Mechatronics
Sensors
Capacitive Proximity
Capacitive Proximity Sensors
Function Description
The operational principle of a capacitive proximity sensor is based
on the measurement of the change of electrical capacitance of a
capacitor in a RC resonant circuit with the approach of any
material.
An electrostatic stray field of a capacitive proximity sensor is
created between an "active" electrode and an earth electrode.
Usually, a compensating electrode is also present which
compensates for any influence of the proximity sensor through
humidity.
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Fig. 1
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Oscillator
Demodulator
Triggering stage
Switching status
Output stage with protective circuit
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External voltage
Internal constant voltage supply
Active zone
Switching output
Block circuit diagram of a capacitive proximity sensor
If an object or medium (metal, plastic, glass, wood, water) is
introduced into the active switching zone, then the capacitance of
the resonant circuit is altered.
This change in capacitance essentially depends on the following
parameters: The distance of the medium from the active surface,
the dimensions of the medium and the dielectric constant of the
medium.
The sensitivity (switching distance) of most capacitive proximity
sensors can be adjusted be means of a potentiometer. In this way
it is possible to suppress the detection of certain media. For
instance, it is possible to determine the fluid level of hydrous
solutions through the wall of a bottle.
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Sensors
Capacitive Proximity
The switching distance of a capacitive proximity sensor is
determined by means of an earthed metal plate. The table below
lists the variation in switching point distances in respect of different
materials.
The maximum obtainable switching distance of
industrial capacitive sensors is approximately 60 mm.
Material Thickness
Switching distance
1.5 mm
-----
3.0 mm
0.2 mm
4.5 mm
1.0 mm
6.0 mm
2.0 mm
7.5 mm
2.3 mm
9.0 mm
2.5 mm
10.5 mm
2.5 mm
Table 1 Variation of switching distance as a function of the material
thickness using a cardboard strip (width = 30 mm)
With capacitive proximity sensors it should be noted that the
switching distance is a function resulting from the type, lateral
length and thickness of the material used. Most metals produce
roughly the same value and a number of different values are listed
in respect of other materials.
Material
Reduction Factor
All metals
1.0
Water
1.0
Glass
0.3… 0.5
Plastic
0.3… 0.6
Cardboard
0.3… 0.5
Wood (dependent on humidity)
0.2… 0.7
Oil
0.1… 0.3
Table 2 Guide values for reduction factor
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Capacitive Proximity
Technical characteristics
Operating voltage
typ. 10… 30 V DC or 20… 250 V DC
Nominal switching
distance
typ. 5… 20 mm
max. 60 mm (usually variable,
adjustable via potentiometer)
Object material
all materials with dielectric constant >1
Switching current
max. 500 mA DC
Ambient operating
temperature
-25C… +70C
Sensitivity to dirt
sensitive
Service life
very long
Switching frequency
up to 300 Hz
Design
cylindrical e.g. M18x1, M30x1, up to
30 mm, block-shaped
Protection to IEC 529,
DIN 40 050
up to IP 67
Table 3 Technical data of capacitive proximity sensors
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Capacitive Proximity
Notes on application
As with inductive position sensors, flush and non-flush fitting
capacitive proximity sensors are to be distinguished. Furthermore,
it should be noted that these sensors can be easily contaminated.
Also, their sensitivity with regards to humidity is very high due to
the high dielectric constant of water ( = 81). On the other hand,
they can be used for the detection of objects through a nonmetallic wall. The wall thickness is this case should be less than
4mm and the dielectric constant of the material to be detected
should be higher by a factor of 4 than that of the wall.
Due to it's ability to react to a wide range of materials, the
capacitive proximity sensor can be used more universally as an
inductive proximity sensor.
On the other hand, capacitive
proximity sensors are sensitive to the effects of humidity in the
active zone. Many manufacturers for instance, use an auxiliary
electrode to reduce the effects of moisture, dew or ice thus
compensating for these disturbances.
Considerations for application
 For cost reasons, the use of inductive as opposed to capacitive
proximity sensors is generally preferred to detect metallic
objects.
 For the detection of non-metallic objects, optical proximity
sensors compete as a viable alternative.
 There is a particular field of application where the use of
capacitive sensors provides a distinct advantage.
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Examples of application
Capacitive proximity sensors for instance are suitable for
monitoring filling levels of storage containers. Other areas of
application include the detection of non-metallic materials.
Detection of matt, black objects
These objects can be made of rubber, leather, plastic and other
materials, which are not detected by diffuse optical sensors and
where ultrasonic proximity sensors are too expensive.
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Fig. 2
Detection of black rubber soles
Detecting filling levels of fluids
In the case of detecting filling levels of fluids through thin walls of
plastic containers, inspection glass etc., the wall thickness must
be limited such as to enable the capacitive proximity sensor to
respond to the contact alone.
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Fig. 3
Detection of filling level inside a steel container
a) Capacitive proximity sensor encapsulated in plastic or quartz glass
b) Detection of liquid level through plastic or glass tube
Detecting filling levels of granular material
Capacitive proximity sensors are suitable for the detection of
powder, grain or granular type bulk goods through containers or
silos.
Checking of packaging contents
For example, it is possible to check the filling volume inside food
containers through the sealed packaging by means of capacitive
proximity sensors.
The illustration below shows four capacitive proximity sensors at
the base of a cardboard box to check that four soft drink bottles
have been inserted.
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Capacitive Proximity
Fig. 4
Checking of packaging contents through cardboard
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Monitoring the winding of electrical wires and cables
Capacitive proximity sensors react to copper containing electrical
wires or cables of relatively small diameter, whereas inductive
proximity sensors react at a smaller switching distance or not at
all. Optical proximity sensors too may fail in this instance.
Fig. 5
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Monitoring for cable breakage be means of a capacitive
proximity sensor
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