Magnetometers

     All of us probably are aware about the fact that the magnets create magnetic fields; earth has a magnetic field; current flowing in a wire also generate magnetic field. But have we ever realized that the fields are generated also by our heart and brain. However, what differs between the magnetic field generated by a magnet and that generated by brain and heart is the magnitude of magnetic field. Following figure shows the magnitude of magnetic fields generated by various sources.

    We all are surrounded by magnetic fields. Magnetic fields are generated by flowing electrical current in various electrical/electronic appliances; TV, computers, power transmission lines, etc.  Earth also has its own magnetic field, though relatively small. Earth magnetic field is largest at the poles (~ 60 000 nT) and smallest as the equator (~ 30 000 nT). The earth’s magnetic field strength is proportional to 1/r3 (until the influence from the solar wind gets noticeable).

    Measurement of the magnetic fields is of interest for various scientific purposes, navigation, etc. Measurement of these fields is done by sensing devices called magnetometers.

    MAGNETOMETER – WHAT IS IT?

    Magnetometers are devices that measure magnetic fields. A magnetometer is an instrument with a sensor that measures magnetic flux density B (in units of Tesla or As/ m2). Magnetometers refer to sensors used for sensing magnetic fields OR to systems which measure magnetic field using one or more sensors.

    Since magnetic flux density in air is directly proportional to magnetic field strength, a magnetometer is capable of detecting fluctuations in the Earth's field.

   Materials that distort magnetic flux lines are known as magnetic, and include materials such as magnetite that possess magnetic fields of their own, as well as very high magnetic conductivity. Such materials create distortions in the Earth's magnetic flux that is flowing around them. Magnetometers detect these distortions.

  A magnetometer measures magnetic flux density at the point in space where the sensor is located. A magnetic field drops in intensity with the cube of the distance from the object. Therefore, the maximum distance that a given magnetometer can detect the object is directly proportional to the cube root of the magnetometer's sensitivity. The sensitivity is commonly measured in Tesla.

    Types of Magnetometers

    Magnetometers are classified into two categories:

   • Vector magnetometers that measure the flux density value in a specific direction in 3 dimensional space. An example is a fluxgate magnetometer that can measure the strength of any component of the Earth’s field by orienting the sensor in the direction of the desired component.
 
  • Scalar magnetometers that measure only the magnitude of the vector passing through the sensor regardless of the direction. Quantum magnetometers are an example of this type of magnetometer.
 
   Various magnetometers used to measure magnetic fields are discussed in following sections.
 
   Coil Magnetometer

   Based on Faraday’s law, coil is the fundamental method of magnetic field sensing.  Faraday’s law states that the induced emf in any      closed conducting coil is equal to the time rate of change of magnetic flux through the circuit.

  With a coil having N turns wounded around a magnetic material with magnetic permeability mr & and a flux f through it, the emf induced in the coil is:

 

   If magnetic field has harmonic variation with time, induced voltage is proportional to the frequency of the magnetic field.
 
   Thus, induced emf depends upon coil’s area. Sensitivity of the small coil magnetometer depends on the size and number of turns. This is used to detect only variations in the field (due to motion or due to the AC nature of the field). Since usually B needs to be measured instead of dB/dt, an integrator is usually used to obtain signal proportional to magnetic field B.
 
  This is useful in such areas as mine detection or buried object detectors (pipe detection, “treasure” hunting, etc.). It is often used because of its simplicity.






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