Induction loop

This article is about a detection system. For the more general electrical component, see Inductor.

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.

Implementation

An example of the Inductance loop installed in the road for cars and bikes

The "aerial" 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.

Other definitions

A different sort of "induction loop" is applied to metal detectors, 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.

Modern day applications

• vehicle detection (e.g. at traffic lights)
• car park Parking Guidance and Information systems
• metal detectors
• audio induction loops

Historical applications

• An Anti-submarine indicator loop was a device used to detect submarines and surface vessels using specially designed submerged cables connected to a galvanometer.

External links

• Loop and infrared systems - for deaf and hard of hearing people (fact sheet) A practical guide from the Royal National Institute for the Deaf in the UK.
• Informational resources about induction loop systems for people with hearing loss.