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Magnetic Field Sensors

There are many types of magnetic field sensors on the market. A brief list would include (but not be limited to) these, with the more exotic devices first:

  • SQUID (superconducting quantum interference device) magnetometer
  • Nuclear precession magnetic field sensor
  • Optically pumped magnetic field sensor
  • Electron tunneling MEMS (micro-electro-mechanical system)
  • AMR (anisotropic magneto-resistance) magnetometer
  • GMR (giant — or occasionally geometrical — magneto-resistance) magnetometer
  • Lorentz Force MEMS
  • MEMS compass
  • Hall effect device
  • Magneto-diode
  • Magneto-transistor
  • Magnetic tunnel junction magnetometer
  • Search coil magnetic field sensor
  • Fluxgate magnetometer
  • Inductive pickups

The MEMS solutions are gaining market share in the smartphone compass market, but in other markets such as automotive, other devices are more often used. Hall effect sensors (and some of the other inexpensive devices) are used as mechanical position and rotation sensors. In medical diagnostic research (e.g., brain research), the exotic sensors such as the SQUID device enable researchers to gain a better understanding of the brain functions in real-time.

In hard-drives, GMR and other sensors are used to achieve what now seems to be a phenomenal storage density. Fluxgate magnetometers have some applications in compasses where MEMS is not a fit. Many of the others have various research applications.

As we all recall from physics, magnetic field strength or flux density is measured in tesla (T) or gauss (G). T=G•104 . One tesla is one weber (Wb) per square meter where a weber is the measure of magnetic flux. Subjecting a one-turn coil to a linearly ramped down change of 1Wb across a 1s span would produce 1V at the coil's terminals — hence 1Wb = 1V•s.

(Source: Rockwell Automation)

(Source: Rockwell Automation)

Sensitivity of sensors can range from hundreds of mT for the most primitive sensors to 1•10-18 T for the SQUID type sensors. Sensitivity, accuracy, operating temperature, ruggedness, resistance to damage at high heat, and a host of other parameters come to play in selecting magnetic sensors for an application.

Hall effect sensors and simple inductive pickup coils are some of the most rugged types of sensors. The simple inductive device still gets used a lot in industrial applications — the classic setup uses a gear or toothed wheel with the pickup nearby.

Inductive pickup form factors can be large (i.e., a large aperture), though their sensitivity is relatively low compared to some of the other devices mentioned. MEMS sensors can be relatively small and quite sensitive.

A Google search for “magnetic sensor selection guide” can bring one to the different vendors' selection guides — the choices seem overwhelming.

Have you used magnetic sensors? If so, what type? How well did they work?

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