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Everyone knows that sensor detection is the most sensitive to interference. Once disturbed, the sensor may be damaged, the measurement system will be affected, inaccurate data will affect the use of the following, the judgment will be biased, the impact can be varied, so Anti-interference is very important. Today we talk about sensor anti-jamming technology and see if we can use those technologies to resist interference.
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1, shielding technology
Made of metal containers. The circuit to be protected is packaged therein, which can effectively prevent electric or magnetic field interference. This method is called shielding. Shield can be divided into electrostatic shielding, electromagnetic shielding and low frequency magnetic shielding.
2, electrostatic shielding According to the principle of electromagnetism, there is no electric field line inside the closed hollow conductor placed in the electrostatic field, and its internal points are equipotential. Using this principle, a metal container with a good conductivity such as copper or aluminum is used as a material to make a sealed metal container, and is connected to a ground wire to protect the circuit value r which needs to be protected so that the external interference electric field does not affect its internal circuit. The electric field generated by the internal circuit will not affect the external circuit. This method is called electrostatic shielding. For example, in a sensor-based measurement circuit, inserting a conductor with a gap between the primary and secondary sides of a power transformer and grounding it prevents electrostatic coupling between the two windings. This method is electrostatic shielding. .
3, electromagnetic shielding
For high-frequency interference magnetic fields, using the eddy current principle, the high-frequency interference electromagnetic field generates eddy currents in the shield metal, consumes the energy of the interfering magnetic field, and the eddy current magnetic field cancels the high-frequency interference magnetic field, thereby protecting the circuit to be protected from the high frequency electromagnetic field. . This shielding method is called electromagnetic shielding. If the electromagnetic shielding layer is grounded, it also has the function of electrostatic shielding. The output cable of the sensor is generally a copper mesh shield, which has both electrostatic shielding and electromagnetic shielding. Shielding materials must be made of low-resistance materials with good conductivity, such as copper, aluminum, or silver-plated copper.
4, low frequency magnetic shielding If the disturbance is a low-frequency magnetic field, the eddy current phenomenon is not obvious at this time. The anti-jamming effect is not very good only with the above method. Therefore, a high-magnetic permeability material must be used as the shielding layer so as to limit the magnetic interference line of low-frequency interference to the magnetic field. Very small magnetic shield inside. Protect the circuit to be protected from interference from low frequency magnetic field coupling. This shielding method is generally called low-frequency magnetic shielding. The metal shell of the sensor detection instrument functions as a low-frequency magnetic shield. If it is further grounded, it also acts as an electrostatic shield and an electromagnetic shield. Based on the above three commonly used shielding techniques, it is possible to use a composite shielded cable on her side where the interference is severe, that is, the outer layer is a low-frequency magnetic shielding layer. The inner layer is an electromagnetic shielding layer. Double shielding effect. For example, the parasitic capacitance of a capacitive sensor during actual measurement is a key problem that must be solved, otherwise its transmission efficiency and sensitivity must be low. The sensor must be electrostatically shielded, and its electrode lead-out line should be double-shielded, commonly known as drive cable technology. This method can effectively overcome the parasitic capacitance of the sensor during use.
5, grounding technology
Grounding technology is one of the effective technologies for suppressing interference and is an important guarantee for shielding technology. Proper grounding can effectively suppress external interference, at the same time, it can improve the reliability of the test system and reduce the interference factors generated by the system itself. The purpose of grounding is twofold: safety and interference suppression. Therefore, the grounding is divided into protective grounding, shielded grounding, and signal grounding. Protective grounding For safety purposes, the housing, chassis, etc. of the sensor measuring device must be grounded. Ground resistance should be below 10Ω. Shield grounding is a low-resistance path to the interference voltage to ground to prevent interference with the measurement device. Grounding resistance should be less than 0.02Ω.
The signal ground is a common line of zero signal potential at the input and output of the electronic device, which itself may be isolated from the ground. The signal ground is divided into analog signal ground and digital signal ground. The analog signal is generally weaker, so the ground wire is more demanding: the digital signal is generally stronger, so the requirement for the ground wire can be lower.
Different sensor detection conditions also have different requirements for the grounding method, and an appropriate grounding method must be selected. The commonly used grounding method is that there is a little grounding and multi-point pressing. The two different ground handling measures are given below.
6, a little earth
A point grounding is generally recommended in low-frequency circuits. It has radial grounding and bus-type grounding lines. Radiated grounding is the direct connection between each functional circuit in the circuit and the zero potential reference point: Busbar grounding is the use of a good conductor with a certain cross-sectional area as the grounding bus, directly connected to the zero potential point, the ground of the functional blocks in the circuit. Can be connected to this bus nearby. At this time, if multi-point grounding is adopted, multiple ground loops will be formed in the circuit. When low-frequency signals or pulsed magnetic fields pass through these loops, electromagnetic induction noise will be caused. Because of the different characteristics of each ground loop, it is closed in different loops. The point produces a potential difference, forming interference. To avoid this situation, it is best to use a little grounding method.
Sensors and measuring devices constitute a complete detection system, but the two may be far apart. Because the earth current in the industrial field is very complicated, the potential between the two parts of the housing is generally not the same. If the zero potential of the sensor and the measurement device is grounded at two points, that is, two points are grounded, there will be Larger currents flow through signal transmission lines with very low internal resistance and cause voltage drops, causing series-mode interference. Therefore, a one-point grounding method should also be used in this case.
7, multi-point grounding
High-frequency circuits are generally recommended to use multi-point grounding. At high frequencies, even a short segment of the ground will have a large impedance drop, plus the effect of the distributed capacitance, it is impossible to achieve a point of grounding, so you can use a planar grounding method, that is, a multi-point grounding method, using a good The conductive plane body (such as one layer of a multilayer circuit board) is connected to the zero potential reference point, and the ground of each high frequency circuit is connected to the conductive plane body. Since the high-frequency impedance of the conductive flat body is small, the consistency of each potential is basically ensured, and bypass capacitors are added to reduce the voltage drop. Therefore, this situation uses a multi-point grounding method.
8, filtering technology The filter is one of the effective means to suppress the AC series interference. The common filter circuit in the sensor detection circuit is Rc filter, AC power filter and true current power filter.
The above technology is an effective way to prevent the sensor from detecting interference and hopes to help everyone.