292
Proximity Sensors
1
Proximity Sensors,
Capacitive Sensors
1
.2
Proximity Sensors,
Magnetic Field Sensors
1
.3
Proximity Sensors,
Magnetic Field Sensors,
Sensors
1
.3.1
Proximity Sensors,
Magnetic Field Sensors,
Accessories
1
.3.2
Proximity Sensors,
Capacitive Sensors,
cylindrical
1
.2.1
Proximity Sensors,
Capacitive Sensors,
rectangular types
1
.2.2
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Introduction to Capacitive Sensors
Capacitive sensors detect both metallic and nonmetallic targets such as
glass, plastic, or wood. Depending on the size and material properties
of the target as well as the size and mounting conditions of the sensor,
switching distances of between 1 mm and 50 mm are possible.
Capacitive proximity switches can be used as "penetrating sensors" to
detect liquid or granular media.They react to the contents of a thin-walled
and electrically nonconductive container made from glass or plastic.This
function is performed by increasing the overall capacitance, consisting of
the container wall and filling media (see also "Functional description of
capacitive proximity switches"). The "penetration" functionality of capaci-
tive sensors enables flow rate and fill level controls to be performed for
liquid media and bulk goods without coming into direct contact with them.
Special technical features of capacitive sensors:
•
They react to both electrically conductive materials and insulating
materials
•
(
ferrous metals, nonferrous metals, glass, wood, plastic, as well as oil
and liquid solutions)
•
The operating distance of the sensor can be adapted to the object
and mounting properties
•
They can be used as "penetrating" sensors through nontransparent
and nonconductive container walls
Common applications for capacitive sensors:
•
Checks for the presence of paper, wood, glass, or plastic materials
•
Fill level control for fluid media and bulk goods (pellets, tablets,
granulate, toner)
•
Flow rate control on plastic piping and hoses
Common areas of application for capacitive sensors:
•
Print and paper processing industry
•
Packaging and food industry
•
Pharmacy, chemical, and process technology
•
Wood and plastic processing
•
Automotive industry
Functional description
Capacitive proximity switches have a sensor electrode (active area) and
a shield ring. The sensor electrode is the active area of the sensor and
emits an electric AC field that forms a capacitance with the surroundings
acting as the counter potential (capacitor principle). If an actuator moves
closer to the active sensor electrode, the counter potential changes.
The capacity between the sensor and the switching flag increases. This
change in external capacity is compared with an internal sensor refer-
ence value. If the reference value is exceeded, the sensor is actuated
and delivers a switch-over signal to the sensor output.The external shield
ring focuses the electric field of the sensor electrode and reduces the
influence of the installation position upon the operating distance of the
sensor.
S
Fig. 1: Principles of the capacitive proximity switch
Target
Sensor electrode
Screen
The rated operating distance s n of a capacitive proximity switch is based
on a grounded metal object of a particular size. In contrast, the capaci-
tance change for nonconductive actuator materials is lower and the ob-
ject must be moved closer to the sensor electrode in order to actuate
the switch. The length of the specific operating distance depends upon
the electrode area, the size and the material (permittivity) of the object
to be detected as well as the sensor's installed position. The lower the
permittivity number of the respective actuator material under otherwise
comparable geometric mounting conditions, the shorter the achievable
operating distance.With most capacitive proximity switches, the sensitiv-
ity can be adjusted within limits using a potentiometer on the sensor. The
factory-set operating distance can be adjusted retrospectively in accor-
dance with the object material and the installation position.
Correlation between material constants and operat-
ing distances
The rated operating distance s n of a capacitive proximity switch is preset
at the factory using a grounded metal object of a particular size and under
defined mounting conditions (flush, non-flush). If other materials are used
with the otherwise unchanged geometrics of the object and the mounting
conditions of the sensor, the preset operating distance decreases. The
table below illustrates the ranges of material-related dielectric constants
and the expected reduction factors for the rated operated distance.
Material
Dielectricity
e
r
Reduction factor
Grounded metal plate
>100 to 10,000
1
Water
80
1
Alcohol
22
0.75
Glass
5
to 16
0.6
Ceramic
4
to 7
0.5
PVC
2.3
to 3.4
0.45
Wood (dry)
3
to 7
0.3
Oil
2.6
to 2.9
0.28
Paper (dry)
1.6
to 2.6
0.25
Air
1
0
The values in the table are reference values and therefore should not be
referred to in terms of physical accuracy.
Special application–penetration functionality
Container wall
(
electrically
non-conductive)
Sensor
(
eg. F46)
ε
r fluid
≥
ε
r wall
ε
r air
≤
ε
r wall
Fill fluid
Air
Sensor
actuated
Sensor
not actuated
The penetration functionality of capacitive proximity switches is based
on the change in the total capacitance of the media to be detected. In
this context, the term "penetration" is not used to describe how a specific
detection range is covered in an optical sense. Rather, an insensitive sen-
sor suppresses the container wall in the sense of a dielectric constant
component (left drawing, sensor does not react to container wall). If the
filling media in the container now rises above the center of the sensor's
electrode, the electric conductivity of the system changes and results in
sensor actuation.
Proximity Switches
