Technical Article

# What Is a Magnetometer?

March 19, 2021 by Ignacio de Mendizábal

## Learn the basics of magnetometers, including their relation to hysteresis curves and their applications.

Magnetometers are devices that can measure the magnitude or direction of a magnetic field. They are present almost everywhere in electronics. They may be as simple as the one that your smartphone uses to detect whether it is upright or as complex as the one NASA uses to measure the magnetic field of Mars.

Here, we will have a look at the fundamentals of magnetometers and their applications. In further articles, we will take a closer look at specific types of magnetometers.

### How Do Magnetometers Work?

Magnetometers, usually through an indirect method, measure what is called the magnetic moment. The magnetic moment of a closed loop with area, A, and current, I, is a vector whose magnitude is equal to I times A. The torque experienced by this loop is equal to the magnetic moment multiplied by the magnetic field.

Mathematically, the magnetic moment is expressed as follows:

$$\overrightarrow{\tau} = \overrightarrow{m} \times \overrightarrow{B}$$

Where

• τ = torque
• m = magnetic moment
• B = external magnetic field

As a vector quantity, the direction of the magnetic field is as important as its amplitude. Some magnetometers can measure both the direction and magnitude of the magnetic field (vector magnetometers), while others can measure only its amplitude (scalar magnetometers).

Regarding units, the International System (SI) unit is Am2. However, it is common to see it expressed in many other units, such as $$\frac{erg}{G}$$, where an erg is a unit of energy equivalent to 10-7 joules and G is one gauss.

### Magnetometers and Hysteresis Curves

The properties of magnetic materials vary when they are immersed in a magnetic field. Depending on how these materials react to a field before and after applying it, they fall into the groups of paramagnetic, diamagnetic, or ferromagnetic materials. Apart from these, there are non-magnetic materials, which present weak magnetic properties.

The best tool to represent magnetic properties is the hysteresis curve. It represents the magnetic flux density, B, against the magnetic field strength force, H.

##### Figure 1. An example hysteresis curve. Image used courtesy of NDT Resource Center

The most salient aspect of magnetic materials is that, even when we remove the applied force, they remain magnetized (that is, they exhibit retentivity). Then, to return the material to its initial point, it is necessary to apply a negative magnetic field strength (H) to demagnetize it (coercivity).

Due to the special aspects of magnetic materials and their plethora of applications, the capability of measuring their properties with a good resolution brought a revolution in the fields of physics and materials.

### High-sensitivity Electronics Measurement

Magnetic sensors, like many other sensors, are accompanied by a set of electronic systems in order to process small electrical signals and generate a readable input by a microcontroller, processor, or humans. The design and build of these systems are a challenge because, most of the time, signals are quite small and very sensitive to noise. Therefore, designers need to reach a trade-off between circuit complexity, sensor capacity, and cost.

There are components frequently used and relatively simple such as filters or amplifiers, but there are also others that are more complex, such as modulation-demodulation chains or lock-In amplifiers

With the progress of miniaturization, it is frequent to find that part of the analog conditioning circuitry is implemented inside an integrated circuit (IC) in the signal chain, since it is less error-prone and more compact compared to a discrete solution.

##### Figure 2. Internal parts of a Hall sensor IC. Image used courtesy of Allegro

On the flip side, an integrated solution can be less flexible. When working in a specific or novel application, designers might prefer a discrete option because they might need to develop a new conditioning chain.

### Magnetometer Applications

Magnetometers are widely used in everyday applications. Usually, the most complex sensors are reserved for highly specialized laboratories like the Physics and Chemistry of Materials in Strasbourg. Such magnetometers may include vibrating sample magnetometers, SQUID magnetometers, and AGFM (alternative gradient field magnetometer), among others.

Hall effect sensors are widely used in applications where the magnetic field is strong. Their biggest applications are motion detection and control in motor drive applications.

##### Figure 3. A Hall sensor IC. Image used courtesy of Microchip

It is common to place some sensors, usually two or three, spatially distributed around a motor axis, and the metallic components that trigger the sensor in front of them. Each time that the metallic parts pass in front of a sensor, they generate a square or sinusoidal signal that permits the calculation of speed or position.

MEMS magnetometers are also easily found on the market. They are usually part of inertial motion units (IMUs), which measure acceleration, angular speed, and magnetic fields. Models such as STMicroelectronics’ eCompasses can be integrated within a PCB and then connected to other parts such as filters or microcontrollers. You may find these magnetometers used for applications such as motion detection in a smartphone screen that automatically adjusts a display's information depending on its horizontal or vertical orientation.

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Chuck(G) March 26, 2021

This article gives very short shrift to the subject.  There are many types of magnetometers, such as proton, SQUID, SERF, optically-pumped, etc.
My first experience with magnetometers was inspired by an old Carl and Jerry story in Popular Electronics, sometime in the 1960s.  It was so fascinating, I had to build one myself.

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