Proximity Sensors are available in models
using high-frequency oscillation to detect ferrous and non-ferrous metal
objects and in capacitive models to detect non-metal objects. Models are
available with environment resistance, heat resistance, resistance to
chemicals, and resistance to water.
What are Proximity Sensors?
"Proximity Sensor" includes all sensors that perform non-contact detection
in comparison to sensors, such as limit switches, that detect objects by
physically contacting them. Proximity Sensors convert information on the
movement or presence of an object into an electrical signal. There are
three types of detection systems that do this conversion: systems that use
the eddy currents that are generated in metallic sensing objects by
electromagnetic induction, systems that detect changes in electrical
capacity when approaching the sensing object, and systems that use magnets
and reed switches.
The Japanese Industrial Standards (JIS) define proximity sensors in JIS C
8201-5-2 (Low-voltage switch gear and control gear, Part 5: Control
circuit devices and switching elements, Section 2: Proximity sensors),
which conforms to the IEC 60947-5-2 definition of non-contact position
JIS gives the generic name "proximity sensor" to all sensors that provide
non-contact detection of target objects that are close by or within the
general vicinity of the sensor, and classifies them as inductive,
capacitive, ultrasonic, photoelectric, magnetic, etc.
This Technical Guide defines all inductive sensors that are used for
detecting metallic objects, capacitive sensors that are used for detecting
metallic or non-metallic objects, and sensors that utilize magnetic DC
fields as Proximity Sensors.
(1) Proximity Sensors detect an object without
touching it, and they therefore do not cause abrasion or damage to the
Devices such as limit switches detect an object by contacting it, but
Proximity Sensors are able to detect the presence of the object
electrically, without having to touch it.
(2) No contacts are used for output, so the Sensor
has a longer service life (excluding sensors that use magnets).
Proximity Sensors use semiconductor outputs, so there are no contacts to
affect the service life.
(3) Unlike optical detection methods, Proximity
Sensors are suitable for use in locations where water or oil is used.
Detection takes place with almost no effect from dirt, oil, or water on
the object being detected. Models with fluororesin cases are also
available for excellent chemical resistance.
(4) Proximity Sensors provide high-speed response, compared with switches
that require physical contact.
For information on high-speed response, refer to Explanation of Terms.
(5) Proximity Sensors can be used in a wide temperature range.
Proximity Sensors can be used in temperatures ranging from -40 to 200°C.
(6) Proximity Sensors are not affected by colors.
Proximity Sensors detect the physical changes of an object, so they are
almost completely unaffected by the object's surface color.
(7) Unlike switches, which rely on physical contact,
Proximity Sensors are affected by ambient temperatures, surrounding
objects, and other Sensors.
Both Inductive and Capacitive Proximity Sensors are affected by
interaction with other Sensors. Because of this, care must be taken when
installing them to prevent mutual interference. Care must also be taken to
prevent the effects of surrounding metallic objects on Inductive Proximity
Sensors, and to prevent the effects of all surrounding objects on
Capacitive Proximity Sensors.
(8) There are two-wire
The power line and signal line are combined. This reduces wiring work to
2/3 of that require for Three-wire Sensors. If only the power line is
wired, internal elements may be damaged. Always insert a load.
Detection Principle of Inductive Proximity Sensors
Inductive Proximity Sensors detect magnetic loss due to eddy currents that
are generated on a conductive surface by an external magnetic field. An AC
magnetic field is generated on the detection coil, and changes in the
impedance due to eddy currents generated on a metallic object are
Other methods include Aluminum-detecting Sensors, which detect the phase
component of the frequency, and All-metal Sensors, which use a working
coil to detect only the changed component of the impedance. There are also
Pulse-response Sensors, which generate an eddy current in pulses and
detect the time change in the eddy current with the voltage induced in the
The sensing object and Sensor form what appears to be a transformer-like
The transformer-like coupling
condition is replaced by impedance changes due to eddy-current losses.
The impedance changes can be viewed as changes in the resistance that is
inserted in series with the sensing object. (This does not actually occur,
but thinking of it this way makes it easier to understand qualitatively.)
Detection Principle of Capacitive Proximity Sensors
Capacitive Proximity Sensors detect changes in the capacitance between the
sensing object and the Sensor. The amount of capacitance varies depending
on the size and distance of the sensing object. An ordinary Capacitive
Proximity Sensor is similar to a capacitor with two parallel plates, where
the capacity of the two plates is detected. One of the plates is the
object being measured (with an imaginary ground), and the other is the
Sensor's sensing surface. The changes in the capacity generated between
these two poles are detected.
The objects that can be detected depend on their dielectric constant, but
they include resin and water in addition to metals.
Detection Principle of Magnetic Proximity Sensors
The reed end of the switch is operated by a magnet. When the reed switch
is turned ON, the Sensor is turned ON.
Selection by Detection Method
Items Requiring Confirmation
Inductive Proximity Sensors
Capacitive Proximity Sensors
Metallic objects (iron, aluminum, brass, copper, etc.)
resins, liquids, powders, etc.
Affected by positional relationship of power lines and signal lines,
grounding of cabinet, etc.
CE Marking (EC Directive compliance)
Sensor covering material (metal, resin).
Easily affected by noise when the cable is long.
AC/DC, DC with no polarity, etc.
Connection method, power supply voltage.
on the power supply, i.e., DC 2-wire models, DC 3-wire models, AC,
DC 2-wire models are effective for suppressing current consumption.
sensing distance must be selected by considering the effects of
factors such as the temperature, the sensing object, surrounding
objects, and the mounting distance between Sensors. Refer to the set
distance in the catalog specifications to determine the proper
distance. When high precision sensing is required, use a Separate
Temperature or humidity, or existence of water, oils, chemicals etc.
Confirm that the degree of protection matches the ambient environment.
extra margin must be provided in the sensing distance when selecting
Sensors for use in environments subject to vibration and shock.
To prevent Sensors from vibrating loose, refer to the catalog values
for tightening torque during assembly.
tightening torque, Sensor size, number of wiring steps, cable length,
distance between Sensors, surrounding objects.
Check the effects of surrounding metallic and other objects, and the
specifications for the mutual interference between Sensors.
This graph shows engineering data from
moving the sensing object parallel to the sensing surface of the
Refer to this graph for Proximity Sensor
applications, such as positioning. When a high degree of precision is
required, use a Separate Amplifier Proximity Sensor.
Sensing Distance vs. Display
This type of graph is used with Separate
Amplifier Proximity Sensors. It shows the values when executing FP (Fine
Positioning) at specified distances. FP settings are possible at any
desired distance, with a digital value of 1,500 as a reference for the
The above graph shows numerical examples
when Fine Positioning is executed at the three points of 0.3, 0.6, and
Effects of Sensing Object
Size and Material
Here, the horizontal axis indicates the
size of the sensing object, and the vertical axis indicates the Sensing
Distance. It shows changes in the Sensing Distance due to the size and
material of the sensing object. Refer to this data when using the same
Sensor to detect various different sensing objects, or when confirming
the allowable leeway for detection.
In contrast with contact-type limit
switches, which have physical contacts, leakage current in a 2-wire
Proximity Sensor is related to an electrical switch that consists of
transistors and other components. This graph indicates the leakage
current characteristics caused by transistors in the output section of
Generally speaking, the higher the
voltage, the larger the leakage current. Because leakage current flows
to the load connected to the Proximity Sensor, care must be taken to
select a load that will not cause the Sensor to operate from the leakage
Be careful of this factor when replacing a
limit switch, micro-switch, or other switch with a Proximity Sensor.
Similar to leakage current
characteristics, residual voltage is something that occurs due to
electrical switches that are comprised of transistors and other
components. For example, whereas the voltage in a normally open switch
should be 0 V in the ON state, and the same as the power supply voltage
in the OFF state, residual voltage refers to a certain level of voltage
remaining in the switch. Be careful of this factor when replacing
a limit switch, micro-switch, or other switch with a Proximity Sensor.
Explanation of Terms
Standard Sensing Object
A sensing object that serves as a reference for measuring basic
performance, and that is made of specified materials and has a specified
shape and dimensions.
The distance from the reference position (reference surface) to the
measured operation (reset) when the standard sensing object is moved by
the specified method.
The distance from the reference surface that allows stable use, including
the effects of temperature and voltage, to the (standard) sensing object
transit position. This is approximately 70% to 80% of the normal (rated)
Hysteresis (Differential Travel)
With respect to the distance between the standard sensing object and the
Sensor, the difference between the distance at which the Sensor operates
and the distance at which the Sensor resets.
t1: The interval from the point when the standard sensing object moves
into the sensing area and the Sensor activates, to the point when the
output turns ON.
t2: The interval from the point when the standard sensing object moves out
of the Sensor sensing area to the point when the Sensor output turns OFF.
The number of detection repetitions that can be output per second when the
standard sensing object is repeatedly brought into proximity.
See the accompanying diagram for the measuring method.
With a Shielded Sensor, magnetic flux is concentrated in front of the
Sensor and the sides of the Sensor coil are covered with metal.
The Sensor can be mounted by embedding it into metal.
With an Unshielded Sensor, magnetic flux is spread widely in front of the
Sensor and the sides of the Sensor coil are not covered with metal.
This model is easily affected by surrounding metal objects (magnetic
objects), so care must be taken in selecting the mounting location.
Expressing the Sensing Distance When measuring the Sensing Distance of
a Proximity Sensor, the reference position and the direction of
approach of the sensing object are determined as follows:
Perpendicular sensing distance
and sensing area diagram
Expressed as the
measured distance from the reference surface when the standard sensing
object approaches from the radial direction (perpendicular to the
Expressed as the measured distance from the reference axis when the
standard sensing objects moved parallel to the reference surface
This distance depends on the transit position (distance from the
reference surface), so it can be expressed as an operating point track
(Sensing Area Diagram)
general-use transistor can be directly connected to a Programmable
Controller or Counter.
Primarily built into machines exported to Europe and other overseas
A 2-wire AC
output that can be used for both AC and DC Sensors. Eliminates the
need to be concerned about reversing the polarity.
Take the following points into account when selecting a DC 2-wire model
A maximum current of 0.8 mA flows to the load current even when the
output is OFF. Check that the load will not operate with this current.
Check that the load will operate with this load voltage.
When the output is ON, voltage remains in the Sensor, and the voltage
applied to the load decreases.
Check that the load will operate with this load voltage.
there is an object in the sensing area, the output switching element
is turned ON.
there is no object in the sensing area, the output switching element
is turned ON.
NO or NC
operation can be selected for the output switching element by a switch
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