Sources of interference for instruments and anti-interference measures (continued)

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Sources of interference for instruments and anti-interference measures: The conditions under which instruments are used in industrial production are often very complex. The parameters to be measured are often converted into weak low-level voltage signals, which are then transmitted over long distances to secondary meters or computer systems. Therefore, in addition to useful signals, voltages or currents unrelated to the signal under test often appear. Such irrelevant voltage or current signals are what we call “interference” (also known as noise). There are many sources of interference. What we usually refer to as interference is electrical interference, but in a broader sense, thermal noise, temperature effects, chemical effects, vibrations, and so on can all affect measurements and cause interference. During the measurement process, if the influence of these disturbances cannot be eliminated, the instrument will not be able to function properly. Based on the mode of interference at the instrument input, it can be divided into common-mode interference and differential-mode interference. Serial mode interference refers to the interference superimposed on the signal being measured ； Common-mode interference is the interference applied between either input terminal of the instrument and ground. 1 Generation of interference: Interference originates from interference sources, which can exist both inside and outside the instrument. Outside the instrument, some high-power electrical devices and power equipment can act as sources of interference, while inside the instrument, items such as power transformers, electromechanical components, switches, and power cables can also be sources of interference. The main ways in which interference is introduced are as follows: 1) Electromagnetic induction, that is, magnetic coupling. The connection wires between the signal source and the instruments, as well as the internal wiring of the instruments, create interference in the circuit through magnetic coupling. In the surrounding space of high-power transformers, AC motors, high-voltage power grids, and similar devices used in engineering, there are strong alternating magnetic fields. The closed circuits of instruments placed in such changing magnetic fields will generate induced electromotive forces. The induced electromotive force can be calculated using the following formula: where en is the induced electromotive force ； B——Magnetic flux density ； A —— Area of the closed loop ； θ — the angle between the magnetic field line and the perpendicular to area A. This magnetic induction electromotive force is connected in series with the useful signal, and this situation is more pronounced when the signal source is far away from the instrument. As shown in Figure 1(a) and Figure 1(b): To reduce the induced electromotive force, values such as B, A, or COSθ must be minimized as much as possible. Therefore, it is necessary to keep the wires away from these high-power electrical devices and power networks, adjust the routing direction of the wires, and reduce the area of the wire loops. Simply by twisting the two signal wires at a short pitch, the magnetomotive force can be reduced to 1/10 to 1/100 of its original value. 2) Electrostatic induction, that is, electrical coupling. In two opposite objects, if the potential of one of them changes, then due to the capacitance between the objects, the potential of the other object also changes. Interference sources create interference in the circuit through capacitive coupling. It is the result of the interaction between two electric fields. As shown in Figure 2: In this figure, the potential of wire 1 induces a voltage ec with respect to ground on wire 2. When the two signal wires are laid parallel to the power wire, the distances from the power wire to each of the signal wires being different results in unequal distributed capacitance. It can generate a potential difference across two signal wires, which can sometimes reach several dozen millivolts or even more. Twisting the signal wires can greatly reduce the potential difference generated across the two signal wires by the electric field. With the use of electrostatic shielding, the induced potential can be reduced to 1/100 to 1/1000. The interference generated by electromagnetic induction and electrostatic induction is mostly 50Hz power-frequency interference voltage. However, other devices such as high-frequency generators and motors with rectifiers can also generate high-frequency interference. Due to discharges between thunderclouds and between thunderclouds and the ground, abnormal voltages can also be induced in the wiring. 3) Additional thermoelectromotive forces and electrochemical potentials arise mainly from the thermoelectromotive forces generated by different metals, as well as the electrochemical potentials resulting from metal corrosion. When these are present in an electrical circuit, they act as disturbances, and such disturbances usually appear in the form of direct current. Thermoelectromotive force is likely to be generated at terminal blocks or reed relays, etc. 4) Vibration. When a wire moves in a magnetic field, an induced electromotive force is generated. Therefore, it is very necessary to secure the signal wires in a vibrating environment. These four types of interference are all in series with the signal, that is, they appear in the form of series interference. 5) Interference caused by different earth potentials. In the ground, there is often a potential difference between various points. Especially near high-power electrical devices, this potential difference is greater when the insulation performance of these devices is poor. In the use of instruments, there are often, intentionally or unintentionally, two or more grounding points in the input circuit. This introduces a potential difference between different ground points into the instrument; such ground potential differences can sometimes exceed 1 to 10 volts, and they appear simultaneously on the two signal wires. As shown in Figure 3, through electrostatic coupling, a common voltage with respect to ground can be induced at the two input terminals, appearing in the form of common-mode interference. Due to common-mode interference, it does not overlap with the signal, so it does not affect the instrument directly. However, it can measure the leakage current from the system to ground, and this leakage current can act directly on the instrument through the coupling of resistors, thereby causing interference. 6) In addition to some pulse voltages that can affect analog circuits, they can also cause interference in digital circuits; the sources of such pulse voltages are inductive loads such as switches, motors, and relays, as well as devices that generate discharges. After understanding the various sources of interference, we can take corresponding measures to eliminate or avoid them in different situations. Since all interference sources affect the instrument through certain coupling channels, we can suppress interference by cutting off these coupling channels. Common methods include twisting signal wires, shielding, grounding, balancing, filtering, isolation, and various other techniques; generally, multiple measures are taken simultaneously. 2 Suppression of interference: There are many common anti-interference measures. To suppress interference, it is necessary to conduct a comprehensive analysis of it; actions must be taken in three areas: eliminating or suppressing noise sources, disrupting the pathways through which interference occurs, and reducing the sensitivity of the receiving circuit to noise interference. Eliminating noise sources is a proactive measure. Issues such as poor contact of connectors or poor soldering can be eliminated; such interference sources can be removed. In principle, noise sources should be eliminated. However, in reality, many noise sources are difficult or impossible to eliminate. For example, sometimes with the instruments in the pump room, the electromagnetic interference generated by the motor during pump operation cannot be eliminated. At this point, protective measures must be taken to suppress interference. 1) Suppression of crosstalk interference: Crosstalk interference occupies the same position as the signal being measured; therefore, once it occurs, it is not easy to eliminate. Therefore, it should be prevented from occurring in the first place. The measures to prevent crosstalk interference generally include the following: twisting of signal wires. Since twisting the signal wires together greatly reduces the area enclosed by the signal loop, and since the distances from the two signal wires to the source of interference can be roughly equal, as well as the distributed capacitance, this helps to significantly reduce the common-mode interference that enters the loop through inductive coupling from magnetic and electric fields. Shielding. To prevent interference from electric fields, the signal wires can be covered with metal. The usual practice is to cover the wire with a metal mesh (or ferromagnetic material), followed by an insulating layer. The purpose of shielding is to prevent the coupling of \"fields\" and to suppress interference from various \"fields\". The shielding layer needs to be grounded in order to prevent interference. As shown in Figure 4, we can clearly see the two scenarios: when the shielding layer is grounded and when it is not grounded. We can analyze these two scenarios: In the figure, wire 1 represents the interference source, while wire 2 is the signal wire; the resistance of wire 2 to ground can be considered to be infinite, and a shielding layer surrounds this wire. As shown in Figure 4, the shielding layer is not grounded; due to the distributed capacitance between the interference source and the shielding layer, a voltage is induced on the shielding layer of wire 2. Since there is no current flowing through the capacitance C2s between wire 2 and the shielding layer, the voltage induced in wire 2, ec, is equal to es. If the shielding layer is grounded, as shown in the figure, es=0, and the induced voltage on wire 2 also decreases to nearly 0. Therefore, in practical use, the shielding layer must be grounded. Otherwise, it has no effect on reducing the induced voltage. However, if the shielding layer is made of a non-ferritic material, it provides no shielding effect against magnetic field interference at power frequency. The wires can be magnetically shielded by running the signal wires inside iron tubes. Filtering. For DC signals with a very slow rate of change, a filtering circuit can be added to the input of the instrument in order to minimize the interference present in the signal. However, in actual engineering design, this method is rarely used; usually, this aspect is already taken into account during the circuit design of the instruments. The methods mentioned above are primarily passive measures aimed at suppressing the interference fields that inevitably arise, but in practice, we should try to avoid the formation of such interference fields as much as possible. For example, be careful to keep the signal wires away from the power wires ； Proper wiring to reduce the generation of stray magnetic fields ； Active isolation measures are taken, such as magnetically shielding electrical components like transformers. 2) Suppression of common-mode interference: Since the signals in instrument systems are usually at low levels, common-mode interference can also cause distortion in these instrument signals, leading to various measurement errors. Common measures taken to prevent common-mode interference are as follows: grounding. Usually, for safety reasons, the enclosures of instruments and signal sources are connected to ground to maintain a zero potential. The signal source circuit and the instrument system also require a stable ground connection. However, if the grounding method is inappropriate, it will create a ground loop that introduces interference. This is the case shown in Figure 3: with two points connected to ground, a common-mode interference is generated due to the potential difference between the grounds. Therefore, typically, the instrument loop uses a single-point grounding at the system. But in reality, it is impossible for the signal source side to be insulated from ground; therefore, in this sense, it is not possible to completely eliminate the interference caused by ground potential differences. Therefore, to improve the interference resistance of instruments, secondary instruments in low-level measuring instruments are usually made \"floating ground\", that is, the secondary instruments are insulated from ground. This is to cut off the pathways through which common-mode interference voltage can leak in, preventing the interference from entering. In practical applications, we usually combine shielding with grounding, which often resolves most interference issues. If the shielding layer is grounded on both the signal side and the instrument side, a potential difference between the grounds will create a circuit through the shielding layer. Since the resistance of the ground is usually much lower than that of the shielding layer, a potential gradient is formed across the shielding layer, which is then coupled into the signal circuit through the distributed capacitance between the shielding layer and the signal wires. Therefore, the shielding layer must also be grounded at one point. Furthermore, the grounding of the signal wire shielding layer should be on the same side as the system grounding. In fact, the enclosure of secondary instruments needs to be grounded for safety reasons. There is always distributed capacitance and leakage impedance between the input terminal of the instrument and the enclosure; therefore, a floating ground cannot completely cut off the leakage paths. As a result, double-layer shielding with floating ground protection is typically used when necessary. In other words, an inner shielding cover is placed inside the instrument’s housing; there is no electrical connection between this inner shielding cover and either the signal input terminal or the housing. A wire is led out from the inner shielding layer and connected to the shielding layer of the signal wire, while the shielding of the signal wire is grounded at the signal source. This arrangement ensures that the input protection shielding and the signal shielding of the instrument maintain a stable electric potential, being at the same potential level. It is possible to **improve the instrument’s resistance to interference (see Figure 5 for details). ) Even so, there is still a certain amount of leakage current. However, the measures taken to suppress interference are intended to reduce the intensity of the interference signals to a level that is negligible compared to the intensity of the actual signals. Additionally, another commonly used anti-interference measure is isolation, which also suppresses interference by preventing the formation of interference loops. The effects of these methods are additive. Usually, we adopt one or several of these methods to improve the interference resistance of signal measurement. Of course, with the development of theory and engineering practice, there are many more anti-interference measures. Here, only a brief introduction is provided to several methods that are commonly used in practical engineering. I hope it can provide some assistance to everyone in practical engineering work.