Search coil magnetometer

 ( Vision ) 

The search coil magnetometer or induction magnetometer, based on an inductive sensor (also known as inductive loop and inductive coil), is a magnetometer which measures the varying magnetic flux. An inductive sensor connected to a conditioning electronic circuit constitutes a search coil magnetometer. It is a vector magnetometer which can measure one or more components of the magnetic field. A classical configuration uses three orthogonal inductive sensors. The search-coil magnetometer can measure magnetic field from mHz up to hundreds of MHz.

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Principle The inductive sensor is based on Faraday's law of induction. The temporal variation of the magnetic flux $\backslash Phi$ through a N turns circuit will induce a voltage $e$ which follows

$e\; =\; -\; N\backslash frac\{\backslash mathrm\; d\; \backslash Phi\}\{\backslash mathrm\; dt\}$

which can be expressed in a simpler way

$e\; =\; -\; N\; S\backslash frac\{\backslash mathrm\; dB\}\{\backslash mathrm\; dt\}$

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

The induced voltage ( $e$) may be increased several ways:

• increase the surface ( S),
• increase the turn number ( N),
• use a ferromagnetic core.

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Search coil using a ferromagnetic core When a coil is wound around a ferromagnetism core, that increases the sensitivity of the sensor thanks to the apparent permeability of the ferromagnetic core.

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Apparent permeability The magnetic amplification, known as apparent permeability $\backslash mu\_\{app\}$, is the result of the magnetization of the ferromagnetic core response to an external magnetic field. The magnetization is reduced by the demagnetizing field.

$\backslash mu\_\{app\}\; =\; \backslash frac\{\backslash mu\_r\}\{1\; +\; N\_z\; (\backslash mu\_r\; -\; 1)\}$

where $\backslash mu\_r$ is the relative permeability, $N\_z$ is the demagnetizing coefficient in the z direction.

The induced voltage will be written

$-Ne\; =\; NS\backslash mu\_\{app\}\backslash frac\{N\backslash mathrm\; dB\}\{\backslash mathrm\; dt\}$

The demagnetizing coefficient can easily be computed in the case of simple shapes (spheres and ellipsoids).

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Applications

• Eye tracker: a search coil is used to measure eye movement using coils that are embedded into a contact lens.
• Education
• Non-destructive testing
• Magnetotellurics
• Space research
• Natural electromagnetic waves observations on Earth.

• Pavel Ripka, Magnetic Sensors and Magnetometers, Artech House Publishers
• S. Tumanski, Induction Coil Sensors - a Review
• [2]

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See also

• Waves (Juno) (Uses a magnetic search coil)

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