Thread Content
Through scientific analysis of the conditions under which interference occurs and methods for suppressing it, this text provides a detailed overview of the eight anti-interference measures employed by automated control systems and instruments to address signal interference, including shielding, isolation, filtering, floating, protection of signal lines against interference, protection against thyristor interference, protection against relay interference, and grounding. For interference signals to affect automated control systems and instruments, three conditions must be met: an interference source that generates such signals, a receiving circuit sensitive to those signals, and a coupling path between the interference-receiving circuit and the rest of the system. Without any one of these elements, interference with electronic devices cannot occur. When addressing interference issues, it is first necessary to identify the source of the interference, the performance of the receiving circuit, and the manner in which the interference source is coupled to the receiving circuit, so that appropriate measures can be taken to mitigate the impact of the interference. Principles for suppressing interference Based on the above analysis, the principles for suppressing interference are as follows: 1. Eliminate or suppress the sources of interference, such as by isolating or keeping power lines and signal lines apart. 2. Pathways of disruption and interference. For interference that intrudes in the form of signals over wires, measures can be taken within the automated control system and the instruments themselves, such as using isolation transformers and optocouplers to cut off certain paths of interference ； For interference that intrudes in the form of a \"field,\" shielding measures are usually employed. 3. Reduce the sensitivity of the receiving circuit (the target affected by interference) to interference. For example, circuits with high input impedance are more susceptible to interference than those with low input impedance, and analog circuits have poorer interference resistance compared to digital circuits. For installation, suppressing interference from sources that affect other circuits is the most effective measure, but it can be difficult to achieve this due to constraints or high costs. At this point, protective measures should be taken for the affected weak signal circuits, automated control systems, and instruments in order to enhance their resistance to interference. Anti-interference measures 1. Shielding: Shielding technology is primarily used to suppress the interference caused by electromagnetic induction on automated systems and instruments. It uses low-resistance materials such as copper or aluminum, or magnetic materials, to enclose components, circuits, assemblies, or transmission lines, in order to isolate mutual electromagnetic interference between the interior and exterior. Shielding includes electrostatic shielding, electromagnetic shielding, low-frequency magnetic shielding, and drive shielding. 1.1 Electrostatic shielding: Under the influence of an electrostatic field, there are no electric field lines inside a conductor, meaning that all points have the same potential. Therefore, a metal with good electrical conductivity is used for the shielding box, and it is grounded. Its internal power lines do not leak out, and at the same time, external power lines do not affect its internal components. Electrostatic shielding can prevent the effects of electrostatic fields, and it can eliminate or reduce interference that arises between two circuits due to coupling caused by parasitic distributed capacitance. 1.2 Electromagnetic shielding: Electromagnetic shielding involves using metal materials with good electrical conductivity to create a shielding layer. High-frequency interference electromagnetic fields generate eddy currents within this shielding layer, and it is these eddy currents that dissipate the energy of the high-frequency interference magnetic fields, thereby reducing the impact of such fields. If the electromagnetic shielding layer is grounded, it also serves as an electrostatic shield. In other words, using a metal material with good electrical conductivity to create a grounding electromagnetic shielding layer can serve both as an electromagnetic shield and an electrostatic shield. 2. Isolation: When the signal measurement circuits and signal sources of automated control systems and instruments are grounded at both ends, it is easy for loop currents to be generated, causing interference. At this point, isolation methods are necessary; especially when automatic devices contain mixed circuits of analog and digital signals as well as high-voltage and low-voltage circuits, it is essential to isolate each component of these circuits, which also helps to suppress drift and provide safety protection. The signal isolator, also known as a signal conditioner, takes in single or dual input current or voltage signals, converts them into isolated current or voltage signals for output, and enhances the electrical isolation between the inputs, outputs, and power supply. The article \"Performance Evaluation Criteria for High-Quality Signal Isolators\" provides an in-depth introduction to signal isolators. http://yunrun.com.cn/upload/201605/31/201605312135186620.jpg 3. Filtering: Filtering is an effective method for suppressing serial mode interference or eliminating the AC signals that result from common mode interference. It involves inserting filters for the corresponding frequency bands into the signal transmission path, based on the frequency distribution of signals and noise, in order to filter out or minimize noise, thereby improving the signal-to-noise ratio and reducing interference. Under the various anti-interference measures mentioned above, if residual AC interference signals remain, they can be eliminated using filtering methods. 4. Floating: Floating is an effective measure to resist common-mode interference. Fully insulating the signal wires and the circuit of automatic devices so that they do not come into contact with the grounded metal casing is what is referred to as \"floating\". A completely floating circuit cannot generate current, even in the presence of a common-mode voltage. Whether the resistances of the two input wires to ground are symmetric or not, it will not convert common-mode interference into differential-mode interference, and pure common-mode signals will not interfere with the proper operation of automated control systems and instruments (up to a certain extent). This is an anti-interference measure taken to prevent conversion. Essentially, the goal of floating and transformer isolation, as well as optoelectronic isolation, is the same: to prevent the propagation of common-mode signals. For passive circuits (such as electronic potentiometers and moving-coil instruments used with thermocouples), floating is achieved simply by keeping the measurement circuit isolated and not in contact with any grounded conductors. But active circuits such as amplifiers are more troublesome, as they require a power supply. The power supply usually comes from the electrical grid, which is grounded. The solution is to use a transformer for isolation, keeping the secondary winding that supplies power to the amplifier floating. 5. Interference resistance of signal wires: The electrical signals in thermal process automation devices are of low voltage and low current; considering the load on the wires, a large cross-sectional area is not necessary. However, the industrial control signals have to be transmitted over long distances in harsh environments. To keep the resistance of the signal wires low and ensure sufficient mechanical strength, multi-strand wires with a cross-sectional area of at least 1 mm2 are typically used. The advantage of multi-strand wires is that they are flexible and easy to bend. According to the anti-interference requirements. Twisted pair, parallel lines, shielded cable, or coaxial cable can be used. From the perspective of reducing interference, the most fundamental measure is to keep the signal wires away from the power wires. Since they must be installed in the same cable trench, they need to be routed along either side of the trench or arranged in two layers, with a grounded metal plate placed between them for shielding purposes. When there is no shielding, the distance between the two types of wires should not be less than 15 cm; ideally, it should be over 60 cm. If wires pass through a conduit, it is absolutely not allowed to run signal and power wires in the same conduit. It should be noted that metal pipes only provide electric field shielding when grounded, and only iron pipes can shield against magnetic field interference. In twisted pair cables, the loop formed by the two wires is very small, and since the wires alternate between opposite directions, the induced electromotive forces generated by electromagnetic interference in each twist can cancel each other out due to their different directions. As for the interference resistance of coaxial cables, especially in terms of the transmission of high-frequency signals, they have a clear advantage over other types of wires. 6. Suppression of thyristor-induced interference: Thyristors are being used more and more widely in automated control systems, yet they represent a significant source of interference among industrial electronic devices. When thyristors are triggered to turn on or off, it causes sharp changes in voltage and current, resulting in interference waves in the power supply network; such interference has an adverse effect on automated control systems and instruments. To prevent or reduce this harmful effect, an RC circuit composed of a resistor and a capacitor in series must be connected in parallel across the load of the thyristor. 7. Suppression of electrical contact interference: When the contacts of switches and relays disconnect inductive loads, a reverse transient high voltage is generated due to the high rate of change of current; typically, this high voltage can reach 10–20 times the supply voltage within a few microseconds. It generates sparks at the relay contacts, which not only erodes the surface of those contacts but also creates electromagnetic interference, which is very harmful. To this end, an electric contact arc-suppression circuit can be installed, which protects the contacts, extends their lifespan, and suppresses interference signals. The most commonly used arc-suppression circuit is shown in the figure below: an RC series circuit with its terminals connected in parallel. When the power supply is DC, no current flows through L after K is opened, and the voltage from the power supply charges the capacitor C. At the moment K is turned on, the resistor R prevents the discharge current from C from becoming too large ； At the moment when K is disconnected, the induced voltage charges C through R, causing the current to decrease more slowly and also protecting the contacts from arcing. 8. Grounding: Grounding is also an important means of resisting interference, but incorrect grounding not only fails to mitigate interference but can actually increase it. As shown in the three circuits below, each of their grounds is connected to a common ground. Due to the resistance present in this common ground, different ground currents flow through the resistances r1, r2, and r3. The voltage drops resulting from this cause interference with the other circuits, thereby providing a pathway for interference within the common impedance. http://yunrun.com.cn/upload/201605/31/201605312137576294.png http://yunrun.com.cn/upload/201605/31/201605312138273045.png Multiple circuits are incorrectly grounded; the correct way to ground multiple circuits is such that the grounding resistance of each circuit is independent of the others, so that no matter what the current in each circuit is, it has no impact on the other circuits. It can be seen that the grounding points can be shared, but the grounding wires cannot; this is the grounding principle. Assuming that the output signal from circuit 1 is supplied to circuit 2, and circuit 2 then sends the signal to circuit 3, in this case the contacts must be combined into one unit; they cannot be located in three different places