Induction loop
Encyclopedia
Induction loop is a term used to describe an electromagnetic
communication- and detection system, relying on the fact that a moving magnet
will induce
an electrical current in a nearby conducting wire. Induction loops are used for transmission and reception of communication signals, or for detection of metal objects in metal detectors or vehicle presence indicators. A common modern use for induction loops is to provide hearing assistance to hearing-aid users.
" system of an induction loop installation can consist of one or more loops of a conductive element.
In industrial applications this might be a large single- or multi-turn, loop, or a complex multi-lobed, phase coincident sub-loop design, most effectively mounted above the required reception area in industrial applications.
An audio induction loop might have one or more loops sometimes with a phase shift between them, and either near to or around the area in which a hearing aid user would be present. Many different configurations can be used depending on the application.
Such an induction loop receiver is classically a very small iron-cored inductor (telecoil), although Rediffusion
demonstrated a prototype Hall-Effect system in its PLL FM system.
The system commonly uses an analogue power amplifier matched to the low impedance of the transmission loop. The transmission is normally direct rather than superimposed or modulated upon a carrier, though multi-channel systems have been implemented using modulation.
Vehicle detection (inductive) loops are used to count vehicles passing or arriving at a certain point, for instance approaching a traffic light
, and in motorway traffic management. An insulated, electrically conducting loop is installed under the road. An electric current
circulates through the loop at all times. When a ferrous (containing iron or steel) body passes close to the wire loop the inductance
of the loop changes and the light controller begins to change state. Contrary to popular belief, the induction loop is not triggered by magnets, nor by the weight of the vehicle.
s, where a large coil, which forms part of a resonant circuit, is effectively "detuned" by the coil's proximity to a conductive object.
The detected object may be metallic, (metal and cable detection) or conductive/capacitive (stud
/cavity detection)
Other configurations of this equipment use two or more receiving coils, and the detected object modifies the inductive coupling or alters the phase angle of the voltage induced in the receiving coils relative to the oscillator coil.
An increasingly common application is for providing hearing aid
-compatible "assistive listening" telecoil. In this application a loop or series of loops is used to provide an audio frequency oscillating magnetic field in an area where a hearing aid user may be present. Many hearing aids contain a telecoil which allows the user to receive and hear the magnetic field and remove the normal audio signal provided from the hearing aid microphone site. These loops are often referred to as a hearing loop or Audio induction loop
.
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
communication- and detection system, relying on the fact that a moving magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
will induce
Faraday's law of induction
Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...
an electrical current in a nearby conducting wire. Induction loops are used for transmission and reception of communication signals, or for detection of metal objects in metal detectors or vehicle presence indicators. A common modern use for induction loops is to provide hearing assistance to hearing-aid users.
Implementation
The "aerialAntenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
" system of an induction loop installation can consist of one or more loops of a conductive element.
In industrial applications this might be a large single- or multi-turn, loop, or a complex multi-lobed, phase coincident sub-loop design, most effectively mounted above the required reception area in industrial applications.
An audio induction loop might have one or more loops sometimes with a phase shift between them, and either near to or around the area in which a hearing aid user would be present. Many different configurations can be used depending on the application.
Such an induction loop receiver is classically a very small iron-cored inductor (telecoil), although Rediffusion
Rediffusion
Rediffusion was a business which distributed radio and TV signals through wired relay networks. The business gave rise to a number of other companies, including Associated-Rediffusion, later known as Rediffusion London, one of the first companies to win a terrestrial ITV franchise in the UK...
demonstrated a prototype Hall-Effect system in its PLL FM system.
The system commonly uses an analogue power amplifier matched to the low impedance of the transmission loop. The transmission is normally direct rather than superimposed or modulated upon a carrier, though multi-channel systems have been implemented using modulation.
Vehicle detection (inductive) loops are used to count vehicles passing or arriving at a certain point, for instance approaching a traffic light
Traffic light
Traffic lights, which may also be known as stoplights, traffic lamps, traffic signals, signal lights, robots or semaphore, are signalling devices positioned at road intersections, pedestrian crossings and other locations to control competing flows of traffic...
, and in motorway traffic management. An insulated, electrically conducting loop is installed under the road. An electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
circulates through the loop at all times. When a ferrous (containing iron or steel) body passes close to the wire loop the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
of the loop changes and the light controller begins to change state. Contrary to popular belief, the induction loop is not triggered by magnets, nor by the weight of the vehicle.
Other definitions
A different sort of "induction loop" is applied to metal detectorMetal detector
A metal detector is a device which responds to metal that may not be readily apparent.The simplest form of a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field...
s, where a large coil, which forms part of a resonant circuit, is effectively "detuned" by the coil's proximity to a conductive object.
The detected object may be metallic, (metal and cable detection) or conductive/capacitive (stud
Stud finder
A stud finder is a handheld device used to determine the location of wood and metal framing studs used in light-frame construction after the walling surface has been installed....
/cavity detection)
Other configurations of this equipment use two or more receiving coils, and the detected object modifies the inductive coupling or alters the phase angle of the voltage induced in the receiving coils relative to the oscillator coil.
An increasingly common application is for providing hearing aid
Hearing aid
A hearing aid is an electroacoustic device which typically fits in or behind the wearer's ear, and is designed to amplify and modulate sound for the wearer. Earlier devices, known as "ear trumpets" or "ear horns", were passive funnel-like amplification cones designed to gather sound energy and...
-compatible "assistive listening" telecoil. In this application a loop or series of loops is used to provide an audio frequency oscillating magnetic field in an area where a hearing aid user may be present. Many hearing aids contain a telecoil which allows the user to receive and hear the magnetic field and remove the normal audio signal provided from the hearing aid microphone site. These loops are often referred to as a hearing loop or Audio induction loop
Audio induction loop
Audio Induction Loop systems, also called audio-frequency induction loops or hearing loops, are an aid for the hard of hearing. They are a loop of cable around a designated area, usually a room or a building, which generates a magnetic field picked up by a hearing aid...
.
Modern day applications
- vehicle detection (e.g. at traffic lightTraffic lightTraffic lights, which may also be known as stoplights, traffic lamps, traffic signals, signal lights, robots or semaphore, are signalling devices positioned at road intersections, pedestrian crossings and other locations to control competing flows of traffic...
s) - car park Parking Guidance and InformationParking guidance and informationParking guidance and information systems, or car park guidance systems, present drivers with dynamic information on parking within controlled areas. The systems combine traffic monitoring, communication, processing and variable message sign technologies to provide the service...
systems - metal detectorMetal detectorA metal detector is a device which responds to metal that may not be readily apparent.The simplest form of a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field...
s - audio induction loopAudio induction loopAudio Induction Loop systems, also called audio-frequency induction loops or hearing loops, are an aid for the hard of hearing. They are a loop of cable around a designated area, usually a room or a building, which generates a magnetic field picked up by a hearing aid...
s - TreadleTreadleA treadle [from OE tredan = to tread] is a part of a machine which is operated by the foot to produce reciprocating or rotary motion in a machine such as a weaving loom or grinder...
on railways - vehicle detection on tramways
Historical applications
- An Anti-submarine indicator loopAnti-submarine indicator loopAn anti-submarine indicator loop was a submerged cable laid on the sea bed and used to detect the passage of enemy submarines. Developed by the Royal Navy during World War I, they were extensively used by the Allies during World War II to protect harbours against submarine attack.They worked as...
was a device used to detect submarineSubmarineA submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...
s and surface vessels using specially designed submerged cables connected to a galvanometerGalvanometerA galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...
.