| How do the sensor techniques compare? This is our guide to choosing a sensor for your application.
Capacitive sensing
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: permittivity variation to background
Pros: 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 uncompressed 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
Cons:
In their simplest form capacitive sensors cannot distinguish between
different objects which present the same relative permittivity.
Again in simplest form can be sensitive to temperature and humidity swing
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.
Pros: Non contact
Using combination of capacitive and electric field sensing it is possible to infer the chemical composition of materials
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
Pros: Non contact
Ignores non metallic objects e.g.: dirt, water lubricating oil.
Cons: Unless shielded, individual sensors must be spaced to avoid cross talk.
No zero speed measurement of moving objects.
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
Pros: Ability to detect target direction, with multiple cells
Passive operation
Low power requirement
No cross talk.
Cons: Temperature sensitive?
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.
Pros: Non-contact electromechanical solution.
Low unit cost.
Cons: Must provide magnetic field to activate reed switch.
Mechanical movement in reed switch limits life.
Limited output possibilities, making or breaking switched output only.
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.
Pros: Ideal and well developed solution for non-invasive non destructive testing within metal parts.
Cons: Limited developments for other applications.
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 miniaturization of storage of magnetic information.
Range: Low
Property in target essential to operation: Specific materials only.
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 LED's 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.
Pros: 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.
Cons: 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.
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 polarization 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).
Pros: Immunity from ambient light interference.
Ability to measure temperature with suitable IR sensors.
Cons: Limited immunity to ambient light interference.
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.
Pros: 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).
Cons: 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.
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 10mm to 1m, 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.
Pros: Ability to determine speed and direction using doppler shift analysis on received data.
Cons: Radar absorbent materials deaden response.
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.
Pros: Ability to determine speed and direction using doppler shift analysis on received data.
Cons: Sound absorbent materials deaden response.
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.
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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