Download How to Use Capacitive Sensors to Help Solve Difficult Level

HOW TO
How to Use Capacitive
Sensors to Help Solve
Difficult Level Applications
G1032
Revised 10/21/2013
C
apacitive sensors can
The electrodes are placed in the feedback loop of a high
sense through lower
frequency oscillator. When no target is present, the sen-
dielectric materials,
sor’s capacitance is low and the oscillation amplitude is
such as plastic or glass, to
small. When a target approaches the face of the sensor it
detect
dielectric
increases the capacitance, thereby increasing the ampli-
materials, like liquids. This
tude of oscillation. The amplitude of oscillation is mea-
allows capacitive sensors to
sured by an evaluating circuit that generates a signal to
detect the level of numerous
turn the output on or off.
higher
materials directly through
Capacitance is a function of the surface area of two
the wall of a plastic or glass
electrodes, the distance between the electrodes, and
container, or by utilizing a
the dielectric constant of the material between the elec-
sight glass or tank well for
trodes. When a conductive target enters the sensor’s field
metal containers.
it forms a counter electrode to the active face of the sen-
Capacitive sensors are used to detect a wide variety of
sor, decreasing the distance between the electrodes and
materials, including liquids of varying viscosities or solids
increasing their average surface area (Figure 1). When a
like powders, rocks and metals. For instance, capacitive
non-conductive target enters the sensors field it acts as an
sensors are often used to detect granular or powered ma-
electrical insulator between the electrodes (Figure 2).
terials like plastic pellets in a hopper for injection molding
processes. And intrinsically safe NAMUR capacitive sensors are used in explosive areas, such as grain elevators, to
detect materials ranging from rice and barley malt to corn
and soybeans.
A capacitive sensor’s active element is formed by two
metallic electrodes positioned like an “open” capacitor.
Figure 1
Figure 1
The dielectric constant of the material being sensed
is a measure of its insulation properties. All liquids and
solids have a greater dielectric constant than air; the ca-
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operating distance.
Since capacitive sensors are used to sense liquids, any
liquid touching the actual sensor will cause it to lock on.
Because capacitive sensors cannot sense through metal,
applications that require liquid level detection through a
metal container wall require special sight glass and tank
well fittings for a capacitive sensor to be effective.
Generally, capacitive sensors have a wide sensitivity
band that makes them capable of performing in difficult
applications, like sensing very small metal parts through
a tube. Capacitive sensors are also extremely versatile in
pacitance of metallic and non-metallic targets is always
their housing options (barrel, rectangular, probe), and are
applicable in many environments and operations. This ver-
greater than the capacitance of a sensor’s circuit in the ab-
satility allows manufacturers to utilize capacitive sensors
sence of the target. Generally, the larger the dielectric con-
in numerous locations, saving expense and helping plants
stant of the material, the greater the sensors achievable
run more efficiently.
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