Electromagnetic interference is seriously affected, how can the sensor resist interference?

“Since there are often a large number of electrical and magnetic interference sources in the production site, they may destroy the normal operation of sensors, computers and even the entire detection system. Therefore, anti-interference technology is an important part of the sensor detection system. For those who are engaged in automatic detection work , it is very necessary to understand anti-jamming technology.

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Since there are often a large number of electrical and magnetic interference sources in the production site, they may destroy the normal operation of sensors, computers and even the entire detection system. Therefore, anti-interference technology is an important part of the sensor detection system. For those who are engaged in automatic detection work , it is very necessary to understand anti-jamming technology.

The unwanted signal that appears in the Circuit of the Electronic measuring device is called noise. When the noise affects the normal operation of the circuit, the noise is called disturbance. The formation of interference in the signal transmission process must have three factors, namely the interference source, the interference path and the receiving circuit with high sensitivity to noise. Therefore, the method of eliminating or reducing noise interference can take measures for any of these three items. The more commonly used method in the sensor detection circuit is to take corresponding measures to the interference path and the receiving circuit to eliminate or weaken the noise interference. The following introduces several commonly used and effective anti-jamming techniques.

1. Shielding technology

Use metal materials to make containers. Enclosing the circuit to be protected can effectively prevent the interference of electric or magnetic fields. This method is called shielding. Shielding 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 airtight hollow conductor placed in the electrostatic field, and each point inside is equipotential. Using this principle, a metal container with good conductivity such as copper or aluminum is used as the material to make a closed metal container, and connect it to the ground wire, and set the value of the circuit to be protected, so that the external interference electric field does not affect its internal circuit, and vice versa. , the electric field generated by the internal circuit will not affect the external circuit. This method is called electrostatic shielding. For example, in the sensor measurement circuit, inserting a conductor with a gap between the primary side and the secondary side of the power transformer and grounding it can prevent the electrostatic coupling between the two windings. This method belongs to electrostatic shielding. .

3. Electromagnetic shielding

For the high frequency interference magnetic field, the principle of eddy current is used to make the high frequency interference electromagnetic field generate eddy current in the shielding metal, consume the energy of the interference magnetic field, and the eddy current magnetic field cancels the high frequency interference magnetic field, so that the protected circuit is not affected by 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 shielded by copper mesh, which has both electrostatic shielding and electromagnetic shielding functions. The shielding material must choose a low-resistance material with good electrical conductivity, such as copper, aluminum or silver-plated copper.

4. Low frequency magnetic shielding

If the interference is a low-frequency magnetic field, the eddy current phenomenon is not obvious at this time, and the anti-interference effect of the above method is not very good. Therefore, a high magnetic permeability material must be used as a shielding layer to limit the low-frequency interference magnetic field line to the magnetic field. Inside the magnetic shield with little resistance. The protected circuit is protected from the influence of low frequency magnetic field coupling interference. This shielding method is generally referred to as low frequency magnetic shielding. The iron shell of the sensor detection instrument plays the role of low-frequency magnetic shielding. If it is further grounded, it will play the role of electrostatic shielding and electromagnetic shielding at the same time. Based on the above three commonly used shielding technologies, composite shielded cables can be used in the other side where the interference is more serious, that is, the outer layer is a low-frequency magnetic shielding layer. The inner layer is an electromagnetic shielding layer. achieve the effect of double shielding. For example, the parasitic capacitance of a capacitive sensor is a key problem that must be solved in actual measurement, otherwise its transmission efficiency and sensitivity will become lower. The sensor must be electrostatically shielded, and its electrode lead wire adopts double-layer shielding technology, which is generally called driving 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 an important guarantee for shielding technology. Correct grounding can effectively suppress external interference, 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, grounding is divided into protective grounding, shielding grounding and signal grounding. Protective grounding is for the purpose of safety, and the casing and chassis of the sensor measurement device must be grounded. The ground resistance is required to be 10? the following. The shield ground is a low-resistance path formed by the interference voltage to ground to prevent interference with the measuring device. Ground resistance should be less than 0.02? . The signal ground is the common line of zero signal potential of the input and output of the Electronic device, and it may be isolated from the ground itself. The signal ground wire is divided into analog signal ground wire and digital signal ground wire. The analog signal is generally weak, so the ground wire is required to be higher; the digital signal is generally stronger, so the ground wire requirement can be lower. Different sensor detection conditions also have different requirements for the grounding method. An appropriate grounding method must be selected. Common grounding methods include one-point grounding and multi-point grounding. The two different grounding treatments are given below.

6. One point grounding

It is generally recommended to use one-point grounding in low-frequency Circuits, which has radial grounding lines and bus-bar grounding lines. Radial grounding means that each functional circuit in the circuit is directly connected to the zero-potential reference point with wires: bus-bar grounding is to use a high-quality conductor with a certain cross-sectional area as the grounding bus, directly connected to the zero-potential point, and the ground of each functional block in the circuit. It can be close to the bus. At this time, if multi-point grounding is used, multiple grounding 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. Due to the different characteristics of each grounding loop, different loops are closed. A potential difference is generated at the point, resulting in interference. To avoid this situation, it is best to use a one-point grounding method. The sensor and the measuring device constitute a complete detection system, but the two may be far apart. Since the ground current in the industrial field is very complex, the potentials between the grounding points of the two parts of the casing are generally different. Larger current flows through the signal transmission line with very low internal resistance, resulting in a voltage drop, resulting in series-mode interference. Therefore, a little grounding method should also be used in this case.

7. Multi-point grounding

It is generally recommended to use multi-point grounding for high-frequency circuits. At high frequency, even a small section of ground wire will have a large impedance voltage drop. With the effect of distributed capacitance, it is impossible to achieve one-point grounding. Therefore, a plane grounding method, that is, a multi-point grounding method, can be used. The conductive plane body (such as a layer in a multi-layer 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 plane body is very small, the same potential is basically guaranteed at each location, and a bypass capacitor is added to reduce the voltage drop. Therefore, in this case, a multi-point grounding method should be adopted.

8. Filtering technology

Filter is one of the effective means to suppress AC series mode interference. Common filter circuits in sensor detection circuits include Rc filter, AC power filter and true current power filter. The application of these filter circuits is described below.

1) RC filter: When the signal source is a sensor with slow signal changes such as thermocouples and strain gauges, the use of a small-volume, low-cost passive Rc filter will have a better suppression effect on the series-mode interference. But it should be mentioned that the Rc filter reduces the series mode interference at the expense of the system response speed.

2) AC power filter: The power network absorbs various high and low frequency noises, and Lc filters are commonly used to suppress the noise mixed into the power supply.

3) DC power filter: DC power is often shared by several circuits. In order to avoid mutual interference between several circuits caused by the internal resistance of the power supply, Rc or Lc decoupling filters should be added to the DC power supply of each circuit. Used to filter out low frequency noise. Photocoupler technology: Photocoupler is an electrical-optical-electrical coupling device, which is composed of light-emitting diode and phototransistor package, and its input and output are electrically isolated, so this device is not only used for In addition to photoelectric control, it is now more and more used to improve the anti-common mode interference capability of the system. When a driving current flows through the light-emitting diode in the photocoupler, the phototransistor is saturated with light. Its emitter output high level, so as to achieve the purpose of signal transmission. In this way, even if there is interference in the input loop. As long as it is within the threshold, it will have no effect on the output. Noise suppression in the pulse circuit, if there is interference noise in the pulse circuit. The input pulse can be differentiated and then integrated, and then a threshold voltage of a certain amplitude can be set, so that the signal less than the threshold voltage is filtered out. For analog signals, A/D conversion can be used first. Then use this method to filter out noise.

When we use these anti-jamming technologies, we need to choose according to the actual situation. Do not use it blindly, otherwise it will not only fail to achieve the purpose of anti-interference, but may also have other adverse effects.