liquid, not the
container. The transparency
of the container has no
effect on the sensing.
For all practical purposes, the
target size can be determined
in the same manner as was
discussed in “Target Size” on
Page V8-T12-18 for inductive
proximity sensors.