The useless signal that appears in the circuit of the electronic measuring device is called noise, and when the noise affects the normal operation of the circuit, the noise is called interference. 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 one of these three items. The more commonly used method in the sensor detection circuit is to take corresponding measures for 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

 

The container is made of metal material. Wrapping the circuit to be protected in it can effectively prevent the interference of electric field or magnetic field. This method is called shielding. Shielding can be divided into electrostatic shielding, electromagnetic shielding and low frequency magnetic shielding.

 

2. 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 with copper mesh, which has the functions of both electrostatic shielding and electromagnetic shielding. The shielding material must be a low-resistance material with good electrical conductivity, such as copper, aluminum or silver-plated copper.

 

3. 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 all points inside it are equipotential. Using this principle, use metals with good conductivity such as copper or aluminum as materials to make airtight metal containers, and connect them to the ground wire, and put the value of the circuit to be protected in it, 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, a conductor with a gap is inserted between the primary side and the secondary side of the power transformer, and it is grounded to prevent electrostatic coupling between the two windings. This method belongs to electrostatic shielding. 

 

4. Low frequency magnetic shielding

 

If the interference is a low-frequency magnetic field, the eddy current phenomenon is not very obvious at this time, and the anti-interference effect is not very good only by the above method, so it is necessary to use a high magnetic permeability material as the shielding layer to limit the low-frequency interference magnetic induction line to the magnetic field. Inside the magnetic shielding layer with little resistance. Protect the circuit from the influence of low-frequency magnetic field coupling interference. This shielding method is generally called 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 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. It achieves 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-out line adopts double-layer shielding technology, which is generally called the 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 to suppress 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. Grounding serves two purposes: safety and suppression of interference. 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 measuring device must be grounded. The grounding resistance is required to be below 10 Ω. Shielding grounding is a low-impedance path formed by interference voltages to ground to prevent interference with measuring devices. Grounding resistance should be less than 0.02 Ω.

 

The signal ground is the common line of the zero signal potential of the input and output of the electronic device, which itself may be insulated from the earth. The signal ground wire is divided into analog signal ground wire and digital signal ground wire. The analog signal is generally weak, so the requirements for the ground wire are higher; the digital signal is generally stronger, so the requirements for the ground wire can be lower.

 

Different sensor detection conditions also have different requirements for grounding methods, and an appropriate grounding method must be selected. Commonly used grounding methods include one-point grounding and multi-point grounding. The two different grounding treatment measures are given below.

 

6. 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 drop, plus the effect of distributed capacitance, it is impossible to achieve one-point grounding, so a planar grounding method, that is, a multi-point grounding method, can be used to utilize a good The conductive plane body (such as one layer of the 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 nearby. Since the high-frequency impedance of the conductive plane is very small, the potential consistency of each place is basically guaranteed, and bypass capacitors are added to reduce the voltage drop. Therefore, in this case, multi-point grounding should be used.

 

7. One point grounding

 

In low-frequency circuits, it is generally recommended to use one-point grounding, which has radial grounding wires and busbar grounding lines. Radial grounding means that each functional circuit in the circuit is directly connected to the zero-potential reference point with wires: the busbar-type grounding is to use a high-quality conductor with a certain cross-sectional area as the grounding busbar, which is directly connected to the zero-potential point, and the grounding of each functional block in the circuit It can be connected to the busbar nearby. 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, it will be closed in different loops. A potential difference is generated at the point, forming interference. To avoid this situation, it is best to use a one-point 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 site is very complicated, the potentials between the grounding points of the two parts of the shell are generally different. If the zero potential of the sensor and the measuring device are grounded at two points respectively, that is, two points are grounded, there will be A large current flows through a signal transmission line with a very low internal resistance to generate a voltage drop, resulting in series mode interference. Therefore, a little grounding method should also be adopted in this case.

 

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 filters, AC power filters and true current power filters.