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| Technique | Summary: &
Pros | Cons |
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| Capacitive
sensing |
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Active: electrodes generate and shape
an electric field. Objects with a dielectric value affect the capacitance
between the electrodes which is detected via the sense electrode. Often only
one electrode, the sense electrode, is required.
Range: low.
Output:
analogue or digital (set-point).
Sensing elements: charged and sense
electrodes, (any material capable of being electrically charged).
Property
in target essential to operation: permitivity variation to background. |
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| | Pros |
Cons |
| | Non-contact.
Detection of metallic or non-metallic
objects.
Light and sound immune.
Ability to detect targets non invasively
(through other materials).
Can distinguish mass, e,g between un compressed
dust powder and greater mass of material.
Can compensate for: dirt build-up,
change in temperature or humidity in sensing field region.
Can shape or
direct sensing field and focus detection into defined areas.
Multiple
receiving electrode arrays can be used without cross talk. |
In their simplest form capacitive sensors cannot
distinguish between different objects which present the same relative
permitivity.
Again in simplest form can be sensitive to temperature and
humidity swings. |
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| Electric Field |
| |
Active or passive: Electrodes
generate an electric field to detect disturbance in the field caused by
objects.
Passive examples measure or detect existent electric
fields.
Range: low
Output: analogue or digital (set-point)
Sensing
elements: charged and sense electrodes.
Property in target essential to
operation: emit or disturb electric fields. |
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| | Pros |
Cons |
| | Non
contact
Using combination of capacitive and electric field sensing it is
possible to infer the chemical composition of materials. |
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| Inductive |
| |
Active: Current is induced in a coil
wound round a ferrite when a ferrous or non-ferrous metallic target passes
through the electromagnetic field in front of the sensor.
Range: low :
Sensing elements: coil wound around a ferrite.
Ideal application: metal
sensing in machine tool industry or engine management where range is low and
lubricating oil or dirt would interfere with other sensing
techniques.
Property in target essential to operation: influence on
electromagnetic fields. |
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| | Pros |
Cons |
| | Non
contact
Ignores non metallic objects e.g: dirt, water lubricating oil. |
Unless shielded, individual sensors must be
spaced to avoid cross talk.
No zero speed measurement of moving
objects. |
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| Hall Effect |
| |
Passive: A Hall cell detects changes
in magnetic field levels.
Range: low, dependent on magnetic field
lines.
Sensing element: Hall cell.
Property in target essential to
operation: influence / disturb magnetic field |
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| | Pros |
Cons |
| | Ability
to detect target direction, with multiple cells
Passive operation
Low
power requirement
No cross talk. | Temperature sensitive? |
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| Reed switch |
| |
A magnetic field pulls together
mechanical contacts making a circuit. A magnet is usually provided within the
target to activate the reed switch.
Suitable for presence detection, some
rpm measurement.
Sensing elements: reed switch, permanent magnet embedded
in target.
Property in target essential to operation: ability to contain
permanent magnet. Note: although a mechanical device (reed switch) is used,
this is the switching element of the sensor, actuation from an external point
of view is non contact with no external moving parts. |
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| | Pros |
Cons |
| | Non-contact electromechanical solution.
Low unit cost. |
Must provide magnetic field to activate reed
switch.
Mechanical movement in reed switch limits life.
Limited output
possibilities, making or breaking switched output only. |
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| Eddy current |
| |
A coil applies alternating magnetic
fields to a target object, this generates electrical currents (eddy currents:
because they flow in circles) below the surface of the target object. These
eddy currents are disturbed by geometry changes including those caused by
corrosion or hairline cracks. Coil impedance responds to changes in the eddy
current flowing in the target object. The change in impedance is measured.
The Eddy current technique only applies to conductive materials and is used
extensively in non destructive testing.
Range: low
Sensing element:
coil
Property in target essential to operation: conductive material. |
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| | Pros |
Cons |
| | Ideal
and well developed solution for non-invasive non destructive testing within
metal parts. | Limited developments for other
applications. |
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| Magneto resistive |
| |
MR Sensors exploit the characteristic
of certain metals, which when exposed to a magnetic field, change their
resistance to the flow of electricity, this is detected by the change in
amperage of the sense current and can be made more sensitive than Hall effect
devices.
Temperature stable, rugged designs withstand impact and
vibration.
Application: non contact rotation sensors.
Used within disk
drives for miniaturisation of storage of magnetic information.
Range:
Low
Property in target essential to operation: Specific materials only. |
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| Optical visual
Light |
| |
Active: Generates visible light to
detect reflections or levels of reflection, or the inverse, from target areas
or objects.
Range: low to medium
Sensing elements: sending and receiving
LEDs and directional lenses.
Basic function making or breaking a beam of
visible light.
Modes: 1:diffuse, 2:reflective, 3:through beam.
Also
used: fixed focus, background suppression
Property in target essential to
operation: reflection or absorption of visible light different to background.
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| | Pros |
Cons |
| | Ability
to focus or constrain beam onto small target areas using shields or
fibres.
Ability to extend sensor via fibre optic cables into hazardous
environments. | Potential for interference
from other ambient light sources.
Dependence on light reflectivity of target
objects.
Opaque or transparent objects can present problems.
Visible
light can aid alignment. |
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| Optical IR |
| |
Active: Generates infra red
wavelength light to detect reflections or levels of reflection, or the inverse,
from target areas or objects.
Passive: responds to changes in received
Infra red light.
Range: low to medium
Sensing elements: IR emitter and
receiver diodes & lenses. May also include polarisation filters and or
discrete reflectors for reflective beam breaking detection modes.
Basic
function making or breaking IR light beams, detection of target
scatter.
Modes 1:diffuse, 2:reflective, 3:through beam.
Property in
target essential to operation: reflection or absorption of IR light different
to background. (note: Sensors using laser light sources work on similar basic
principles). |
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| | Pros |
Cons |
| | Immunity from ambient light interference.
Ability to measure
temperature with suitable IR sensors. | Limited immunity to ambient light interference. |
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| Optical Colour
recognition |
| |
Sensing elements: transmitter LED,
receiving RGB LEDs and or light filters and lenses.
Property in target
essential to operation: reflection of light spectrum sufficient for detection
of scatter.
Colour recognition for fruit ripeness, component sorting, colour
coding, object recognition. |
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| Ultrasonic |
| |
Active: A piezo transducer generates
ultrasonic waves (short wavelength, high frequency normally outside the audible
frequency) which are reflected by suitable target objects (echo). The
reflection is received by a piezo transducer and time of flight is calculated
to determine range to object.
Can be used internally if coupled to a surface
where internal reflection will occur at changes in material consistency.
(medical ultrasound)
Passive? See acoustic emission.
Range: to 20m
Sensing elements: transmitting piezo transducer, receiving transducer
(often same transducer when not transmitting).
Property in target essential
to operation: ability to reflect ultrasound waves, or if used as through beam,
to absorb or break waves. |
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| | Pros |
Cons |
| | Vibration of piezo transducer has self cleaning effect.
Longer
range capability normally than capacitive or inductive techniques.
Can do
background suppression. (able to tune out range >X). |
Single transducer types feature a dead band over
the time period to dampen transmission oscillations.
The speed of sound
varies according to temperature and pressure of the medium, changes or
gradients affect accuracy and may require compensation.
Freezing can affect
transducers esp. if ice forms. |
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| Acoustic emission |
| |
Passive devices monitor acoustic
(sound) levels emitted by target objects or materials especially under stress.
Compare results as materials age.
Range: varies.
Sensing elements:
Acoustic sensor.
Property in target essential to operation: produces
detectable sounds, (need not be audible).
Ideal application: maintenance /
monitoring of part wear / ageing for replacement of items "when required"
rather than on a "routine schedule". |
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| Radar |
| |
Radio Detection and Ranging for long
range target detection, measures the strength and round-trip time of microwave
signals emitted by an antenna and reflected off a distant surface or object.
Using microwave wavelengths in the range 1 cm to 1 m, frequency range of
about 300 MHz to 30 GHz and various polarizations.
Detection and or imaging
of targets uses combinations of echo and doppler shift.
Range: long (note:
range in a straight line).
Sensing elements: directional sending and
receiving microwave frequency antenna.
Property in target essential to
operation: differential reflection of radar waves to background. |
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| | Pros |
Cons |
| | Ability
to determine speed and direction using doppler shift analysis on received
data. | Radar absorbent materials deaden
response. |
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| Sonar |
| |
Similar to radar but using sound
waves through water.
Measures the strength and round-trip time of sound
waves reflected off a distant surface or objects through water.
Fish
finders and submarine range finding and bottom scanning / mapping.
Detection and or imaging of targets again uses combinations of echo and
doppler shift.
Range: long (in water).
Sensing elements: directional
transducer.
Property in target essential to operation: differential
reflection of sonar waves. |
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| | Pros |
Cons |
| | Ability
to determine speed and direction using doppler shift analysis on received
data. | Sound absorbent materials deaden
response. |
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| Conductive |
| |
Generates an AC potential difference
between two electrodes, to detect when a conductive material fills the gap
closing the circuit.
Range: depends, normally contact.
Application:
level of conductive fluids using two electrodes, (brake fluid). AC to avoid
electrolytic corrosion.
Sensing elements: two electrodes.
Property in
target essential to operation: electrical conductance.
Low cost level
detection solution. |
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| Resistive |
| |
Generates a current to detect changes
in the resistance of target materials or as caused by target materials.
Range: contacts.
Sensing elements: electrodes to transmit current to
target.
Property in target essential to operation: conductive material with
resistance change according to event. |
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