Magnetic field sensors are used to measure magnetic flux and/or the strength and direction of a magnetic field. They are used mainly in scientific measurement, navigation, and industrial applications.

Operation Principle of a Resonant Magnetic Field Microsensor. Graphic credit: Wikimedia Commons

Types of Magnetic Field Sensors

The Engineering360 SpecSearch database organizes magnetic field sensors by technology or type.

• Magnetoresistive sensors measure electrical resistance as a function of the applied or ambient magnetic field.
• Flux gate or coil sensors measure differences in the magnetic field at the ends of a vertical rod and then plot this information on a grid.
• Hall Effect sensors convert the energy stored in a magnetic field to an electrical signal by developing a voltage between the two edges of a current-carrying conductor whose faces are perpendicular to a magnetic field.
• Magnetoinductive sensors consist of a coil that surrounds a ferromagnetic core whose permeability changes within the earth's magnetic field
• Proton precession sensors use liquids such as kerosene and methanol that have high densities of hydrogen atoms.
• Overhauser or nuclear precession sensors combine an electron-rich liquid with hydrogen and subject the mixture to a radio frequency (RF) signal.
• Optically-pumped sensors polarize a gaseous alkali with a specific wavelength of light.
• SQUID sensors are very sensitive to low magnetic fields and can measure fields as low as the femtoTesla range. They are used mainly in medical, particularly neuroscientific applications, and have a very wide range of magnetic field instrument. SQUID stands for superconducting quantum interference devices (SQUID).

Sensor Performance

Selecting magnetic field sensors requires an analysis of performance specifications. Flux density, the total measurement range in gauss (G), often corresponds to the linear output region of the sensing technology. Resolution is the smallest measurable increment. Accuracy is a percentage of full-scale. Bandwidth is the frequency range over which magnetic field sensors meet their accuracy specifications. The number of axes indicates the number of possible, simultaneous measurements for magnetic field sensors. Single-axis devices are common, but two-axis and three-axis devices are also available.

Measurement Features and Electrical Outputs

Magnetic field sensors may incorporate directional or magnitude measurement features.

• Directional magnetic field sensors measure the direction of the magnetic field and may provide bipolar outputs.
• Magnitude sensors simply measure the magnitude of the magnetic field.

Magnetic field sensors differ in terms of electrical outputs.

• Analog current levels such as 4- 20 mA are suitable for sending signals over long distances.
• Analog voltages are simple, usually linear functions.
• Modulated analog output signals are encoded, but still analog in nature. Examples include sine wave, pulse wave, amplitude modulation (AM), and frequency modulation (FM) signals.
• RS232, RS422, and RS485 are common serial, digital protocols.
• Parallel protocols include the general-purpose interface bus (GPIB), a standard which is also known as IEEE 488.

Other digital outputs for magnetic field sensors include transistor-transistor logic (TTL) signals. Outputs that change the state of a switch or alarm are also available.

Resources

Magnetic Field Sensors for Detection of Pneumatic Cylinder Piston Position

Exploring Optical and Magnetic Sensors

Image credit:

pddnet.com