| When an application calls for
detecting a metallic target that falls within an inch of the sensing
surface, inductive proximity sensors are apt for the task. First
introduced in the early 1960s, these durable components have proven
their mettle in the sensing arena. In fact, they're the best-selling
sensing technology in the world. Their immunity to dust and dirt
buildup suits them well for harsh industrial environments.
Additionally, the standardized physical and electrical
characteristics of the general-purpose, cylindrical types of these
sensors simplify their use.
An inductive proximity sensor has four
components: the coil, oscillator, detection circuit, and output
circuit. The target material, environment, and mounting restrictions
all have an influence on these items and on the senor's operation,
magnetic nature, and shielding. The oscillator generates a
fluctuating, doughnut-shaped magnetic field around the winding of
the coil, which is located in the device's sensing face. When a
metal object moves into the sensor's field of detection, Eddy
currents build up in the object, magnetically push back, and finally
dampen the sensor's own oscillation field. The sensor's detection
circuit monitors the amplitude of the oscillation and, when it
becomes sufficiently damped, triggers the output circuitry.
There are five categories of inductive
proximity sensors: cylindrical, rectangular, miniature, harsh
environment, and special purpose. Cylindrical threaded-barrel
sensors account for 70% of all inductive proximity sensor purchases.
Years ago, this style's behavior was standardized by the CENELEC
organization, which determined characteristics such as body size,
sensing distances, and output levels. |
Capacitive proximity switches
contain 4 main components: plate, oscillator, detection circuit and
solid state switch. The sensor operates in a similar fashion to a
simple capacitor.When the power is turned on, the oscillator detects
the amount of capacitance between the external target and the metal
plate in the sensor. As the target approaches, the capacitance
increases. When the capacitance reaches a certain point, the
oscillator begins to operate (the opposite of inductive sensors, in
which the oscillator stops operating when the target approaches).
Once the oscillation reaches a preset point, the detection circuit
senses this, and signals the switch to change state.
A capacitor or condenser is a passive
electronic component consisting of a pair of conductors separated by
a dielectric (insulator). When a potential difference (voltage)
exists across the conductors, an electric field is present in the
dielectric. This field stores energy and produces a mechanical force
between the conductors. The effect is greatest when there is a
narrow separation between large areas of conductor, hence capacitor
conductors are often called plates. An ideal capacitor is
characterized by a single constant value, capacitance, which is
measured in farads. This is the ratio of the electric charge on each
conductor to the potential difference between them. In practice, the
dielectric between the plates passes a small amount of leakage
current. The conductors and leads introduce an equivalent series
resistance and the dielectric has an electric field strength limit
resulting in a breakdown voltage.
A Capacitive Proximity Switch is a good choice
to detect level, waer or even solids such as plastic pellets for
injection molding. They are also used on sight glass tubes for level
detection on bulk liquid storage tanks. The sensor will ignore the
sight glass itself and detect the liquid inside of it, providing
feedback for the liquid level in the bulk tank. More often, however,
the capacitive sensor is used as a replacement sensor for
applications ill-suited to photoeye detection. Photoelectric sensors
have difficulty with materials that are dark, highly reflective, or
clear, but a capacitive sensor ignores all these properties. Unlike
the inductive proximity sensor, which only detects metals,
capacitive sensors function as efficient, all-purpose sensors to
detect nearly any material.
|
Both Inductive and Capacitive
Proximity can both be termed as NAMUR switches. NAMUR switches are
designed to be used in hazardous locations with very low voltage
requirements.
NAMUR capacitive sensors are designed for use
in hazardous areas. Our NAMUR sensors are ATEX approved. NAMUR
sensors are also FM (Factory Mutual) approved for use in potentially
explosive atmospheres.
NAMUR inductive proximity sensors are low
energy devices designed for use in hazardous areas. When connected
to intrinsically safe approved switch isolators, NAMUR inductive
proximity sensors are approved for all hazardous area
classifications.
NAMUR (Normenarbeitsgemeinschaft für Mess- und
Regeltechnik in der Chemischen Industrie)
DIN EN 60947-5-6
|
Ultrasonic sensors (also known
as tranceivers when they both send and receive) work on a principle
similar to radar or sonar which evaluate attributes of a target by
interpreting the echoes from radio or sound waves respectively.
Ultrasonic sensors generate high frequency sound waves and evaluate
the echo which is received back by the sensor. Sensors calculate the
time interval between sending the signal and receiving the echo to
determine the distance to an object.
This technology can be used for measuring:
wind speed and direction (anemometer), fullness of a tank and speed
through air or water. For measuring speed or direction a device uses
multiple detectors and calculates the speed from the relative
distances to particulates in the air or water. To measure the amount
of liquid in a tank, the sensor measures the distance to the surface
of the fluid. Further applications include: humidifiers, sonar,
medical ultrasonography, burglar alarms and non-destructive testing.
Systems typically use a transducer which
generates sound waves in the ultrasonic range, above 20,000 hertz,
by turning electrical energy into sound, then upon receiving the
echo turn the sound waves into electrical energy which can be
measured and displayed.
The technology is limited by the shapes of
surfaces and the density or consistency of the material. For example
foam on the surface of a fluid in a tank could distort a reading.
Ultrasonic sensors are used to detect the
presence of targets and to measure the distance to targets in many
automated factories and process plants. Sensors with an on or off
digital output are available for detecting the presence of objects,
and sensors with an analog output which varies proportionally to the
sensor to target separation distance are commercially available.
Because ultrasonic sensors use sound rather
than light for detection, they work in applications where
photoelectric sensors may not. Ultrasonics are a great solution for
clear object detection and for liquid level measurement,
applications that photoelectrics struggle with because of target
translucence. Target color and/or reflectivity don't affect
ultrasonic sensors which can operate reliably in high-glare
environments. |
