The magnetic sensor is a device that converts various magnetic fields and their changes into electrical signals. We can manually set up permanent magnets and use the magnetic field generated by them as carriers of various information, so that the magnetic sensor can undertake the task of detecting, storing, converting, reproducing and monitoring various magnetic fields and information in the magnetic field. In today's information society, magnetic sensors have become an indispensable cornerstone element in information technology and information industry. At present, people have developed magnetic sensors that utilize various physical, chemical and biological effects, and have been widely used in various aspects of scientific research, production and social life.
Today, the editor will mainly introduce 4 kinds of magnetic sensors, Hall element, anisotropic magnetoresistance AMR element, giant magnetoresistance GMR element and tunnel magnetoresistance TMR element, and briefly discuss their applications.
Hall sensors, as the name suggests, are made based on the Hall effect. The Hall effect is a basic method for studying the performance of semiconductor materials. The Hall coefficient measured by the Hall effect experiment can determine the conductivity type, carrier concentration and carrier mobility of semiconductor materials and other important parameters.
Hall sensors are divided into linear Hall sensors and switching Hall sensors.
1. Linear Hall sensor is composed of Hall element, linear amplifier and emitter follower, and outputs analog quantity.
2. The switch-type Hall sensor is composed of a voltage regulator, a Hall element, a differential amplifier, a Schmitt trigger and an output stage, and outputs a digital quantity.
An element made of semiconductor material based on the Hall effect is called a Hall element. It has the advantages of being sensitive to magnetic field, simple structure, small size, wide frequency response, large output voltage change and long service life. Therefore, it is widely used in the fields of measurement, automation, computer and information technology.
Anisotropic magnetoresistance AMR
Magnetoresistive sensors, for the same reason, are made using the magnetoresistive effect. In 1857, Thomson discovered the anisotropic magnetoresistance effect.
The anisotropic magnetoresistance effect (namely AMR) means that when the external magnetic field and the built-in magnetic field of the magnet are at a zero degree angle, the resistance does not change with the change of the external magnetic field; but when the external magnetic field is at an angle to the built-in magnetic field of the magnet, A phenomenon in which the magnetization vector inside the magnet shifts and the film resistance decreases.
AMR is good at sensing the measurement of weak magnetic fields within the range of the earth's magnetic field, and is often made into speed sensors, proximity switches, and isolation switches. And the compass in the navigation system, the disk drive of the computer, various magnetic card machines, rotation position sensing, drilling orientation, etc.
Giant magnetoresistance GMR
The giant magnetoresistance effect refers to the phenomenon that the resistivity of a magnetic material changes greatly when there is an external magnetic field than when there is no external magnetic field. Giant magnetoresistance is produced by a layered magnetic film structure, which is formed by alternately stacking thin layers of ferromagnetic materials and non-ferromagnetic materials. When the magnetic moments of the ferromagnetic layers are parallel to each other, the spin-related scattering of carriers is the smallest, and the resistance of the material is also the smallest. When the magnetic moment of the ferromagnetic layer is antiparallel, the spin-related scattering is the strongest, and the resistance of the material is the largest at this time.
Based on the giant magnetoresistance effect, extremely sensitive magnetic heads have been developed, which can clearly read weak magnetic signals and convert them into clear current changes. The emergence of this kind of magnetic heads has triggered the revolution of "large capacity and miniaturization" of hard disks. At the same time, giant magnetoresistance can also be used in MRAM, angle or position sensors, current sensors, biological detection, and military fields.
Tunnel magnetoresistance TMR
TMR element is a new type of magnetoresistance effect sensor that has been industrially applied in recent years. It uses the tunnel magnetoresistance effect of magnetic multilayer film materials to sense the magnetic field, and has a greater resistance change rate than AMR elements and GMR elements.
At present, AMR sensors have already been used on a large scale, and GMR sensors are developing rapidly. TMR sensing technology was first applied to hard disk drive read heads, which greatly increased the recording density of hard disk drives. TMR integrates the high sensitivity of AMR and the wide dynamic range of GMR. Therefore, TMR magnetic sensors have unparalleled technical advantages in various magnetic sensor technologies. Therefore, TMR sensors can be widely used in industrial control, financial instruments, biomedicine, consumer electronics, and automotive fields.
1. auto industry
The application of magnetic sensors in automobiles is mainly the detection of parameters such as vehicle speed, inclination, angle, distance, proximity, and position, as well as applications in navigation and positioning. In the energy saving and consumption reduction, especially in the current situation of reducing carbon emissions or other pollutants, the motor will become the top priority. The motor will change from a frictional "non-stop" pulley system to an electronic motor. People can control on demand. In addition, electronic motors will shift to brushed DC motors that are more efficient and more reliable, and the role of magnetic sensors is to make motor control or commutation more accurate.
2. Industrial application
In other industrial applications, magnetic sensors can be used in uninterrupted power supplies and welding systems for computers, servers, and other machines. In high-current applications such as large variable frequency motors, open-loop and closed-loop Hall sensors can use Hall-effect ICs in a small package, or include an application specific integrated circuit in the package to improve integration. Hall IC is also suitable for industrial inverter control applications. In addition to current sensors, independent Hall-effect ICs or magnetoresistive sensor switches also have motor rectification to reduce ripple and improve performance, or for applications such as position measurement.
3. Medical application
The application of magnetic sensors in the medical field, although the scale is smaller than the industrial field, can assist patient care and monitoring in many occasions: ambulances, hospitals and home care. Whether it is during surgery, intensive care unit, or general home care, it can provide effective ways to control movement, airflow, and detect blood pressure to save lives or improve the quality of life. However, magnetic sensors are mainly used for motor control in medical equipment with reversing sensors, such as ventilators, infusions, insulin, and kidney dialysis machines.
4. Consumer Electronics
Magnetic sensors are also widely used in consumer electronics. For example, mobile phones, laptops, electronic compasses, etc. The application of MEMS sensors and magnetic sensors in electronic compasses promotes each other, which can make navigation products more accurate, which will become a bright spot in the navigation market. If the three sensors of gyroscope, acceleration and magnetic sensor are integrated, and the three of them assist each other in function, they form a more powerful inertial navigation product.
5. Aerospace and military industry
Sensors with high sensitivity and low magnetic field are very important materials in aviation, aerospace and satellite communication technology. With the development of absorbing technology in the military industry, military objects can be covered by a layer of absorbing material to hide from the sky, but in any case these objects will generate magnetic fields, so these hidden objects can be found through GMR magnetic field sensors. GMR magnetic field sensors can also be used on satellites to detect objects on the surface of the earth and the distribution of mineral deposits underneath. The electronic compass is mainly used for weapon/missile navigation, and is indispensable in high-performance navigation equipment for navigation and aviation.