Do you know the 4 main anti-interference measures of PLC? How to improve operational efficiency?
Controllable programming system (PLC), as a mainstream control product in industrial automation, has been in existence for half a century. With the development of semiconductor technology, computer technology and communication technology, the field of industrial control has undergone tremendous changes. Function, ease of use and product form have gone through five generations of changes. Today we will talk about solutions to PLC problems.
PLC is a kind of programmable memory, which is used for its internal storage of programs, executes user-oriented instructions such as logical operations, sequence control, timing, counting and arithmetic operations, and controls 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, the application of PLC in industrial control is more and more extensive, 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. .
【The main source of electromagnetic interference】
1. Radiation interference from space
Radiated electromagnetic fields (EMI) in space are mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., and are often referred to as radiation interference.
Mainly through two paths of interference:
1) Directly interfere with the radiation inside the PLC and the induction of the postal circuit
2) The radiation to the PLC communication network 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 and PLC partial shielding and high-voltage relief components.
2. Intrference from 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 PLC control system failures caused by the interference introduced by the power supply. After replacing the PLC power supply with higher isolation performance, the problem can be solved.
PLC power supply usually adopts 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 from signal lines
All kinds of signal transmission lines connected with the PLC control system, in addition to transmitting various types of effective information, there will always be external interference signals intrusion.
There are two main ways for this interference: one is the grid interference connected in series through the power supply of the transmitter or the power supply of the common signal instrument, which is often ignored;
Second, the signal line is disturbed by space electromagnetic radiation induction, that is, the external inductive 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. Correct grounding can not only restrain the influence of electromagnetic interference, but also restrain the equipment from emitting external interference; but wrong grounding will introduce serious interference signals, which will make the PLC system unable to work normally.
The ground wire of the PLC control system includes system ground, shield ground, AC ground and protection ground. The disturbance of the grounding system to the PLC system is mainly due to the uneven potential distribution of each grounding point and the ground potential difference between different grounding points, which causes ground loop current and affects the normal operation of the system.
3. Interference from within the PLC system
It is 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 its influence on the analog circuit, the mutual influence of the analog ground and the logic ground, and the mismatch between the components. This belongs to the content of the electromagnetic compatibility design inside the system by the PLC manufacturer, which is relatively complex and cannot be changed as an application department.
1. Equipment selection
When choosing equipment, firstly choose products with high anti-interference ability, including electromagnetic compatibility (EMC), especially anti-external interference ability, such as PLC system with floating technology and good isolation performance; Understand the anti-interference indicators given by the manufacturer, such as the common analog system ratio, the poor analog system ratio, the withstand voltage capability, the allowable electric field strength and high frequency magnetic field strength environment; the other is to check its performance in similar work. application performance.
2. Comprehensive anti-interference design
Mainly consider several suppression measures from outside the system, the main contents include: shielding the PLC system and the outer leads to prevent electromagnetic interference from space radiation; isolation and filtering of the outer leads, especially the principle power cables, layered arrangement to prevent Conducted electromagnetic interference is introduced through the outer lead; the grounding point and grounding device are properly designed to improve the grounding system. In addition, software means must be used to further improve the safety and reliability of the system.
【Main anti-interference measures】
1. Use a power supply with excellent performance to suppress the interference introduced by the power grid
In the PLC control system, the power supply occupies a very important position. The power grid interference into the PLC control system is mainly coupled into the PLC system power supply (such as CPU power supply, I/O power supply, etc.), the transmitter power supply and the instrument power supply that has a 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 used, while the power supply of the transmitter and the power supply of the instrument with direct electrical connection to the PLC system have not received enough attention, although certain measures have been taken. The isolation measures are generally not enough. The main reason is that the isolation transformer used has large distribution parameters and poor interference suppression capability. Common mode interference and differential mode interference are connected in series through power coupling.
Therefore, for the power supply of the transmitter and the common signal instrument, the distributor with small distributed capacitance and large suppression band (such as using multiple isolation and shielding and leakage inductance technology) should be selected to reduce the interference of the PLC system.
2. The selection of cables
Different types of signals are transmitted by different cables. Signal cables should be arranged in layers according to the type of transmission signals. It is strictly forbidden to use different wires of the same cable to transmit power supply and signals at the same time. interference.
3. Hardware filtering and software anti-if measures
Before the signal is connected to the computer, a capacitor should be connected in parallel between the signal line and the ground to reduce the common mode interference; adding a filter between the two poles of the signal can reduce the differential mode interference.
4. Correctly select the grounding point and improve the grounding system
The purpose of grounding is usually two, one is for safety, and the other is to suppress interference. A perfect grounding system is one of the important measures for PLC control system to resist electromagnetic interference. There are three types of system grounding methods: floating grounding, 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 multi-point grounding must be avoided; When the shielded twisted pair of the point signal is connected with the multi-core twisted pair total shielded cable, the shielding layers should be well connected to each other and treated with insulation.
2. Improve operating efficiency
1. According to the actual needs of the project, carry out function block planning
Writing subprograms Subprograms are relatively independent programs compiled for some specific control purposes in PLC. Execute the subprogram call instruction CALL, etc. If the condition does not meet the subprogram call, the program scan is only performed in the main program, and no longer scans this subprogram, thus reducing unnecessary scan time.
2. Use word or double word data to transmit to the DO point to control the output
In the application of PLC, there are usually a large number of output controls. Using word or double word data to transmit to the DO point to control the output can increase the speed. As long as the output address is reasonably allocated according to the requirements of the actual application, the control output control word can be changed. The number of steps executed by the PLC program is greatly reduced, 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 to be used in conjunction with the pulse output (PLS/PLF) instruction. Some engineers ignore this problem and use the conventional method to drive the SET instruction, inadvertently increasing the PLC program scan runtime.
To improve the reliability of PLC control system, on the one hand, PLC manufacturers are required to improve the anti-interference ability of equipment; anti-interference performance.
[Interference sources and general classification of interference]
The interference sources that affect the PLC control system are the same as the interference sources that generally affect industrial control equipment. Most of them are generated in the parts where the current or voltage changes drastically. The parts where these charges move violently are the noise sources, that is, the interference sources.
Interference sources are usually divided according to the causes of interference, noise interference patterns and noise waveform properties.
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 connection, the ground potential difference and the space electromagnetic radiation. The common mode voltage is sometimes relatively large, especially It is a power supply room of a distributor with poor isolation performance. The common mode voltage of the output signal of the transmitter is generally high, 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 signal and causes damage to the components. This common mode interference can be either direct current or alternating current.
Differential mode interference refers to the interference voltage acting between the two poles of the signal. It is mainly caused by the coupling induction of the space electromagnetic field between the signals and the voltage formed by the unbalanced circuit converting the common mode interference. This is directly superimposed on the signal and directly affects the measurement and control. precision