Search coil magnetometer explained

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.

Principle

\Phi through a N turns circuit will induce a voltage e which follows e = - N (d \Phi)/(dt) which can be expressed in a simpler waye = - N S (dB)/(dt) by assuming that the induced magnetic field B is homogeneous over a surface S (the magnetic flux will be expressed \Phi = B x S ).

The induced voltage (

e ) may be increased several ways:

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

Search coil using a ferromagnetic core

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

Apparent permeability

The magnetic amplification, known as apparent permeability

\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.\mu _{app} = (\mu _{r})/(1+ N _{z} (\mu _{r} -1 ))

where

\mu _{r} is the relative permeability, N _{z} is the demagnetizing coefficient in the z direction.

The induced voltage will be written

- Ne = NS\mu _{app} (N dB)/(dt)

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

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.

References

• Pavel Ripka, Magnetic Sensors and Magnetometers, Artech House Publishers
• S. Tumanski, Induction Coil Sensors - a Review
• http://www.haarp.alaska.edu/haarp/magind.html

See also

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