Page 354 - Sensors and Systems - North American Catalog
Basic HTML Version
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
352
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Proximity Switches
Introduction to Capacitive Sensors
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, detection
distances of between 1 mm and 50 mm are possible.
Capacitive proximity sensors can be used as "penetrating sensors" to
detect liquid or granular media. They react to the contents inside 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 sensors"). The "penetration" functionality of capacitive
sensors enables fill level controls to be performed for liquid media and bulk
goods without coming into direct contact with the material.
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)
•
Detect presence of liquid in 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 sensors 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 the target material moves
closer to the active sensor electrode, the counter potential changes. The
capacity between the sensor and the target increases. This change in
external capacity is compared with an internal sensor reference value. If the
reference value is exceeded, the sensor is actuated and delivers a transition
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 sensor is based on
a grounded metal object of a particular size. In contrast, if the capacitance
change for nonconductive actuator materials is lower, then the object must
be moved closer to the sensor electrode in order to actuate the sensor. 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 target material under otherwise comparable physical mounting
conditions, the shorter the achievable operating distance. With most
capacitive proximity sensors, the sensitivity can be adjusted within limits
using a potentiometer on the sensor.The factory-set operating distance can
be adjusted in accordance with the object material, the installation position,
and the application requirements.
Correlation between material constants and operating
distances
The rated operating distance s n of a capacitive proximity sensor 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 object size 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–thru-wall sensing
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 thru-wall sensing functionality of capacitive proximity sensors is based
on the change in the total capacitance of the media to be detected. In this
context, the sensor is not sensitive enough to detect the wall beacuse of a
low relative dielectric constant component (right diagram, 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.
