This chapter is composed by three parts. The first is an introductory part, providing general information about magnetism and related phenomena. Magnetic materials are also discussed and presented. Afterwards, the magnetic field and various measurement techniques are discussed. In the second part, different magnetic sensors used in a laboratory or space are presented. Magnetic sensors that are discussed include anisotropic magneto-resistance (AMR), giant magneto-resistance (GMR), giant magneto-impedance (GMI), flux-gate, and superconducting quantum interference device (SQUID). Although some of them may be outdated and well known, they are widespread, and they still pose an excellent choice for certain applications. Advances in magnetometers also presented in order to provide the reader with the recent trends in the field. Magnetic cleanliness is an important factor both in calibration and in normal operation of a system; in the third part, current techniques to isolate a system from the external magnetic field providing cleanliness are discussed.
TopIntroduction
During the last few decades, the continuously increasing demand for accurate and reliable AC/DC magnetic field measurements has paved the way for the development of various types of magnetic sensing systems as well as different measurement techniques. The sensor’s sensitivity and linearity, signal-to-noise ratio, measurement range, cross-talk between sensors in multi-sensor applications are only some of the aspects that have been examined in the past.
Magnetic sensors are categorized primarily based on their sensing principle, which impacts directly the performance characteristics like measurement range, resolution, frequency response, working temperature, and manufacturing cost. For example, Hall Effect and “pick up coils” based sensors have been proven to be practical and cost-effective solutions in many common applications. Nevertheless, “pick up coils” are not appropriate for measuring low-frequency magnetic fields, whilst Hall-effect devices cannot provide the resolution levels needed in demanding applications. On the other hand, Anisotropic Magnetoresistance (AMR), Giant Magnetoresistance (GMR), Giant Magneto-Impedance (GMI), Magnetostrictive, and Flux-Gate sensors, although available at an elevated cost, are frequently employed in cases where higher resolution is required. Moreover, Superconducting Quantum Interference Devices (SQUID) offer the highest resolution and lower noise levels but requires cryogenic refrigeration.
Many industrial applications nowadays use magnetic sensors as measuring systems. These sensors have several advantages such as the fact that they can easily operate under harsh environmental conditions and elevated temperatures, they are very reliable due to the absence of moving parts and they can be embedded inside building materials. Magnetic Sensors are commonly used for laboratory measurements and satellite magnetic measurements. Understanding the various types of magnetometers and their limitations is crucial for the interpretation of the satellite magnetometer results.
For space use, magnetic sensors must have some unique characteristics. For example, SQUID sensors may not be suitable for this type of application due to the need for continuous cooling in cryogenic temperatures. In space, magnetometers can observe the fluctuations of a planet’s magnetic field, from the planet’s core or the solar wind. On the other hand, in some cases, there is a need to protect certain instruments from electromagnetic radiation. That is achieved by using a magnetic sensor to measure this disturbance and a closed-loop technique to cancel it. The most common types of magnetic sensors for space, are the fluxgate and search coils magnetometers.
In the laboratory, usually, there is the flexibility of using more complex and sensitive equipment. For accurate laboratory measurements, there is a need for a magnetically clean environment. The magnetically clean room or facility can be used for numerous applications such as:
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calibration of magnetic sensors
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noise measurements for the magnetic sensor
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identification of the magnetic signature of the Equipment Under Test (EUT)
There are passive and active techniques to create a magnetically clean room and they depend on the shielding specifications/requirements.
This chapter will explore the different types of magnetic sensors and their applications, as well as some techniques for electromagnetic shielding at low frequencies.
TopMagnetic Field
The definition of the magnetic field is most probably the most fundamental concept in magnetism. The magnetic field, which is generated by a magnetic source, covers a certain (infinite but reciprocal dampened) area around the source. In the vicinity of the magnetic field, there is an energy gradient, which can lead to the generation of a force in the presence of another magnetic field. For example, the needle of a compass is a magnetic dipole, which has its own magnetic field, which in turn interacts with the earth’s magnetic field. Therefore, a torque is applied to the needle in order to come to rest, at the point with the least energy. (Daniel, 1988; Fraden, 2010; Jiles, 2015).