Controllable programming system (PLC) is the mainstream control product in industrial automation. It has been half a century since its birth. With the development of semiconductor technology, computer technology and communication technology, the field of industrial control has undergone earth-shaking changes. Functions, ease of use and product form have undergone five generations of changes. Today we will talk about the solution to the PLC problem. First, repeat the old tune and talk about its definition: It is a kind of programmable memory, used for its internal storage program, execute logic operation, sequence control, timing, counting and arithmetic operation and other user-oriented instructions, and control various types of machinery or production through digital or analog input and output process. 1. Anti-interference problem With the development of science and technology, PLC is more and more widely used in industrial control, and its reliability also directly affects the safe production and economic operation of industrial enterprises. The anti-interference ability of the system is the key to the reliable operation of the entire system. . To improve the reliability of the PLC control system, on the one hand, the PLC manufacturer is required to improve the anti-interference ability of the equipment; on the other hand, it is required to attract great attention in engineering design, installation, construction, and use and maintenance, and multi-party cooperation can solve the problem and effectively enhance the system. The anti-jamming performance. [Interference source and general classification of interference] The interference sources that affect the PLC control system are the same as those that generally affect industrial control equipment. Most of them are generated in the parts where the current or voltage changes drastically. The parts where the charge moves drastically are the noise source, that is, the interference source. Interference sources are usually divided according to the causes of interference, noise interference patterns, and the nature of the noise waveform. 1. According to the different causes of noise: discharge noise, surge noise, high frequency oscillation noise 2. According to the waveform and nature of the noise: continuous noise, occasional noise 3. Different interference modes according to noise: common mode interference, differential mode interference Among them, common mode interference and differential mode interference are a relatively common classification method. Common mode interference is the potential difference between the model and the ground, which is mainly formed by the superposition of the common mode (same direction) voltage induced on the signal line by the grid string, the ground potential difference and the space electromagnetic radiation. The common mode voltage is sometimes relatively large and special It uses a power distribution room with poor isolation performance. The common mode voltage of the transmitter output signal is generally higher, and some can be as high as 130V. The common-mode voltage can be converted into a differential-mode voltage through an asymmetrical circuit, which directly affects the measurement and control signal and causes damage to the components. This common-mode interference can be DC or AC. Differential mode interference refers to the interference voltage acting between the two poles of the signal, which is mainly caused by the coupling and induction of spatial electromagnetic field between signals and the voltage formed by the unbalanced circuit converting common mode interference. This direct superimposition on the signal directly affects measurement and control. Accuracy. [The main source of electromagnetic interference] 1. Radiation interference from space The radiated electromagnetic field (EMI) of space is mainly produced by the transient process of power network, electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc. It is usually called radiated interference. Mainly through two paths of interference: 1) Directly interfere with the radiation inside the PLC and induction of the mail circuit 2) For the radiation of the PLC communication network, interference is introduced by the induction of the communication line Radiation interference is related to the layout of field equipment and the size of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by setting shielded cables, PLC local shielding and high-voltage relief components. 2. Interference from the external leads of the system Mainly introduced through power and signal lines, usually called conducted interference. This kind of interference is more serious in my country's industrial scene. 1) Interference from the power supply Practice has proved that there are many cases of PLC control system failure due to the interference introduced by the power supply. After replacing the PLC power supply with higher isolation performance, the problem can be solved. The PLC power supply usually uses isolated power supply, but its mechanism and manufacturing process factors make its isolation not ideal. In fact, due to the existence of distributed parameters, especially distributed capacitance, absolute isolation is impossible. 2) Interference introduced from the signal line The various signal transmission lines connected with the PLC control system, in addition to transmitting effective various types of information, will always have external interference signals invade. There are two main ways of this interference: one is the power grid interference that comes in series through the power supply of the transmitter or the power supply of the shared signal instrument, which is often ignored; Second, the signal line is interfered by space electromagnetic radiation induction, that is, the external induction interference on the signal line, which is very serious. 3) Interference from the chaos of the grounding system Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper grounding can not only suppress the influence of electromagnetic interference, but also restrain the equipment from sending out interference; and wrong grounding will introduce serious interference signals and make the PLC system unable to work normally. The ground wire of PLC control system includes system ground, shield ground, AC ground and protection ground. The main interference of the chaotic grounding system to the PLC system is the uneven distribution of the potential of each grounding point, and the ground potential difference between different grounding points, causing ground loop currents and affecting the normal operation of the system. 3. Interference from inside the PLC system Mainly caused by the mutual electromagnetic radiation between the internal components and circuits of the system, such as the mutual radiation of logic circuits and their influence on analog circuits, the mutual influence of analog ground and logic ground, and the mismatching use of components. This is the content of the electromagnetic compatibility design of the internal system by the PLC manufacturer. It is more complicated and cannot be changed as an application department, but it is not too much to consider, but it is necessary to choose a system with more application performance or tested. ã€Anti-interference design】 1. Equipment selection When selecting equipment, first choose products with higher anti-interference ability, including electromagnetic compatibility (EMC), especially anti-interference ability, such as the use of floating technology, good isolation performance of the PLC system; secondly, it should Understand the anti-jamming indicators given by the manufacturer, such as the total analog system ratio, the differential analog system ratio, the pressure resistance, the allowable electric field strength and the high frequency magnetic field strength environment to work; in addition, it depends on the examination of its similar work Application performance. 2. Comprehensive anti-jamming design Mainly consider several suppression measures from the outside of the system. The main content includes: shielding the PLC system and outer leads to prevent electromagnetic interference from space radiation; isolating and filtering the outer leads, especially the principle power cable, layered arrangement to prevent Introduce conducted electromagnetic interference through the outer lead; properly design the grounding point and grounding device to improve the grounding system. In addition, software means must be used to further improve the safety and reliability of the system. ã€Main anti-interference measures】 1. Use a power supply with excellent performance to suppress the interference introduced by the grid In the PLC control system, the power supply occupies an extremely important position. The grid interference into the PLC control system is mainly coupled into the PLC system's power supply (such as CPU power supply, I/O power supply, etc.), transmitter power supply and instrument power supply with direct electrical connection with the PLC system. Now, for the power supply of the PLC system, the power supply with better isolation performance is generally adopted, and the power supply for the power supply of the transmitter and the power supply of the instrument that has a direct electrical connection to the PLC system has not received enough attention, although certain However, the isolation measures are generally not enough. The main reason is that the isolation transformer used has large distribution parameters and poor interference suppression capabilities. Common mode interference and differential mode interference are serially coupled through power supply. Therefore, power distributors with small distributed capacitance and large suppression band (such as multiple isolation and shielding and leakage inductance technologies) should be selected for the power supply of transmitters and shared signal instruments to reduce the interference of the PLC system. 2. Cable selection Different types of signals are transmitted by different cables. Signal cables should be arranged in layers according to the types of transmission signals. It is strictly forbidden to use different wires of the same cable to transmit power and signals at the same time. Avoid signal lines and power cables close to parallel installations to reduce electromagnetic waves. interference. 3. Hardware filtering and software anti-if measures Before the signal is connected to the computer, connect a capacitor in parallel between the signal line and the ground to reduce common mode interference; install a filter between the signal poles to reduce differential mode interference. 4. Choose the grounding point correctly and perfect the grounding system There are usually two purposes of grounding, one is for safety, and the other is for interference suppression. A perfect grounding system is one of the important measures for the PLC control system to resist electromagnetic interference. There are three ways to ground the system: floating ground, direct grounding and capacitive grounding. When the signal source is grounded, the shielding layer should be grounded on the signal side; when it is not grounded, it should be grounded on the PLC side; when there is a connector in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multiple grounding must be avoided; multiple measurements When the point signal shielded twisted pair is connected to the multi-core paired total shield cable, the shielding layers should be connected to each other and be insulated. 2. Improve operation efficiency 1. Carry out function block planning according to the actual needs of the project Writing subroutines In PLC, subroutines are relatively independent programs that are compiled for some specific control purposes. Execute the subroutine call instruction CALL, etc., if the condition is not met, the scan of the program is only performed in the main program, and the subroutine is no longer scanned, thus reducing unnecessary scan time. 2. Use word or double word data to transfer to DO point method to control output In PLC applications, there are usually a large number of output controls. Using word or double-word data to transfer to the DO point to control the output can increase the speed. As long as the output address is reasonably allocated and the control output control word is changed according to the requirements of the actual application, Greatly reduce the number of steps executed by the PLC program, thereby speeding up the running speed of the PLC program. 3. Pulse trigger SET, RESET In PLC, the SET instruction can be executed only once, and it is not necessary to execute this instruction every scan. It is very suitable for use with pulse output (PLS/PLF) instructions. 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