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With the advancement of modern technology, the level of production automation has also been continuously improved. In industrial production, various sensors and automatic detection devices are widely used to monitor various aspects of production, and some also require computers to control the entire process of production. In such systems, hundreds of different sensors are generally required to be different. The non-electric parameters are converted into electricity for computer processing. However, since there are often a large number of electrical and magnetic sources of interference at the production site, they will not be able to disrupt the normal operation of sensors, computers, and even the entire detection system. Therefore, anti-jamming technology is an important part of the sensor detection system for those who are engaged in automatic detection work. It is necessary to understand anti-jamming technology.
The unwanted signal appearing in the circuit of the electronic measurement device is called noise. When the noise affects the normal operation of the circuit, the noise is called interference. The formation of interference in the process of signal transmission must have three factors, namely, the source of interference, the interference path, and the receiver circuit with high sensitivity to noise. Therefore, the method of eliminating or reducing noise interference can take measures against any of these three items. The commonly used method in the sensor detection circuit is to take corresponding measures for the interference path and the receiving circuit to eliminate or reduce the noise interference. Here are some common, effective anti-jamming techniques.
1, shielding technology
The container is made of metal material. The circuit to be protected is packaged therein, which can effectively prevent the electric or magnetic field from being interfered with. Such a 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 disturbance 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 side and the secondary side 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 chosen from low resistance materials with good electrical 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 highly conductive material must be used as the shielding layer so as to limit the low-frequency interference magnetic induction line 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 in her severely disturbed side, ie, the outer layer is a low-frequency magnetic shielding layer. The inner layer is an electromagnetic shielding layer. It achieves double shielding effect. For example, the parasitic capacitance of a capacitive sensor during actual measurement is a key issue 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 required at 10? the following. Shielded grounding is a low-resistance path to ground voltage that interferes with the measurement device. Ground resistance should be less than 0.02? .
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 lines 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. As the characteristics of each ground loop are different, they are 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 of the industrial site is very complex, 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 in two places, 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 potential of each location 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.
Here are the applications of these filter circuits.
RC filter
When the signal source is a sensor whose signal changes slowly, such as a thermocouple and a strain gauge, using a small-sized, low-cost passive Rc filter will have a good suppression effect on the series mode interference. However, it should be mentioned that the Rc filter reduces the serial mode interference at the expense of the system response speed.
AC power filter
The power network absorbs a variety of high and low-frequency noise, and Lc filters are commonly used to suppress the noise mixed into the power supply.
DC power filter
The DC power supply is often shared by several circuits. In order to avoid mutual interference caused by the internal resistance of the power supply, an Rc or Lc decoupling filter should be added to the DC power supply of each circuit to filter out low frequency noise.
Photoelectric coupling technology
An optocoupler is an electrical-optical-electrical coupling device consisting of a light emitting diode and a phototransistor package. The input and the output are electrically insulated. Therefore, this device is not only used for photoelectric control, but also Now more and more are used to improve the system's resistance to common-mode interference. When a driving current flows through the light emitting diode in the optical coupler, the phototransistor is saturated with light. Its emitter output is high, so as to achieve the purpose of signal transmission. This even interferes with the input circuit. As long as it is within the threshold, it will not affect the output.
Snore suppression in pulse circuits
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 is set so that a signal smaller than the threshold voltage is filtered out. For analog signals, A/D conversion can be used first, and then noise can be filtered out in this way.
When we use these anti-jamming technologies, we must choose according to the actual situation. It must not be used blindly, otherwise it will not only fail to achieve anti-interference purposes, but may also have other adverse effects.
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