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 $\Phi$ through a N turns circuit will induce a voltage $e$ which follows:

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

which can be expressed in a simpler way:

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

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

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 is a type of non-contact electronic proximity sensor that is used to detect the position 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, can reduce the sensing range by up to 60 percent.

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

The sensor consists of an induction loop. Electric current generates a magnetic field, which collapses generating a current that falls toward zero from its initial trans when the input electricity ceases. The inductance of the loop changes according to the material inside it and since metals are much more effective inductors than other materials the presence of metal increases the current flowing through the loop. This change can be detected by sensing circuitry, which can signal to some other device whenever metal is detected.

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.

• Magnetic Resonance Imaging

• Non Destructive Testing

• Nuclear Magnetic Resonance

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.

See also

• List of sensors

References

• Pavel Ripka, Magnetic Sensors and Magnetometers, Artech House Publishers
• S. Tumanski, Induction Coil Sensors - a Review
• C. Coillot & P. Leroy, Induction Magnetometers: Principle, Modelling and ways of improvement, Intech Open Access Publisher
• C. Coillot et al., Signal modeling of an MRI ribbon solenoid coil dedicated to spinal cord injury investigations