Working Principle and Development of Magnetic Sensors

Catalog

Ⅰ Introduction

Magnetic sensors are widely used in modern industrial and electronic products to measure physical parameters such as current, position, and direction with the intensity of the induced magnetic field. In the prior art, there are many different types of sensors for measuring magnetic fields and other parameters. It has the characteristics of non-contact measurement, durable and long life. The detection signal is almost not affected by the measured object. It is resistant to pollution and strong noise and can work reliably even in very harsh environmental conditions. Because of this, such sensors are used in various sectors from the defense, aerospace to the national economy, from medical care to everyday life.

The magnetic sensor starts with the guidance of the magnet as a compass for navigation. Later, as components for sensing magnetic fields and magnetic fluxes, detection coils, fluxgate magnetometers, semiconductor Hall elements and magnetoresistive elements, ferromagnetic thin film anisotropic magnetoresistive (AMR) components were developed. There are also stress sensors using bulk ferrite cores, temperature sensors using thermosensitive ferrite cores, fiber optic current sensors using ferromagnetic garnet magneto-optic effect, high sensitivity superconducting quantum interference devices (SQUID), and many more. In short, there are many types of magnetic sensors and frequent replacements.

compass

Magnetic sensors are usually assembled and used inside machines and equipment. Modern complete machines are rapidly developing towards small, lightweight, multi-functional, and intelligent. They require sensors to be able to respond with high sensitivity and high speed even if they change physical quantities in a small space. That is, while the sensor itself needs to be small and lightweight, it is also eager to improve its working speed, detection resolution, and sensitivity.

The promotion and application of Semiconductor large-scale integrated circuit manufacturing technology, micro-electromechanical system (MEMS) manufacturing technology, micro-assembly technology, and new materials such as magnetic thin films, multilayer films, and nano-magnetic wires have laid the foundation for miniaturization and miniaturization of magnetic sensors basis. Many new high-performance, miniaturized, and miniaturized magnetic sensors that use various new effects are being continuously put on the market. The AMR thin-film sensitive components and sensors that were launched in the early days, the newly launched GMI sensors, SI sensors, SV-GMR sensors, and the upcoming thin-film fluxgate magnetometer and wireless magneto-elastic microsensor array are typical examples.

Magnetic sensors are ubiquitous, compact, and affordable. They can be easily integrated with other circuits on the chip. Therefore, magnetic sensors are widely used in various applications. The advantages of magnetic sensing technology in robots and factory automation are obvious. Since the magnetic sensor has higher reliability and precision in the detection of part position and speed, it is crucial for motion control. This advantage also allows the robot arm and other components to move smoothly and accurately, thereby ensuring a high-quality and high-security manufacturing process.

Ⅱ Working principle of magnetic sensors

The working principle of the magnetic sensor is by converting the magnetic field into voltage or current information. Because the internal operation of the sensor and external components do not require physical contact, the magnetic sensor is ideal for reducing environmental pollution in automotive and industrial environments. At the same time, it can also reduce the wear caused by friction between components, thereby reducing equipment maintenance costs.

Magnetic sensors have a variety of functions, but it should be particularly pointed out that there are two types of sensors that can be widely integrated into other circuits, that is, Hall effect sensors and fluxgate sensors.

Hall effect sensors can reliably operate in various electromagnetic environments and are highly respected and loved by manufacturers. Common applications include oil gauges, which are mainly used to detect floats in oil tanks. It can also be used for brushless motors to detect rotor position and time the current. When the magnetic sensor is integrated with other circuits, the manufacturing process does not require special materials or special processing, which helps reduce design costs.

Hall effect sensor

Compared with Hall-effect sensors, fluxgate sensors are more sensitive and can detect very subtle changes in the magnetic field. Common applications for fluxgate sensors include electronic compasses for ships and aircraft and instruments used by geologists to detect underground structures.

fluxgate sensor

Ⅲ Four stages of magnetic sensor development

There are many types of magnetic sensors with different performances and applications. People have been pursuing a magnetic sensor with superior performance. Among them, the type of sensors with high performance, miniaturization, low power consumption, and low-cost development potential are more concerned, such as superconducting quantum interference (SQUID) magnetometer, optical pump atomic magnetometer, TMR magnetic sensor, fluxgate sensor, etc.

1 High-temperature superconducting materials

SQUID can be used as a magnetic sensor based on the principle of magnetic flux passing through the superconducting ring to induce the superconducting current. Its resolution can reach the fT level, and the magnetic field resolution of general commercial SQUID can also reach the order of 10fT. It can be said that SQUID is the current magnetic sensor that has a strong detection capability. However, because the superconducting ring can only work at extremely low temperatures, SQUID must be equipped with a refrigeration device, which leads to a substantial increase in its volume, weight, and cost of use, which restricts the application range of SQUID. The emergence of high-temperature superconducting materials has brought new vitality to SQUID.

In 1987, MKWu and others reported a YBaCuO high-temperature superconducting material system with a critical temperature of 93K, which means that the material system can enter the superconducting state under a liquid nitrogen environment (77K). Compared with the superconducting materials working in the liquid helium (4K) environment, it has obvious advantages, because the preparation difficulty and cost of liquid nitrogen are much smaller than that of liquid helium. Therefore, SQUID based on high-temperature superconducting materials is soon used in biomedicine, geophysical prospecting, material research, and other fields.

At present, it has become a hot spot to manufacture high-quality high-temperature superconducting thin films through micro-nano processing technology to manufacture miniaturized SQUID probes. It is reported that a SQUID magnetic sensor combining a MEMS planar coil and a YBaCuO thin film has come out. The resolution reaches about 50fT/sqr (Hz) (@1Hz). Although the electrothermal noise of the detection coil and the temperature fluctuation of the superconducting ring in the high-temperature SQUID increase, the key indicators are lower than those of the SQUID working in a liquid helium environment, it is undeniable that SQUID is striving for miniaturization and high-temperature development. Thin-film materials and noise reduction technology have become the main breakthrough direction of SQUID.

2 MEMS technology

The optical pump atomic magnetometer is a total magnetic sensor. Its basic principle is to use the optical pump to excite the alkali metal gaseous atoms in the closed cavity to be in the same spin precession state, and the atomic spin precession frequency generated linear change by the external magnetic field. Through the optical detector to detect the spectral frequency shift to obtain the measured magnetic field. The resolution of the optical pump atomic magnetometer can also reach 1fT/sqr (Hz) (@1Hz), which is equivalent to SQUID. The performance of the optical pump atomic magnetometer mainly depends on the consistency of the spin states of the alkali metal atoms in the sealed cavity.

At present, most optical pump atomic magnetometers are expensive and relatively large in volume and weight. It is limited in application, but the miniaturization of optical pump atomic magnetometers has become an attractive direction. The National Institute of Standards and Technology (NIST) has successfully developed a millimeter-level optical pump atomic magnetometer using MEMS technology, effectively reducing the size and cost. However, due to the small volume of the sealed cavity and the small amount of alkali metal that can be poured, the magnetic field resolution is reduced, about 6pT/sqr (Hz) (in the frequency band of 10Hz-1kHz). Fortunately, a new mechanism called "Spin Exchange-Release Freedom" (SERF) is being used in MEMS optical pump atomic magnetometers to improve spin consistency and greatly increase the magnetic field resolution to tens of fT /sqr (Hz) (@1Hz). Although the SERF mechanism can only be formed under extremely weak magnetic field conditions, its emergence is of great significance in itself, opening up a possible way to realize a high-performance MEMS optical pump atomic magnetometer.

3 The miniaturization of fluxgate sensors

Fluxgate sensor was born in the 1930s. It is a component-type magnetic sensor developed to overcome the inability of the electromagnetic induction coil to measure the DC magnetic field. It uses the principle that the external magnetic field affects the magnetic core magnetization to achieve magnetic field measurement. It is mainly composed of a magnetic core, excitation coil, and detection coil, and the key to determining the detection ability is the magnetic core. When the high-frequency magnetic field generated by the excitation coil repeatedly magnetizes the magnetic core, the detection coil can induce a distorted voltage signal. When the external magnetic field changes, the signal will also change. The measured magnetic field can be detected according to the change of the even-order component of the distortion signal. By improving the soft magnetic performance of the magnetic core, the resolution of the fluxgate sensor can reach the pT level.

With the development of micro-nano technology, the concept of micro-flux gate appears. People expect to reduce the volume and power consumption of high-performance fluxgate sensors through micro-nano processing technology. However, a large number of studies have shown that as the size of the miniature fluxgate sensor decreases, its sensitivity and resolution will decrease rapidly. The main reason is that the coil and magnetic core of the miniature fluxgate sensor are generally in the form of thin films, and the thermal noise level is significantly higher than that of the usual coil and magnetic core. In recent years, A. Baschirotto et al. have carried out systematic research on miniature fluxgate sensors, from PCB fluxgates to IC fluxgates based on CMOS processes with high levels, with a resolution of about a few nT/sqr (Hz ) (@1Hz). There is still a big gap compared to traditional fluxgate sensors. Therefore, it is a general trend to reduce the noise level of miniature fluxgate sensors through improved design and manufacturing processes, but the challenges are still great.

4 GMR (MTJ) magnetic sensor research

GMR magnetic sensor has the characteristics of small size, low power consumption, and high sensitivity. It is expected to be used in the fields of UAV anti-submarine, micro-nano satellite, and intelligent fuze.

Since the discovery of the GMR effect in 1988, the structure and theory of GMR have been rapidly developed. From the initial sandwich structure to the multilayer film structure to the spin valve structure and the new magnetic tunnel junction (MTJ) structure, etc., the rate of the magnetic resistance changes and magnetic field sensitivity continue to increase. In 2006, S. Yuasa et al. published the results of a large magnetoresistance change rate of 410% in a room-temperature magnesium oxide-based MTJ. In 2007, RCChaves et al. obtained a very high magnetic field sensitivity (87%/Oe) in a 26-micron-diameter columnar magnesium oxide-based MTJ (with magnetic bias and magnetic field line concentrator), but its low-frequency magnetic field resolution is still only 330pT /sqr (Hz) (@2.5Hz). After studying a variety of GMR magnetoresistive materials, people found that with the increase of the sensitivity of the magnetic field, the magnetic noise also increased, and showed a 1/f characteristic change with frequency. In 2009, the National Institute of Standards and Technology, the University of Delaware, and the US Army Laboratories collaborated to obtain the MTJ noise theoretical model. The model shows that increasing the sensitivity of the magnetic field cannot suppress magnetic thermal noise and magnetic 1/f noise. The smaller the volume, the worse the noise characteristic.

However, the existing magnetic force line modulation method has the problems of low modulation efficiency and complicated structure. For this reason, scholars have proposed a vertical magnetic force line modulation method and developed a prototype of a three-axis integrated magnetic sensor based on GMR. At the same time, new low-dimensional nanomaterials such as graphene are being used to develop a new generation of highly sensitive MTJ, which is expected to increase the resolution of magnetic sensors to about 1pT/sqr (Hz) (@1Hz). At present, the tunnel magnetoresistance effect of graphene-based magnetic tunnel junctions has been experimentally verified, which opens a new door for the development of high-performance magnetic sensors.

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