Capacitive Proximity Sensors
4–1
General Information Quick Selection Guide page 4–2. . . . . . . . . . . . . . . . . . . . . . .
Technical Definitions and Terminology page 4–3. . . . . . . . .
Introduction page 4–5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Products 875C General Purpose Tubular page 4–9. . . . . . . . . . . . . . .
875CP Plastic Barrel Tubular page 4–13. . . . . . . . . . . . . . . .
Accessories Mounting Brackets, Sight Glass Style page 4–21. . . . . . . . .
Sensor Wells page 4–22. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Indexes Catalog Number Index page 9–1. . . . . . . . . . . . . . . . . . . . . . .
Comprehensive Product Index page 10–1. . . . . . . . . . . . . . .
Contents
Capacitive Proximity Sensors
4–2
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Quick Selection Guide
Capacitive Proximity Sensors
4–3
Axial Approach: The approach of the
target with its center maintained on the
reference axis.
Complementary Outputs: (N.O. &
N.C.) A proximity sensor that features
both normally open and normally closed
outputs, which can be used
simultaneously.
Correction Factors: Suggested
multiplication factors taking into account
variations in the target material
composition. When figuring actual
sensing distance this factor should be
multiplied with the nominal sensing
distance.
Current Consumption: The current
consumed by the proximity switch when
the output device is in the off condition.
Differential Travel: See Hysteresis.
Dual Output: Sensor which has two
outputs which may be complementary
or may be of a single type (i.e. two
normally open or two normally closed).
Effective Operating Distance: (Sr)
The operating distance of an individual
proximity switch measured at stated
temperature, voltage, and mounting
condition.
False Pulse: An undesired change in
the state of the output of the proximity
switch that lasts for more than two
milliseconds.
Flush Mounting: A shielded proximity
sensor which can be flush mounted in
metal up to the plane of the active
sensing face.
Free Zone: The area around the
proximity switch which must be kept
free from any damping material.
Hysteresis: The difference, in
percentage (%), of the nominal sensing
distance between the operate (switch
on) and release point (switch off) when
the target is moving away from the
sensors active face. Without sufficient
hysteresis a proximity sensor will
“chatter” (continuously switch on and
off) when there is significant vibration
applied to the target or sensor.
Isolation Voltage: Maximum rated
voltage between isolated outputs or
input and output.
Lateral Approach: The approach of
the target perpendicular to the
reference axis.
Leakage Current: Current which flows
through the output when the output is in
an “off” condition or de-energized. This
current is necessary to supply power to
the electronics of the sensor.
LED: Light Emitting Diode used to
indicate sensor status.
Maximum Load Current: The
maximum current level at which the
proximity sensor can be continuously
operated.
Maximum Inrush Current: The
maximum current level at which the
proximity sensor can be operated for a
short period of time.
Minimum Load Current: The minimum
amount of current required by the
sensor to maintain reliable operation.
Sensing Distance: The distance at
which an approaching target activates
(changes state of) the proximity output.
Normally Closed: Output opens when
an object is detected in the active
switching area.
Normally Open: Output closes when
an object is detected in the active
switching area.
NPN: The sensor switches the load to
the negative terminal. The load should
be connected between the sensor
output and positive terminal.
Operating Distance, Rated: The
operating distance specified by the
manufacturer and used as a reference
value. Also known as nominal sensing
distance.
PNP: The sensor switches the load to
the positive terminal. The load should
be connected between the sensor
output and negative terminal.
Programmable Output: (N.O. or N.C.)
Output which can be changed from
N.O. to N.C. or N.C. to N.O. by way of a
switch or jumper wire. Also known as
selectable output.
Repeatability: The variation of the
effective operating distance measured
at room temperature and constant
supply voltage. It is expressed as a
percentage of the sensing distance.
Residual Voltage: The voltage across
the sensor output while energized and
carrying maximum load current.
Response Time: See Switching
Frequency.
Reverse Polarity Protection: Proximity
sensors which are protected against a
reversal in voltage polarity.
Ripple: The variance between
peak-to-peak values in DC voltage. It is
expressed in percentage of rated
voltage.
Sensing Range: The rated operating
distance.
Shielded: Sensor which can be flush
mounted in metal up to the plane of the
active sensing face.
Short Circuit Protection: (SCP)
Sensor protected from damage when a
shorted condition exists for an indefinite
or defined period of time.
Sinking: See NPN.
Sourcing: See PNP.
Switching Frequency: The maximum
number of times per second the sensor
can change state (ON and OFF) usually
expressed in Hertz (Hz). As measured
in DIN EN 50010.
Target: Object which activates the
sensor.
Three-Wire Proximity Switch: An AC
or DC proximity sensor with three leads,
two of which supply power and a third
that switches the load.
Two-Wire Proximity Switch: A
proximity sensor which switches a load
connected in series to the power supply.
Power for the proximity switch is obtained
through the load at all times.
Voltage Drop: The maximum voltage
drop across a conducting sensor.
Technical Definitions and Terminology
Capacitive Proximity Sensors
4–4
Notes
Capacitive Proximity Sensors
4–5
Principles of Operation for
Capacitive Proximity Sensors
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Capacitive proximity sensors are
designed to operate by generating an
electrostatic field and detecting
changes in this field caused when a
target approaches the sensing face.
The sensor’s internal workings consist
of a capacitive probe, an oscillator, a
signal rectifier, a filter circuit and an
output circuit.
In the absence of a target, the oscillator
is inactive. As a target approaches, it
raises the capacitance of the probe
system. When the capacitance reaches
a specified threshold, the oscillator is
activated which triggers the output
circuit to change between “on” and “off.”
The capacitance of the probe system is
determined by the target’s size,
dielectric constant and distance from
the probe. The larger the size and
dielectric constant of a target, the more
it increases capacitance. The shorter
the distance between target and probe,
the more the target increases
capacitance.
Standard Target and Grounding
for Capacitive Proximity Sensors
The standard target for capacitive
sensors is the same as for inductive
proximity sensors. The target is
grounded per IEC test standards.
However, a target in a typical
application does not need to be
grounded to achieve reliable sensing.
Shielded vs. Unshielded
Capacitive Sensors
Shielded capacitive proximity sensors
are best suited for sensing low dielectric
constant (difficult to sense) materials
due to their highly concentrated
electrostatic fields. This allows them to
detect targets which unshielded
sensors cannot. However, this also
makes them more susceptible to false
triggers due to the accumulation of dirt
or moisture on the sensor face.
The electrostatic field of an unshielded
sensor is less concentrated than that of
a shielded model. This makes them well
suited for detecting high dielectric
constant (easy to sense) materials or
for differentiating between materials
with high and low constants. For the
right target materials, unshielded
capacitive proximity sensors have
longer sensing distances than shielded
versions.
Unshielded capacitive sensors are also
more suitable than shielded types for
use with plastic sensor wells, an
accessory designed for liquid level
applications. The well is mounted
through a hole in a tank and the sensor
is slipped into the well’s receptacle. The
sensor detects the liquid in the tank
through the wall of the sensor well. This
allows the well to serve both as a plug
for the hole and a mount for the sensor.
Target Correction Factors for
Capacitive Proximity Sensors
For a given target size, correction
factors for capacitive sensors are
determined by a property of the target
material called the dielectric constant.
Materials with higher dielectric constant
values are easier to sense than those
with lower values. A partial listing of
dielectric constants for some typical
industrial materials follows. For more
information, refer to the CRC Handbook
of Chemistry and Physics (CRC Press),
the CRC Handbook of Tables for
Applied Engineering Science (CRC
Press), or other applicable sources.
Dielectric Constants of
Common Industrial Materials
Acetone 19.5
Acrylic Resin 2.7–4.5
Air 1.000264
Alcohol 25.8
Ammonia 15–25
Aniline 6.9
Aqueous Solutions 50–80
Bakelite 3.6
Benzene 2.3
Carbon Dioxide 1.000985
Carbon Tetrachloride 2.2
Celluloid 3.0
Cement Powder 4.0
Cereal 3–5
Chlorine Liquid 2.0
Ebonite 2.7–2.9
Epoxy Resin 2.5–6
Ethanol 24
Ethylene Glycol 38.7
Fired Ash 1.5–1.7
Flour 1.5–1.7
Freon R22 & 502 (liquid) 6.11
Gasoline 2.2
Glass 3.7–10
Glycerine 47
Marble 8.0–8.5
Melamine Resin 4.7–10.2
Mica 5.7–6.7
Nitrobenzine 36
Nylon 4–5
Oil Saturated Paper 4.0
Paraffin 1.9–2.5
Paper 1.6–2.6
Perspex 3.2–3.5
Petroleum 2.0–2.2
Phenol Resin 4–12
Polyacetal 3.6–3.7
Polyamide 5.0
Polyester Resin 2.8–8.1
Polyethylene 2.3
Polypropylene 2.0–2.3
Polystyrene 3.0
Polyvinyl Chloride Resin 2.8–3.1
Porcelain 4.4–7
Powdered Milk 3.5–4
Press Board 2–5
Quartz Glass 3.7
Rubber 2.5–35
Salt 6.0
Sand 3–5
Shellac 2.5–4.7
Shell Lime 1.2
Silicon Varnish 2.8–3.3
Soybean Oil 2.9–3.5
Styrene Resin 2.3–3.4
Sugar 3.0
Sulphur 3.4
Teflon 2.0
Toluene 2.3
Transformer Oil 2.2
Turpentine Oil 2.2
Urea Resin 5–8
Vaseline 2.2–2.9
Water 80
Wood, Dry 2–7
Wood, Wet 10–30
Introduction
Capacitive Proximity Sensors
4–6
Shielded vs. Unshielded Construction
Each capacitive sensor can be
classified as having either a shielded or
unshielded construction.
Shielded Probe
Shielded sensors are constructed with a
metal band surrounding the probe. This
helps to direct the electrostatic field to
the front of the sensor and results in a
more concentrated field.
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Shielded construction allows the sensor
to be mounted flush in surrounding
material without causing false trigger.
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Shielded capacitive proximity sensors
are best suited for sensing materials
with low dielectric constants (difficult to
sense) as a result of their highly
concentrated electrostatic fields. This
allows them to detect targets that
unshielded sensors cannot.
Unshielded Probe
Unshielded sensors do not have a
metal band surrounding the probe and
hence have a less concentrated
electrostatic field. Many unshielded
models are equipped with
compensation probes, which provide
increased stability for the sensor.
Compensation probes are discussed
later in this section.
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Unshielded capacitive sensors are also
more suitable than shielded types for
use with plastic sensor wells, an
accessory designed for liquid level
applications. The well is mounted
through a hole in a tank and the sensor
is slipped into the well’s receptacle. The
sensor detects the liquid in the tank
through the wall of the sensor well.
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The electrostatic field of an unshielded
sensor is less concentrated than that of
a shielded model. This makes them well
suited for detecting high dielectric
constant (easy to sense) materials or
for differentiating between materials
with high and low constants. For certain
target materials, unshielded capacitive
proximity sensors have longer sensing
distances than shielded versions.
Introduction
Capacitive Proximity Sensors
4–7
Wood Industry
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Applications
Capacitive Proximity Sensors
4–8
Notes
Capacitive Proximity Sensors
4–9
Description
Bulletin 875C and 875CP capacitive
proximity sensors are self-contained
solid-state devices designed for
noncontact sensing of a wide range of
materials.
Unlike inductive proximity sensors, the
875C and 875CP can detect nonmetal
solids and liquids in addition to standard
metal targets. They can even sense the
presence of some targets through
certain other materials, making them an
ideal choice in some applications where
inductive proximity and photoelectric
sensors cannot be used.
Each unit has an adjustable sensing
distance and is equipped with two LEDs
to indicate power and output. They are
housed in either a nickel-plated brass
barrel (shielded models) or a plastic
barrel (unshielded models) which meets
NEMA 12 and IP67 (IEC 529) enclosure
standards. Connection options include
PVC cable as well as micro and pico
quick-disconnect.
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Features
� Metal, nonmetal solid and liquid
sensing capability
� Adjustable sensing distance
� Cable or quick-disconnect styles
� Short circuit�, overload�, reverse
polarity�, and transient noise
protection
� Plastic models have glass filled nylon
housings
� Meets NEMA 12 and IP67 (IEC 529)
enclosure standards
� CE marked for all applicable
directives
Styles
DC 3-Wire Nickel-Plated
Brass Barrel page 4–10. . . . . . . . . . . . .
DC 3-Wire Plastic Barrel page 4–13. . .
AC 2-Wire Plastic Barrel page 4–16. . .
AC 2-Wire Nickel-Plated
Brass Barrel page 4–19. . . . . . . . . . . . .
Accessories
Quick-Disconnect Cables page 7–1. . .
Mounting Brackets
Sight Glass Style page 4–21. . . . . . . . .
Sensor Wells page 4–22. . . . . . . . . . . . .
Bulletin 875C and 875CP
Plastic Face/Plastic Barrel or Nickel-Plated Brass Barrel
Capacitive Proximity Sensors
4–10
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Features
� Metal, nonmetal solid and liquid
sensing capability
� Adjustable sensing distance for
18mm and 30mm models
� 3-wire operation
� 3-conductor, 3-pin or 4-pin
connection
� Normally open or normally closed
output
� Short circuit, overload, reverse
polarity, and transient noise
protection
� CE marked for all applicable
directives
Specifications
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Capacitive Proximity Sensors
4–11
Product Selection
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