# Inductive sensor

Elements of a simple inductive proximity sensor.
1. Field sensor
2. Oscillator
3. Demodulator
4. Flip-flop
5. Output

## Principle

The inductive sensor is based on Faraday's law of induction. The temporal variations of the Magnetic Flux ${\displaystyle \Phi }$ through a N turns circuit will induce a voltage ${\displaystyle e}$ which follows:

${\displaystyle e=-N{\frac {\mathrm {d} \Phi }{\mathrm {d} t}}}$

which can be expressed in a simpler way:

${\displaystyle e=-N\times S{\frac {\mathrm {d} B}{\mathrm {d} t}}}$

by assuming that the induced magnetic field B is homogeneous over a section S (the Magnetic flux will be expressed ${\displaystyle \Phi =B\times S}$).

## Applications

### Search Coil Magnetometer

Inductive sensors constitute the main element to build a search coil magnetometer also known as search coil. These are used in many fields of research: magnetotellurics, electromagnetic waves measurement, space magnetometers to investigate electromagnetic waves in space plasma as well as natural electromagnetic waves observations on Earth.

### Inductive Proximity Sensor (Proxy Switch)

An inductive proximity sensor belongs to the category of non-contact electronic proximity sensor. It is used for positioning and detection of metal objects. The sensing range of an inductive switch is dependent on the type of metal being detected. Ferrous metals, such as iron and steel, allow for a longer sensing range, while nonferrous metals, such as aluminum and copper, may reduce the sensing range by up to 60 percent.[1]

Since the output of an inductive sensor has two possible states, an inductive sensor is sometimes referred to as an inductive proximity switch.[1][2]

The sensor consists of an induction loop or detector coil. Most often this is physically a number of turns of insulated magnet wire wound around a high magnetic permeability core, such as a ferrite ceramic rod or coil form, and the winding may or may not have a feedback tap some number of turns from one end of the total winding. It is connected to a capacitance to form a tuned frequency oscillator tank circuit. In conjunction with a voltage or current gain device like a transistor or operational amplifier, this forms a tuned frequency oscillator. When power is applied, the resulting oscillation is a high frequency alternating electric current in the coil that has a constantly changing magnetic field able to induces eddy currents in proximal (target) conductors. The closer the target is and the greater its conductivity (metals are good conductors, for example), the greater the induced eddy currents are and the more effect their resulting opposing magnetic fields have on the magnitude and frequency of the oscillation. Its magnitude is reduced as the load is increased in a non-magnetic conductor like aluminum because the induced field in the target opposes the source induction field, lowering net inductive impedance and therefore simultaneously tuning the oscillation frequency higher. But that magnitude is less affected if the target is a highly magnetically permeable material, like iron, as that high permeability increases the coil inductance, lowering the frequency of oscillation.

A change in oscillation magnitude may be detected with a simple amplitude modulation detector like a diode that passes the peak voltage value to a small filter to produce a reflective DC voltage value, while a frequency change may be detected by one of several kinds frequency discriminator circuits, like a phase lock loop detector, to see in what direction and how much the frequency shifts. Either the magnitude change or the amount of frequency change can serve to defined a proximity distance at which the sensors go from on to off, or vice versa.

Common applications of inductive sensors include metal detectors, traffic lights, car washes, and a host of automated industrial processes. Because the sensor does not require physical contact it is particularly useful for applications where access presents challenges or where dirt is prevalent.

Inductive sensors, also referred (in this area) as NMR coils or radiofrequency coils, are used to detect the magnetic component of the electromagnetic field associated to the nuclear spin precession in Nuclear Magnetic Resonance.