The level of automation is constantly improving. Many metallurgical industrial control companies are using a large number of instrumentation tests to check the level of automation and provide further guidance. In this process, problems often arise. The main problem is the interference problem. Then we should solve this problem. Here are some common methods.
It is customary for us to refer to various external or internal unwanted signals that affect the measurement results of an electrical test system or instrument as interference. The false signal caused by the interference not only causes damage to the equipment itself, but also causes the control system to malfunction due to logical confusion, resulting in production accidents or even production suspension.
1 Type of interferenceAccording to different noise interference modes, it is divided into common mode interference and differential mode interference. Common mode interference and differential mode interference are a common classification method.
1.1 Common mode interference
Common mode interference is the potential difference of the signal to the ground, which is mainly formed by the grid-in, ground potential difference and the common-state (same-direction) voltage induced by the space electromagnetic radiation on the signal line. The common mode voltage is sometimes large, especially in the power supply room of the distributor with poor isolation performance. The common mode voltage of the transmitter output signal is generally higher, and some can be as high as 130V or more. The common mode voltage can be converted into a differential mode voltage through an asymmetric circuit, which directly affects the measurement and control signals, causing component damage (this is the main reason for the high damage rate of some system I/O devices). This common mode interference can be DC, It can also be an exchange.
1.2 differential mode interference
Differential mode interference refers to the interference voltage acting between the two poles of the signal, also called series mode interference. It is mainly caused by the coupling of the space electromagnetic field between the signals and the conversion of the common mode interference by the unbalanced circuit. This interference is directly superimposed on the signal. On, directly affects measurement and control accuracy. Differential mode interference is transmitted between two signal lines and belongs to symmetric interference 1.
2 Generation of interferenceIn instrumentation systems, the most common signal system is 4 to 20 mA DC or 1 to 5 V DC. The measured quantity is first converted into a milliampere or millivolt signal. Since the secondary meter is far away from the field, in addition to the useful signal transmitted to the control system, there are often voltages or currents that are independent of the measured signal. Exist, this is interference.
There are three links in the formation of interference: (1) interference source; (2) receiving circuit sensitive to interference; (3) transmission path of interference. Cutting off any link will eliminate the interference. The main ways to introduce interference are as follows.
2.1 Electromagnetic coupling
When the sensor signal line is directly in a strong magnetic field or near a large current grid, since the signal line forms a closed loop through the signal source, an induced current will be generated in the loop (Fig. 1), and the induced electromotive force EM and magnetic field The conversion frequency is proportional to the signal line loop area and decreases as the distance of the grid increases. The relationship is:
2.2 Electrostatic coupling
If the potential of one of the two objects changes, the potential of the other object also changes due to the capacitance between the objects. If the signal line is close to the grid, electrostatic coupling is formed through the capacitor, so that the signal line is formed. Interference voltage is generated. Induced electromotive force when the signal line is parallel to the grid 
The value is
Equations (1) and (2) are the interference voltages generated by the interference source to a single signal line. When the signal line is composed of two parallel wires, the interference voltage is the difference between the induced electromotive forces on the two wires. Graduation thesis, grounding. Therefore, the use of stranded or shielded wires can reduce electromagnetic coupling and electrostatic coupling, thereby reducing the effects of interference 2 .
2.3 Resistance coupling
In the measurement system, when there is more than one grounding point, the earth circuit caused by the potential difference of the earth circuit, the leakage caused by the insulation resistance between the signal source and the power supply source, and the leakage caused by the recording system to earth leakage Ground interference, etc. are all resistance coupling, and this interference voltage belongs to the common mode interference voltage.
2. 4 other
Some of the pulse voltages generated by relays, contactors, etc., which are used extensively by some enterprises, can interfere with digital circuits in addition to analog circuits. These switching inductive components sometimes generate transient voltages up to 4kV and frequencies up to 200 MHz. . After understanding the different sources of interference, it is possible to eliminate or avoid corresponding measures for different situations.
3 interference suppressionThere are three methods to suppress interference: 1 to weaken or eliminate the source of interference. 2 Attenuate the coupling due to the interference source to the signal loop, that is, the propagation path of the cutoff. 3 Improve the anti-interference ability of the device and system. Comparing the three measures, eliminating interference sources is the most effective and thorough method. However, there are many sources of interference in the industrial field that cannot be eliminated, so it is necessary to choose the most suitable method based on the actual situation.
3. 1 shielding technology
Shielding technology is to use metal materials to have better absorption and reflection ability for electromagnetic waves to prevent interference. Shielding is generally divided into three types: electrostatic shielding, magnetic shielding, and electromagnetic shielding. According to the mechanism of mutual interference between the electrical coupling, magnetic coupling and electromagnetic coupling between the wires, the wiring of the instrument should be: the cable of the strong electricity must be walked alone, and must not be tied with the signal wire; The cable is orthogonal to the weak signal line; parallel lines that cannot be avoided should maintain the distance between the strong cable and the weak signal line. The use of the connecting line should be: for the interference caused by the electric field coupling, the meshed aluminum foil shielded cable is used, and the exposed portions of the lead wires at both ends are as short as possible, and the coaxial cable is grounded at one end; in addition, a shield is added around the interference source and Grounding the shield at one point can shield the interference source formed by the electric field so that it does not interfere with adjacent wires or circuits and can suppress the interference that the magnetic field may cause on the weak signal loop.
3. 2 Grounding anti-jamming technology
Various instruments and equipment need to be grounded. The so-called grounding is to connect a point with an equipotential point or an equipotential surface with a low-resistance wire to form a reference potential. The purpose of grounding is to eliminate the common-impedance coupling interference generated by the common ground impedance and to avoid the influence of the magnetic field and potential difference, and to prevent the magnetic current coupling between the ground current loop and other circuits. But it is worth noting that the ground line is also an important channel for introducing interference. In a large measurement and control system, it often includes various test instruments, including high-frequency signals and low-frequency signals; both high-power circuits and weak-circuit circuits; devices with switching actions and extremely sensitive Weak electrical signal device. Therefore, different types of signal circuits should have different ground lines, such as signal ground, signal source ground, and load ground. For the same type of signal circuit, there is generally a common grounding system, but sometimes different grounding forms are required depending on the signal circuit, such as series single-point grounding, parallel single-point grounding, and multi-point grounding. Graduation thesis, grounding.
1 series single point grounding
The common ground wire is not an ideal pure wire and has a certain resistance, which is easily overlooked. Even if the resistance of the wire is small, interference can form between the circuits. The ground voltage of each circuit is affected by the current of other circuits. With this grounding method, the weak signal circuit is placed in the nearest ground. However, from the perspective of suppressing the resistance coupling, this grounding method is the most undesirable.
2 parallel single point grounding
This way, resistance coupling interference can be avoided because the ground potential of each circuit is only related to its own current and is not affected by other circuit currents. This grounding method is best suited for low frequencies.
3 multi-point grounding
For high frequency circuits, multi-point grounding should be used. The grounding system is generally a flat metal conductor or a casing itself connected to the casing, and is also commonly used as a grounding wire or a metal meshing plate as a grounding wire. In order to reduce the ground potential of the circuit, the ground of each circuit should be shortened as much as possible to reduce the impedance of the ground line. The advantage of the multi-point grounding system is that the circuit configuration is simpler than a single point grounding, and the high-frequency standing wave phenomenon that may occur on the grounding line is significantly reduced due to the short grounding line. However, it should be noted that due to the multi-point grounding, many ground loops will be added inside the device, and they will cause interference to the circuit of the low-level signal, which will have adverse effects. Graduation thesis, grounding. Taken together, the grounding method can be selected as follows: one grounding method can be used when the frequency is lower than 1MHz, and multi-point grounding should be used when the frequency is higher than 10MHz. Between 1~10MHz, if one point is grounded, the ground length should not exceed 120 of the wavelength. Otherwise, multi-point grounding should be used.
3. 3 filtering method
Filtering is one of the important means to suppress differential mode interference. The so-called filtering method is to use a capacitor and an inductor coil or a capacitor and a resistor to be connected to the input end of the measuring line to suppress the interference signal from entering the secondary instrument or the control system such as PLC or DCS, so that the interference signal is attenuated. This method is particularly effective for DC signals that change slowly. Most commonly used analog filters, it can attenuate impulse noise, spike noise, harmonics and other clutter signals. For common mode noise, the filter forms a path between the power line (signal line) and the ground line to introduce noise current into the ground. For series mode noise, the filter forms a path between the lines, so that the noise current forms a short circuit between the lines, thereby eliminating interference. .
In practical engineering design, this method of hardware filtering is generally seldom used. When designing the instrument circuit, the AC line and the DC line, the input line and the output line are preferably separated. The switching quantity and the analog I/O line are preferably laid separately, and the line for transmitting the analog signal should be shielded. For instruments that are vulnerable to lightning strikes, such as instruments at high places and instruments in open areas, surge protectors should be installed in the interior. More common in engineering is software filtering, which is to write a corresponding program in PLC or DCS to preprocess the input analog signal. Commonly used software filtering methods are:
(1) Mean filtering, that is, averaging the values ​​of N samples as the output of the filter, or increasing the proportion of freshly sampled values ​​as needed to form a weighted average filter.
(2) Median filtering, that is, sorting N consecutive sampled values ​​and taking the median value as the output of the filter. This method has a good effect on the impulse interference filtering of the ramping process. Graduation thesis, grounding.
(3) Limiting filtering, this method is to determine the maximum possible difference Δ of the adjacent two samples according to the sampling period and the normal rate of change of the real signal, and the difference between the current sampling and the last sampling is less than or equal to Δ. It is considered to be a valid signal, and a signal larger than Δ is treated as noise.
3.4 using twisted pair
The use of twisted pairs allows the interference voltage to be greatly reduced because the twisted pairs cancel the currents in the same direction. Figure 2 shows the parallel-line twisted pair magnetic induction.
Figure 2 Electromagnetic induction of different wires a) Parallel line b) Twisted pair
It can be seen from the figure that the direction of the magnetic induction current in the parallel line circuit is the same; and the direction of the magnetic induction current of the twisted pair circuit is opposite and cancels each other. In order to reduce the interference, it can be seen from the equations (1) and (2) that the laying length of the signal line should be minimized.
It is best not to use a coaxial cable for the output line of the sensor and its connection to the measuring system, but to use a two-core cable.
3.5 isolation
Isolation technology is one of the effective means to suppress interference. In order to prevent various interference signals generated on high-voltage, high-current, high-power and other high-voltage or long-distance transmission lines from entering the instrumentation and affecting normal operation, the path of information transmission can be electrically isolated, that is, two parts before and after isolation. There is no electrical connection between the lines, and the two systems are electrically independent of each other, each having an independent power supply and a reference potential, which are independent of each other and transmit information in a non-electrical manner. For analog input signals, modulation-demodulation isolation amplifiers, operational amplifiers, etc. are used in many cases. DC output isolation and conversion isolation methods can be used for analog output signal isolation. For digital signals, optical isolation can be used. Method.
Compared with analog signals, the anti-interference ability of digital signals is strong, but there are also methods of analog-to-digital conversion isolation, that is, in-situ analog-to-digital conversion, using analog-to-digital converters to convert susceptible analog signals into digital signals. For transmission, opto-isolation is used at the receiving end to enhance its anti-interference ability during signal transmission.
The function of the above method is superimposed. Usually, one or several methods are taken at the same time to improve the anti-interference ability of signal measurement.
4 ConclusionThe instrument is an important part of the metallurgical automation system. A large number of low-voltage analog quantities are used as transmission signals. In the harsh working environment, there are various sources of interference, which can easily interfere with the transmission of measurement signals. In the design and use of instrumentation, it is necessary to conduct a comprehensive analysis of the working environment, determine the nature of the interference, and take appropriate anti-interference measures.
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