Understanding Magnetometers and Their Uses

From guiding spacecraft through the vastness of the cosmos to pinpointing buried treasure beneath our feet, magnetometers are the unsung heroes of countless technologies we rely on. But what exactly is a magnetometer? In essence, it’s a device that measures the strength and direction of magnetic fields, those invisible forces that surround us.

But their applications go far beyond just detecting metal objects. Magnetometers are essential in navigation systems, robotics, medical devices, and even scientific research. In this article, we’ll delve into the different types of magnetometers, their varied uses, and how they are revolutionizing industries from consumer electronics to space exploration.

Magnetometers are used in conjunction with gyroscopes and accelerometers for accurate attitude determination and motion tracking. This algorithmic combination of 3-axis magnetometers, 3-axis gyroscopes, and 3-axis accelerometers is referred to as 9-axis sensor fusion. The gist of sensor fusion is that the clever combination of the information from the three sensors that form the basis of 9-axis motion sensing—magnetometer, accelerometer, and gyro—provides more than the sum of its constituent parts. Strengths shore up weaknesses, and weaknesses rely on forces in some magic and artful combination between the three.

Magnetometer Quality in Sensor Fusion and Motion Tracking

The accuracy of sensor fusion only goes as far as its weakest sensor link. Put a crappy gyro into the equation, and it becomes immediately obvious that motion tracking suffers commensurately. The same goes for the accelerometer and, of course, the magnetometer as well. But the magnetometer has always been the most mysterious and squirrelly of the three from the very beginning, and the fewest engineers thoroughly understand its contributions, dynamics, and idiosyncrasies. How does a good magnetic sensor affect the results of fusion and motion tracking? And what, exactly, are good versus not-so-good magnetometers to begin with? It is the intent of this blog to lay some of the foundations with respect to the different types of magnetometers that now exist for high-volume applications, while it is the subject of another blog on what the impact is to sensor fusion that varying levels of magnetometer performance brings.

What are Magnetometers, and What are they used For?

Magnetometers are devices used for measuring magnetic fields. The main purpose of a magnetometer is to detect magnetic variations accurately, and their output is used in – among other things – navigation, object detection, and positional tracking. There are several types of magnetometers available today, including Fluxgate, optically pumped, superconducting quantum interference device (SQUID), Hall effect sensors, Magneto-Resistive sensors, Lorentz Force, and Magneto-inductive sensors.

1. Fluxgate Magnetometers: The fluxgate technology uses magnetic materials that experience hysteresis, allowing them to easily measure even the slightest changes in magnetic fields. While offering good sensitivity, fluxgate magnetometers tend to be bulky and consume considerable power, limiting their applicability in compact devices.

2. Optically Pumped Magnetometers: These magnetometers utilize an atomic vapor cell and lasers to measure magnetic fields. Optically pumped magnetometers are known for their high sensitivity and accuracy, making them suitable for scientific research. However, their complex design and relatively high costs restrict their widespread adoption for commercial applications.

3. SQUID Magnetometers: SQUID magnetometers are renowned for their extreme sensitivity. These sensors utilize superconducting materials and measure the changes in magnetic fields by detecting quantum interference in the superconducting circuits. However, their reliance on cryogenic temperatures and the need for careful handling make them very difficult to employ in most applications.

4. Hall Effect Magnetometers: Hall effect sensors detect the strength of a magnetic field using the Hall effect. The generation of an electric potential difference across a conductor is known as the production of voltage. Situated perpendicular to a magnetic field. Although Hall effect magnetometers offer compact sizes and low power consumption, they provide limited sensitivity and are mainly used to detect on or off types of applications.

5. Magneto-Resistive (MR) Sensors: All magneto-resistive sensors work on the principle that certain magnetic materials deposited on a semiconductor substrate change their ability to resist current flow in proportion to an applied magnetic field. The main forms of magneto-resistive sensors are Anisotropic Magneto-resistive (AMR), Tunneling Magnetic-resistive (TMR), and Giant Magneto-resistive (GMR). All three of these technologies exhibit changes in conduction based upon applied magnetic field, although they achieve them in slightly different ways. All three-exhibit similar magnetic measurement capability, and the reason to choose one over the others usually revolves around issues of manufacturability for a particular supplier. AMR is by far the most common form of magneto-resistive magnetic sensor that is used.

6. Lorentz Force Magnetometers: Today, Asahi Kasei Micro-devices (AKM) produces the market-leading chip-scale magnetometer, and its technology is based upon the Lorentz force principle that is manifested in their proprietary Hall-effect sensor.

The concept is simple. A current is generated in a semiconductor material by applying a voltage across the desired path of current flow. As the electrons flow through the semiconductor media, another passive set of electrodes is positioned orthogonally to the direction of the current flow so that any transverse magnetic field applied to the path of the current flow causes a slight deflection of the electrons toward one or the other of the sense electrodes. This deflection of the electrons is caused by the Lorentz force generated by the movement of current through a magnetic field.

As one can imagine, the amount of deflection caused by a field as weak as the Earth’s is over the minute distance that the current travels between the electrodes placed on a chip no larger than 1.5 mm x 1.5mm is almost undetectable. In fact, only very noisy signals can be achieved by Silicon-based Hall effect sensors, so AKM uses a more exotic semiconductor substrate as well as an exotic dopant material. This helps enhance the signal just enough so that AKM’s Hall effect sensor can be used as a geomagnetic compass. In the spectrum of geomagnetic field measurement devices, AKM’s sensor rates are the bare minimum in terms of acceptable performance. In addition, because the Hall effect sensor’s natural axis of measurement is orthogonal to the plane of the chip, the X and Y component measurements are accomplished by a clever toroidal flux concentrator that bends the X and Y components of the field into the sensitive portion of the sensor. This further degrades some of the measurement capability of the sensor.

Understanding the different types of magnetometers and their unique characteristics is crucial when selecting the right sensor for a particular application. Each technology offers distinct advantages and limitations, which we’ll explore in the following comparison

Type of Magnetometer	Pros	Cons
Fluxgate	High sensitivity, wide range, can measure static fields	Bulky, high power consumption
Optically Pumped	Very high sensitivity, very high accuracy	Complex design, expensive, requires controlled environment
SQUID	Extremely high sensitivity, very low noise	Requires cryogenic cooling, delicate handling
Hall Effect	Compact, low cost, low power consumption	Limited sensitivity, accuracy impacted by temperature
Magneto-Resistive	Compact, low cost, low power consumption, good sensitivity	Sensitive to temperature variations, require calibration, susceptible to magnetic interference
Lorentz Force	Compact, low cost, good sensitivity	Some sensitivity to temperature, may require calibration
Magneto-Inductive	High sensitivity, high accuracy, very large dynamic range, less susceptible to magnetic interference, low power consumption	Larger size than chip-scale sensors

Magneto-Inductive Sensors

Magneto-inductive sensors, like the RM3100 magnetometer from PNI Sensor, have emerged as an excellent alternative to Hall effect magnetometers and Magneto-Resistive sensors due to their accurate and reliable magnetic field measurements. PNI’s RM3100 magneto-inductive sensors are currently made from bulk manufacturing techniques, very much in the same style as chip inductors and chokes currently are made.

They are larger than Hall-effect and MR sensors. Still, despite their size, they remain very cost-competitive to the lowest-cost magnetic sensors of the competing technologies and perform well over an order of magnitude better in terms of signal-to-noise and sensitivity. They also possess an extremely large measurement dynamic range that makes them ideally suited for geomagnetic field measurement within challenging magnetic environments, such as found in many real-world applications.

Magnetometers Applications

While their role in motion tracking is essential, magnetometers are versatile sensors with applications that span industries and scientific disciplines. Let’s dive into some key areas where they’re making a difference:

1. Navigation & Orientation:

Smartphones & Tablets: That compass app you use for hiking? It’s powered by a magnetometer, sensing the Earth’s magnetic field to point you in the right direction. But it goes beyond just a simple compass. In AR games like Pokémon Go, the magnetometer helps align the virtual world with your real surroundings, letting those digital creatures appear as if they’re right in front of you.
Drones & Robotics: Magnetometers provide essential heading information for drones and robots, ensuring they stay on course even when GPS signals are unavailable (think indoor warehouses or disaster zones). Some high-end drones use magnetometers for obstacle detection, sensing changes in the magnetic field caused by large metal objects.

2. Object Detection:

Metal Detectors: Airport security, treasure hunting, even finding pipes buried underground – all rely on magnetometers to detect the presence of metal. The change in the magnetic field when a metal object is nearby triggers an alert.
Vehicle Detection: Embedded in roads, magnetometers can sense the passage of vehicles, providing data for traffic management, parking systems, and even toll collection.

3. Scientific Research:

Geological Surveys: Magnetometers mounted on aircraft or ships are used to map the Earth’s magnetic field and discover hidden geological structures, like mineral deposits or underwater volcanoes.
Space Exploration: Satellites use magnetometers to study the magnetic fields of other planets and moons, giving us clues about their composition and potential habitability.

4. Medical Applications:

Heartbeat Monitoring: Extremely sensitive magnetometers can detect the faint magnetic field generated by the human heart, offering a non-invasive way to monitor heart health.
Brain Imaging (MEG): Magnetoencephalography (MEG) uses arrays of magnetometers to measure the magnetic fields produced by brain activity, aiding in the diagnosis of neurological disorders.

5. Niche and Emerging Applications:

Archeological Surveys: Magnetometers help locate buried artifacts or structures without the need for excavation.
Animal Tracking: Some studies use magnetometers to track the migratory patterns of animals that rely on Earth’s magnetic field for navigation.
Submarine Detection: Military magnetometers are used to detect the distortions in the Earth’s magnetic field caused by submarines, crucial for naval operations.

This is just a glimpse into the vast world of magnetometer applications. As sensor technology continues to advance, we can expect even more creative and impactful uses to emerge.

Conclusion

Magnetometers play an integral role in countless applications that rely on precise measurement of magnetic fields. Among the various types of magnetometers available, the magneto-inductive sensor technology stands out for its accuracy, power efficiency, and versatility. PNI Sensor’s RM 3100 magnetometer is a prime example of the remarkable sensitivity and reliability delivered by these sensors. With their ability to provide highly accurate magnetic field measurements, magneto-inductive sensors continue to revolutionize the field of magnetic sensing, enabling advancements in fields ranging from military positioning navigation work to automotive, robotics, consumer electronics, and wearables applications.