Figure 2- 1 magnetoresistance effect
Magnetoresistive sensors have been fabricated on silicon wafers and products have been formed. Its sensitivity and linearity can meet the requirements of magnetic compass, and its performance in all aspects is obviously attributed to Hall element. Hysteresis error and zero temperature drift can also be eliminated by alternating forward magnetization and reverse magnetization of the sensor. Because of these excellent performances, magnetoresistive sensors can compete with fluxgates in some applications.
The main problem of magnetoresistive sensor is its flip effect, which is inherent in its principle. As mentioned above, the magnetic material is magnetized before use, and then if it encounters a strong magnetic field in the opposite direction (greater than 20 gauss), the magnetization of the material will be affected, thus affecting the performance of the sensor. In extreme cases, the magnetization direction will be reversed 180. Although this danger can be eliminated by periodic magnetization, there are still problems. The magnetic field of magnetized materials must be very strong. If external coils are used to generate periodic magnetized magnetic fields, the significance of miniaturization will be lost. A patent of Honeywell Company solved this problem. They made a current band on the silicon wafer to produce a magnetized magnetic field. The resistance of the current band is only about 5 ohms. Although the magnetization current is only 1-2 milliseconds, the current intensity is as high as 1 to 1.5 amps. However, this scheme requires higher driving circuit. If it is integrated into a micro-system, such a strong pulse current will threaten the reliability of other circuits such as microprocessors in the system. The working principle of Hall effect magnetic sensor is shown in Figure 2-2. If a current I is applied in the long direction of a rectangular metal sheet, and an enhanced magnetic field B is applied in the direction perpendicular to the plane of the sheet due to the Lorentz force on the carriers, a voltage difference U will be generated in its transverse direction, the magnitude of which is directly proportional to the current I, the magnetic field B and the Hall coefficient r of the material, and inversely proportional to the thickness d of the metal sheet. /kloc-hall effect discovered more than 0/00 years ago is difficult to apply, because the hall coefficient of general materials is very small, and it was not really used for magnetic field measurement until semiconductors appeared. This is because the number of carriers in the semiconductor is very small. If the current applied to it is the same as the metal material, the carrier velocity in the semiconductor will be faster, the Lorentz force will be greater, and the coefficient of Hall effect will be greater.
Hall effect magnetic sensor has the advantages of small size, light weight, low power consumption, low price and simple interface circuit, and is especially suitable for measuring strong magnetic field. However, it has the disadvantages of low sensitivity, high noise and poor temperature performance. Although some high-sensitivity Hall elements or magnetic concentration measures can also be used to measure the geomagnetic field, they are generally used in places with low requirements.
Magnetic saturation method is based on the principle of magnetic modulation, that is, under the saturation excitation of alternating magnetic field, the weak magnetic field is measured by using the nonlinear relationship between the magnetic induction intensity of ferromagnetic material core and the magnetic field intensity in the measured magnetic field. The magnetometer that uses magnetic saturation method to measure magnetic field is called magnetic saturation magnetometer, also known as fluxgate magnetometer or ferromagnetic probe magnetometer. Magnetic saturation method can be roughly divided into two categories: harmonic selection method and harmonic non-selection method. The harmonic selection method only considers the even harmonics (mainly the second harmonics) of the electromotive force induced by the probe, and filters out other harmonics; Harmonic non-selection method is to directly measure the full spectrum of induced electromotive force of probe without filtering. The differential magnetic saturation probe can be used to form a magnetic saturation gradiometer, which can measure the non-uniform magnetic field. At the same time, the gradiometer can overcome the influence of geomagnetic field and restrain external interference. This magnetometer has been continuously developed and improved since it was used in geomagnetic measurement in the 1930s, and it is still one of the basic instruments for measuring weak magnetic fields. Magnetic saturation magnetometer has the advantages of high resolution, wide range of measuring weak magnetic field, reliability, simplicity, cheapness and durability. It can directly measure the component of magnetic field and is suitable for high-speed motion systems. Therefore, it is widely used in various fields, such as geomagnetic research, geological exploration, weapon reconnaissance, nondestructive testing of materials, space magnetic field measurement and so on. In recent years, magnetic saturation magnetometer has been widely used in aerospace engineering, such as controlling the attitude of satellites and rockets, mapping the "solar wind" from the sun and the interaction mode between space magnetic field, lunar magnetic field, planetary magnetic field and interplanetary magnetic field and charged particles.
Although fluxgate still has some problems, such as relatively complex processing circuit, large volume and relatively large power consumption, these problems can be solved with the research of micro-system, micro-fluxgate and low-power fluxgate.
From the comparison of the three, the electronic compass based on magnetoresistive sensor has the advantages of small volume, fast response and obvious advantages, which is the development direction of electronic compass.