Inductive sensors, capacitive sensors and ultrasonic
sensors are three different proximity sensors. These devices detect
targets in industrial applications. They differ for technology, capability to
detect different type of targets and working distance.
All three, on the other hand, can detect object without any
contact, in spite of surface color.
Inductive sensors and capacitive sensors can operate at few
millimeters, whilst ultrasonic sensors can reach several meters.
What is an inductive sensor?
Inductive sensor are industrial
electronic devices used to detect the presence of metal objects, without any
interference of other different materials (like plastic, wood, water or oil).
Datasensing qualifies as one of the main world manufacturers of
sensors, after the merger of Datalogic and M.D. Micro Detectors, two companies
that have made the history of sensors in Italy since the early 1970s.
So,
who better to explain this matter?
Inductive sensor working principle
Inductive sensors rely on emitting
an oscillating electromagnetic field. When the metal target approaches the
sensor, drain currents are generated in the target. This current drains energy
from the electromagnetic field and so the amplitude of the oscillation
decreases.
A threshold detector inside the sensor detects when the
amplitude is below a defined value and so the output switches. When the target
moves away, the drain currents in the target decrease and so the
electromagnetic field amplitude increases. Over a defined threshold, the sensor
output switches again in the initial working condition.
Inductive sensor: how is it made?
Inductive sensors consist basically
in the following stages:
1) a coil and a ferrite, that generate the electromagnetic field
2) an oscillator used to drive
electromagnetic field generation
3) a comparator stage that compares
the received signal with a switching threshold
4) a transistor that drives an
external actuator or directly the load.
Inductive sensor types
Inductive sensors can be
distinguished by dimensions. The classification goes from cylindrical
miniaturized models with 3mm diameter to M30 housing and rectangular models.
Sensing distance, installation (shielded or unshielded) and advanced functions
(analogue output, weld field immune, …) are other means of classification.
What metals does an inductive sensor sense?
As mentioned before, inductive
sensors detect only metal targets. Metal targets respond in different
way to the electromagnetic field according to the different type of materials.
The sensor detects
different materials such as steel, stainless steel, copper, brass, or aluminum
at varying maximum distances.
The standard target, according to the Standard ISO60947-5-2,
is a Fe360, 1mm thickness square plate with a side accordingly to sensor
dimensions and working distance. In the inductive
sensor product specifications, it is possible to find the reduction factor
table for different materials. Users must take care of this effect when
selecting the right inductive sensors for their
application.
What is a capacitive sensor?
Capacitive sensors are industrial electronic device that can
detect any kind of material (metal, plastic, wood, water, …).
Capacitive sensor working principle
Capacitive sensors are based on the
emission of an oscillating electric field. Capacitive sensor and target act as
the plates of a capacitor.
When
the target approaches the sensor, the electric field amplitude decreases until
that, below a defined threshold, the trigger increases. Over a defined
threshold, the sensor switches the outputs. When the target moves away from the
sensor comparator makes switch again the sensor in the initial conditions.
Capacitive sensor material detection
Capacitive sensors can detect very
different types of materials. The actual detection distance depends on the
dielectric constant of the material itself:
-materials with
high dielectric constant are detected at higher distances
-materials with
low dielectric constant are detected only when they are close to the sensor
For this reason, materials such as metals and water are well
detected by a capacitive sensor, while materials as flour and nylon are
detected only at short distance.
Another effect that can change the effective working distance of
a capacitive sensor is the humidity rate of the material. For example, it is
possible to observe a different sensing detection distance between dry wood and
wet wood, due to the different presence of water inside.
Further
effects on the sensing distance are due to the compactness of the material
itself.
The sensor is well able to detect water. Meanwhile, because ice
contains air bubbles, sensors can detect it at a shorter distance. The same
could happen for plastic materials: compacted panels are detected at higher
distance compared to granular pellets.
Capacitive liquid level sensor
Capacitive sensors are widely used for liquid level detection.
If the tank walls are made of plastic or glass, it is possible to adjust the
sensitivity to detect the presence of the liquid inside.
Capacitive
sensors are widely used to detect aggressive liquids or liquids that must not
be contaminated getting in touch with the sensor itself. Of course, the higher
the difference in terms of dielectric constant between the tank wall and the
content, the better the performance
What is an ultrasonic sensor?
Ultrasonic sensors are industrial
electronic devices that can detect any kind of materials (metal, plastic, wood,
water, …). The detection can happen independently from color, transparency,
shininess, and state (liquid, solid, and powder).
Of course, the performances depend on the
sound-absorbance capability of the target surface material. The higher the
absorbance, the lower the actual sensing distance. For instance, materials like
spoons, powders and foams are sound absorbing.
Ultrasonic sensor working principle
Ultrasonic sensors are based on the emission of an ultrasonic
beam. The beam is reflected by the target towards the sensor. The sensor
measures the time delay between the emission of the ultrasonic signal and the
returning echo. Since the speed of the sound in the air is well known, it is
possible to infer not only if the target is present, but also the distance
between sensor and target.
Since the same ultrasonic transducer is used for the emission of
the signal and the detection of the echo, a particular space is present just in
front of the sensor, where the detection does not happen. This area is called
the ultrasonic sensor blind zone.
The blind zone is the minimum distance between the sensor head
surface and the position of target with the first identifiable echo. In the
blind zone, no material must be present.
In ultrasonic sensor specifications, maximum working distance
and blind zone are always indicated. For obvious physical reasons, the higher
the working distance the higher the blind zone.
How is an ultrasonic sensor working with different targets?
Ultrasonic sensors can detect every kind of material, but the
actual working distance depends on the sound absorbing capability of the
materials.
-Compact
materials reflect the ultrasonic signal toward the sensors. These materials
include metals, plastics or wood panels, flat liquid surfaces with no foam. In
this case, the sensor detects target at the full operative distance.
-Sound absorbing
materials or irregular surfaces reflect only a part of the incident signal. The
former are materials like foams, textiles, plastic pellet; the latter are
objects like vegetables layers, potatoes or apples. In this case, these
materials absorb or spread away part of the ultrasonic beam. This means that,
to increase the energy signal received from the sensor, the right choice would
be a model with higher working distance and more beam energy.
Another aspect to take into account is the target's position. If
the target aligns perpendicularly with the ultrasonic beam direction, it
reflects all the energy towards the sensor. In contrast, if the target is
tilted, part of the energy is deflected away. If the inclination increases, the
sensor does not receive any energy and does not detect the target.
In case of inclined target, a possible solution is a
retroreflective ultrasonic sensor. The retroreflective ultrasonic sensor
requires a fixed, flat, perpendicular target to work with. The sensor is set to
this reference position and every object in the middle is detected, independently
from position and sound absorbing material characteristics.
How is an ultrasonic sensor influenced by environmental factors
Ultrasonic sensors are designed in a specific environment: air
with standard atmospheric pressure. So, what is happening if there is a change
in:
-Transmission
gas: ultrasonic sensors are designed to work in air. If there is a change in
the transmission gas (nitrogen, helium, …), the speed of the ultrasonic beam
changes. Given that, the distance measured at the same time can vary. The
sensor may give a different reading than expected in the air.
-Temperature:
the speed of the ultrasonic beam in air is dependent from the temperature.
UK/UT series are fully compensated in the whole working temperature range;
-Pressure:
ultrasonic sensors are designed to work in standard atmospheric pressure
(1atm). Changes in environmental pressure affects the speed of the ultrasonic
beam. By that, the measured distance could differ from the measure in standard
conditions. The sensors are not suitable to work correctly in high or low
environmental pressure;
-Dust,
drops and humidity on the sensing head: ultrasonic sensors are very resistant
to the presence of dirty on the sensor head. In any case if the accumulation of
dirt is over a defined threshold, it influences the sensor correct working. It
is necessary to keep the detection head as clean as possible;
-Air flow:
ultrasonic sensors are very resistant to the presence of air flow and wind in
outdoor use. In any case, it is always necessary to remind that the ultrasonic
beam is moving in the air. So, if the air flow is excessive, the ultrasonic
beam is deviated from the correct path to/from the target and so the detection
could be unstable. For this reason, it necessary to check the position of
pneumatic valves outlet;
-Environmental
condition for outdoor use: ultrasonic sensor can work outdoor since they are
IP67 and not affected by sunlight. In any case, harsh environmental conditions
like heavy rain, snow and storm wind, can affect the correct detection of the
target.
What products does Datasensing recommend?
In case you have to detect metal target at short distance, the
best solution is a inductive sensor. They are not influenced by the
environmental conditions (presence of water, oil, grease, nonmetal materials,
…) and have a very high repeatability. See for example Datasensing series AD (M5
miniaturized), AE (M8), AM (M12), AK (M18) and AT (M30).
For level detection applications and detection of granular
materials (like plastic pellet, seeds, …), the right choice is a capacitive sensor.
By the sensitivity adjustment, it is possible to set the sensor to detect the
target materials.
Datasensing recommended product lines are C18
series (M18) and C30 (M30).
For long distance detection, ultrasonic sensors are the right
solution. Ultrasonic sensors are also available with analogue output for
measurement and level applications from the top of tank.
You can start your product search with
Datasensing series UK (M18) and UT (M30).