Abstract: This paper introduces the application of programmable controller in the field of industrial control and the series of problems that PLC should pay attention to in the process of application, and give some reasonable suggestions.
Keywords: PLC industrial control anti-interference wiring grounding suggestion
I. Brief description
For many years, programmable controllers (hereinafter referred to as PLC) have realized the leap from wiring logic to storage logic from its generation to the present; its functions from weak to strong, achieving the progress of logic control to digital control; its application field from small to small Large, it realizes the simple control of single equipment to the various tasks such as competent motion control, process control and distributed control. Today's PLC has greatly improved its ability to handle analog, digital computing, human-machine interface and network, and has become the mainstream control device in the field of industrial control, playing an increasingly important role in all walks of life.
Second, the application field of PLC
At present, PLC has been widely used in various industries such as steel, petroleum, chemical, electric power, building materials, machinery manufacturing, automobile, textile, transportation, environmental protection and cultural entertainment. The usage is mainly divided into the following categories:
1. Switching logic control
It replaces the traditional relay circuit and realizes logic control and sequence control. It can be used for control of single equipment, multi-machine group control and automatic assembly line. Such as injection molding machines, printing machines, bookbinding machinery, combination machine tools, grinding machines, packaging production lines, electroplating lines and so on.
2. Industrial process control
In the industrial production process, there are some continuous changes such as temperature, pressure, flow, liquid level and speed (ie analog quantity), PLC adopts corresponding A/D and D/A conversion modules and various controls. The algorithm program processes the analog quantity and completes the closed loop control. PID regulation is an adjustment method used in general closed-loop control systems. Process control has a wide range of applications in metallurgy, chemical, heat treatment, boiler control and other occasions.
3. sport control
The PLC can be used for the control of circular motion or linear motion. Special motion control modules are generally used, such as single-axis or multi-axis position control modules that can drive stepper motors or servo motors, and are widely used in various machines, machine tools, robots, elevators, etc.
4. data processing
PLC has mathematical operations (including matrix operations, function operations, logic operations), data transfer, data conversion, sorting, table lookup, bit manipulation and other functions, can complete the data collection, analysis and processing. Data processing is commonly used in some large control systems such as paper, metallurgy, and food industries.
5. Communication and networking
PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of the factory automation network, the current PLC has a communication interface, and communication is very convenient.
Third, the application characteristics of PLC
1. High reliability and strong anti-interference ability
High reliability is a key feature of electrical control equipment. PLC adopts modern large-scale integrated circuit technology and is manufactured by strict production process. The internal circuit adopts advanced anti-interference technology and has high reliability. The use of PLC to form a control system, compared with the same size of the relay contactor system, electrical wiring and switch contacts have been reduced to hundreds or even thousands of, the fault is greatly reduced. In addition, the PLC has a hardware fault self-detection function that can issue an alarm message in time when a fault occurs. In the application software, the application can also be programmed into the fault self-diagnosis program of the peripheral device, so that the circuit and the device other than the PLC in the system also obtain the fault self-diagnosis protection. In this way, the entire system will be extremely reliable.
2. Completely furnished, fully functional, and highly adaptable
Since the development of PLC to date, serialized products of various scales have been formed, which can be used in industrial control occasions of various scales. In addition to the logic processing functions, PLCs have perfect data computing capabilities and can be used in various digital control fields. A large variety of functional units have emerged, allowing PLCs to penetrate into various industrial controls such as position control, temperature control, and CNC. Coupled with the enhancement of PLC communication capabilities and the development of human-machine interface technology, it is very easy to use PLC to form various control systems.
3. Easy to learn and use, popular with engineers and technicians
PLC is an industrial control equipment for industrial and mining enterprises. Its interface is easy and the programming language is easy for engineers to accept. The graphical symbols of the ladder language are quite similar to the expressions and relay circuit diagrams. It opens the door for industrial control who are not familiar with electronic circuits, who do not understand computer principles and assembly language.
4. System design, small workload, easy maintenance, easy to transform
PLC replaces the wiring logic with storage logic, which greatly reduces the wiring outside the control device, shortens the design and construction cycle of the control system, and makes the daily maintenance easier. More importantly, the same device changes after changing the program. The production process is possible. This is especially suitable for production occasions of many varieties and small batches.
Fourth, the need to pay attention to the PLC application
PLC is a kind of equipment used for industrial production automation control. Generally, it can be used directly in industrial environment without any measures. However, although the reliability is higher as described above and the anti-interference ability is strong, when the production environment is too harsh, the electromagnetic interference is particularly strong, or the installation is improperly used, it may cause a program error or an operation error, thereby causing a false input and Causes false output, which will cause the equipment to run out of control and malfunction, thus can not guarantee the normal operation of the PLC, to improve the reliability of the PLC control system, on the one hand requires PLC manufacturers to improve the anti-interference ability of the equipment; on the other hand, requires design In the installation, use and maintenance, it is highly valued, and multi-party cooperation can solve the problem and effectively enhance the anti-jamming performance of the system. Therefore, you should pay attention to the following issues during use:
1. working environment
(1) Temperature
The PLC requires an ambient temperature of 0~55oC. It should not be placed under the component with large heat output during installation. The space for ventilation and heat dissipation should be large enough.
(2) Humidity
In order to ensure the insulation performance of the PLC, the relative humidity of the air should be less than 85% (no condensation).
(3) Vibration
The PLC should be kept away from strong vibration sources to prevent frequent or continuous vibrations with a vibration frequency of 10 to 55 Hz. When the use environment is unavoidable, shock absorption measures must be taken, such as damping rubber.
(4) Air
Avoid corrosive and flammable gases such as hydrogen chloride, hydrogen sulfide, etc. For environments with more dust or corrosive gases in the air, the PLC can be installed in a well-closed control room or control cabinet.
(5) Power supply
PLC has a certain resistance to the interference caused by the power line. In environments with high reliability requirements or particularly severe power disturbances, an isolation transformer with a shield can be installed to reduce interference between the equipment and the ground. General PLC has DC 24V output provided to the input terminal. When the input terminal uses external DC power supply, DC stabilized power supply should be selected. Because of the ordinary rectified and filtered power supply, it is easy for the PLC to receive error information due to the influence of ripple.
2. Interference and its source in the control system
On-site electromagnetic interference is one of the most common and most likely factors affecting system reliability in PLC control systems. The so-called palliative treatment of the first problem, to find out the problem, can propose a solution to the problem. It is therefore necessary to know the source of the on-site interference. (1) Interference sources and general classification
The interference sources that affect the PLC control system are mostly generated in places where the current or voltage changes drastically. The reason is that the current changes to generate a magnetic field, which generates electromagnetic radiation to the device; the magnetic field changes to generate current, and the electromagnetic high-speed generates electromagnetic waves. Generally, electromagnetic interference is classified into common mode interference and differential mode interference according to different interference modes. Common mode interference is the potential difference of the signal to the ground, which is mainly formed by the superposition of the common state (same direction) voltage induced by the power grid, the ground potential difference and the space electromagnetic radiation on the signal line. 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 modules). This common mode interference can be DC. It can also be an exchange. Differential mode interference refers to the interference voltage acting between the two poles of the signal. It is mainly caused by the coupling of the space electromagnetic field between the signals and the common mode interference caused by the unbalanced circuit. This interference is superimposed on the signal and directly affects the measurement and control. Precision.
(2) Main sources and ways of interference in PLC system
Strong electrical interference
The normal power supply of the PLC system is powered by the grid. Due to the wide coverage of the grid, it will be subject to electromagnetic interference from all spaces and induce voltage on the line. In particular, internal changes in the power grid, surges in knife switch operation, start and stop of large-scale power equipment, harmonics caused by AC and DC transmissions, and short-circuit transients of power grids are transmitted to the primary side of the power supply through transmission lines.
Interference in the cabinet
High-voltage electrical appliances in the control cabinet, large inductive loads, and chaotic wiring are all likely to cause a certain degree of interference to the PLC.
Interference from signal line introduction
In addition to transmitting various types of information, various types of signal transmission lines connected to the PLC control system always have external interference signals intruded. There are two main ways of this interference: one is the power grid interference through the power supply of the transmitter or the power supply of the shared signal meter, which is often neglected; the second is that the signal line is interfered by the space electromagnetic radiation, that is, the signal line The external inductive interference, which is very serious. The introduction of interference from the signal will cause the I/O signal to work abnormally and the measurement accuracy will be greatly reduced. In severe cases, the component will be damaged.
Interference from grounded system chaos
Grounding is one of the effective means to improve electromagnetic compatibility (EMC) of electronic equipment. The correct grounding can not only suppress the influence of electromagnetic interference, but also suppress the external interference of the equipment; and the wrong grounding will introduce serious interference signals, so that the PLC system will not work normally.
Interference from the PLC system
It is mainly generated by mutual electromagnetic radiation between internal components and circuits of the system, such as the mutual radiation of logic circuits and their influence on analog circuits, the interaction between analog ground and logic ground, and the mismatch between components.
Inverter interference
First, the harmonics generated during the startup and operation of the inverter cause conduction interference to the power grid, causing voltage distortion of the power grid and affecting the power supply quality of the power grid. Second, the output of the frequency converter will generate strong electromagnetic radiation interference, affecting the normal operation of peripheral equipment. .
3. Main anti-jamming measures
(1) Reasonable handling of power supply to suppress interference introduced by power grid
For the grid interference introduced by the power supply, an isolation transformer with a shielding ratio of 1:1 can be installed to reduce the interference between the equipment and the ground, and the LC filter circuit can be connected in series at the input end of the power supply. As shown in Figure 1
(2) Installation and wiring
â— The power line, control line, and PLC power line and I/O line should be separately wired. The double-glue line should be used between the isolation transformer and the PLC and I/O. Separate the IO line and the high power line of the PLC separately. If it is necessary to bundle the AC line and the DC line separately in the same line slot, if the conditions permit, the slot line is the best, which not only makes it as large as possible. Space distance and minimize interference.
â— The PLC should be kept away from strong interference sources such as electric welders, high-power silicon rectifiers and large-scale power equipment. It cannot be installed in the same switch cabinet as high-voltage electrical appliances. The PLC in the cabinet should be away from the power line (the distance between the two should be greater than 200mm). Inductive loads installed in the same cabinet as the PLC, such as relays with larger power and coils of contactors, should be connected in parallel with the RC arc-extinguishing circuit.
â— It is best to separate the input and output of the PLC separately. The switch quantity and analog quantity should also be laid separately. The transmission of analog signals should be shielded. The shield should be grounded at one or both ends. The grounding resistance should be less than 1/10 of the resistance of the shield.
â— Do not use the same cable for the AC output line and DC output line. The output line should be as far away as possible from the high voltage line and the power line to avoid parallelism.
(3) Wiring at the I/O side
Input wiring
â— The input wiring should not be too long. However, if the environmental interference is small and the voltage drop is not large, the input wiring can be longer.
â— The input/output lines cannot use the same cable, and the input/output lines should be separated.
â— Connect to the input terminal as often as possible with the normally open contact, so that the programmed ladder diagram is consistent with the relay schematic and is easy to read.
Output connection
â— The output wiring is divided into independent output and common output. Different types and voltage levels of output voltage can be used in different groups. However, the output in the same group can only use the same type of power supply of the same voltage level.
â— Since the output component of the PLC is packaged on the printed circuit board and connected to the terminal block, if the load connected to the output component is short-circuited, the printed circuit board will be burnt.
â— When the relay output is used, the inductive load will affect the service life of the relay. Therefore, when using the inductive load, it should be reasonably selected or an isolation relay.
â— The output load of the PLC may cause interference, so measures should be taken to control it, such as the freewheeling protection of the DC output, the RC absorption circuit of the AC output, and the bypass resistor protection of the transistor and the triac output.
(4) Correctly select the grounding point and perfect the grounding system
Good grounding is an important condition to ensure reliable operation of the PLC, which can avoid accidental voltage shock hazards. There are usually two purposes for grounding, one for safety and the other for interference suppression. The perfect grounding system is one of the important measures for the anti-electromagnetic interference of the PLC control system.
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 distribution of potential at each grounding point. There is a ground potential difference between different grounding points, causing ground loop current and affecting the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends A and B of the cable shielding layer are grounded, there is a ground potential difference, and a current flows through the shielding layer. When an abnormal state such as a lightning strike occurs, the ground current will be larger.
In addition, the shielding layer, the grounding wire and the earth may form a closed loop. Under the action of the changing magnetic field, an induced current will appear in the shielding layer, and the signal loop is interfered by the coupling between the shielding layer and the core wire. If the system is confused with other grounding, the generated ground loop may generate unequal potential distribution on the ground, which affects the normal operation of the logic circuit and analog circuit in the PLC. The logic voltage interference tolerance of PLC operation is low, and the distribution interference of logic ground potential easily affects the logic operation and data storage of PLC, resulting in data confusion, program running or crash. The distribution of the analog ground potential will result in a decrease in measurement accuracy, causing severe distortion and malfunction of the signal measurement and control.
â— Safely or power grounded
Ground the power cord grounding terminal and the cabinet wiring to a safety ground. If the power supply is leaking or the cabinet is charged, it can be introduced into the ground from the safety ground without causing harm to people.
â— System grounding
The PLC controller is grounded in order to be at the same potential as the controlled devices, and the system is grounded. The grounding resistance value shall not be greater than 4Ω. Generally, the PLC equipment system ground and the negative terminal of the switching power supply in the control cabinet shall be connected together as the control system ground.
â— Signal and shield grounding
Generally, the signal line must have a unique reference ground. When the shielded cable encounters a possibility of conductive interference, it must be grounded locally or in the control room to prevent the formation of a "ground loop." 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 joint 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 shielded cable, the shield layers should be connected to each other and insulated, and a single point contact at the appropriate grounding point should be selected.
(5) Suppression of inverter interference
The interference handling of the inverter is generally as follows:
The isolation transformer is mainly used for conducting interference from the power supply, and can block most of the conducted interference before the isolation transformer.
The filter is used, the filter has strong anti-interference ability, and also prevents the interference of the device itself from being transmitted to the power supply, and some also have the function of peak voltage absorption.
The use of output reactors, adding AC reactors between the inverter and the motor is mainly to reduce the electromagnetic radiation generated by the inverter output during the energy transmission process, affecting the normal operation of other equipment.
V. Conclusion
The interference in the PLC control system is a very complicated problem. Therefore, in the anti-interference design, all aspects should be considered comprehensively, and the anti-interference should be effectively and effectively suppressed to enable the PLC control system to work normally. With the continuous expansion of PLC application fields, how to use PLC efficiently and reliably has become an important factor in its development. In the 21st century, PLC will have greater development, the product variety will be more abundant, the specifications are more complete, through the perfect human-machine interface, complete communication equipment will better adapt to the needs of various industrial control occasions, PLC as automatic control An important part of the network and the international common network will play an increasingly important role in the field of industrial control.
Keywords: PLC industrial control anti-interference wiring grounding suggestion
I. Brief description
For many years, programmable controllers (hereinafter referred to as PLC) have realized the leap from wiring logic to storage logic from its generation to the present; its functions from weak to strong, achieving the progress of logic control to digital control; its application field from small to small Large, it realizes the simple control of single equipment to the various tasks such as competent motion control, process control and distributed control. Today's PLC has greatly improved its ability to handle analog, digital computing, human-machine interface and network, and has become the mainstream control device in the field of industrial control, playing an increasingly important role in all walks of life.
Second, the application field of PLC
At present, PLC has been widely used in various industries such as steel, petroleum, chemical, electric power, building materials, machinery manufacturing, automobile, textile, transportation, environmental protection and cultural entertainment. The usage is mainly divided into the following categories:
1. Switching logic control
It replaces the traditional relay circuit and realizes logic control and sequence control. It can be used for control of single equipment, multi-machine group control and automatic assembly line. Such as injection molding machines, printing machines, bookbinding machinery, combination machine tools, grinding machines, packaging production lines, electroplating lines and so on.
2. Industrial process control
In the industrial production process, there are some continuous changes such as temperature, pressure, flow, liquid level and speed (ie analog quantity), PLC adopts corresponding A/D and D/A conversion modules and various controls. The algorithm program processes the analog quantity and completes the closed loop control. PID regulation is an adjustment method used in general closed-loop control systems. Process control has a wide range of applications in metallurgy, chemical, heat treatment, boiler control and other occasions.
3. sport control
The PLC can be used for the control of circular motion or linear motion. Special motion control modules are generally used, such as single-axis or multi-axis position control modules that can drive stepper motors or servo motors, and are widely used in various machines, machine tools, robots, elevators, etc.
4. data processing
PLC has mathematical operations (including matrix operations, function operations, logic operations), data transfer, data conversion, sorting, table lookup, bit manipulation and other functions, can complete the data collection, analysis and processing. Data processing is commonly used in some large control systems such as paper, metallurgy, and food industries.
5. Communication and networking
PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of the factory automation network, the current PLC has a communication interface, and communication is very convenient.
Third, the application characteristics of PLC
1. High reliability and strong anti-interference ability
High reliability is a key feature of electrical control equipment. PLC adopts modern large-scale integrated circuit technology and is manufactured by strict production process. The internal circuit adopts advanced anti-interference technology and has high reliability. The use of PLC to form a control system, compared with the same size of the relay contactor system, electrical wiring and switch contacts have been reduced to hundreds or even thousands of, the fault is greatly reduced. In addition, the PLC has a hardware fault self-detection function that can issue an alarm message in time when a fault occurs. In the application software, the application can also be programmed into the fault self-diagnosis program of the peripheral device, so that the circuit and the device other than the PLC in the system also obtain the fault self-diagnosis protection. In this way, the entire system will be extremely reliable.
2. Completely furnished, fully functional, and highly adaptable
Since the development of PLC to date, serialized products of various scales have been formed, which can be used in industrial control occasions of various scales. In addition to the logic processing functions, PLCs have perfect data computing capabilities and can be used in various digital control fields. A large variety of functional units have emerged, allowing PLCs to penetrate into various industrial controls such as position control, temperature control, and CNC. Coupled with the enhancement of PLC communication capabilities and the development of human-machine interface technology, it is very easy to use PLC to form various control systems.
3. Easy to learn and use, popular with engineers and technicians
PLC is an industrial control equipment for industrial and mining enterprises. Its interface is easy and the programming language is easy for engineers to accept. The graphical symbols of the ladder language are quite similar to the expressions and relay circuit diagrams. It opens the door for industrial control who are not familiar with electronic circuits, who do not understand computer principles and assembly language.
4. System design, small workload, easy maintenance, easy to transform
PLC replaces the wiring logic with storage logic, which greatly reduces the wiring outside the control device, shortens the design and construction cycle of the control system, and makes the daily maintenance easier. More importantly, the same device changes after changing the program. The production process is possible. This is especially suitable for production occasions of many varieties and small batches.
Fourth, the need to pay attention to the PLC application
PLC is a kind of equipment used for industrial production automation control. Generally, it can be used directly in industrial environment without any measures. However, although the reliability is higher as described above and the anti-interference ability is strong, when the production environment is too harsh, the electromagnetic interference is particularly strong, or the installation is improperly used, it may cause a program error or an operation error, thereby causing a false input and Causes false output, which will cause the equipment to run out of control and malfunction, thus can not guarantee the normal operation of the PLC, to improve the reliability of the PLC control system, on the one hand requires PLC manufacturers to improve the anti-interference ability of the equipment; on the other hand, requires design In the installation, use and maintenance, it is highly valued, and multi-party cooperation can solve the problem and effectively enhance the anti-jamming performance of the system. Therefore, you should pay attention to the following issues during use:
1. working environment
(1) Temperature
The PLC requires an ambient temperature of 0~55oC. It should not be placed under the component with large heat output during installation. The space for ventilation and heat dissipation should be large enough.
(2) Humidity
In order to ensure the insulation performance of the PLC, the relative humidity of the air should be less than 85% (no condensation).
(3) Vibration
The PLC should be kept away from strong vibration sources to prevent frequent or continuous vibrations with a vibration frequency of 10 to 55 Hz. When the use environment is unavoidable, shock absorption measures must be taken, such as damping rubber.
(4) Air
Avoid corrosive and flammable gases such as hydrogen chloride, hydrogen sulfide, etc. For environments with more dust or corrosive gases in the air, the PLC can be installed in a well-closed control room or control cabinet.
(5) Power supply
PLC has a certain resistance to the interference caused by the power line. In environments with high reliability requirements or particularly severe power disturbances, an isolation transformer with a shield can be installed to reduce interference between the equipment and the ground. General PLC has DC 24V output provided to the input terminal. When the input terminal uses external DC power supply, DC stabilized power supply should be selected. Because of the ordinary rectified and filtered power supply, it is easy for the PLC to receive error information due to the influence of ripple.
2. Interference and its source in the control system
On-site electromagnetic interference is one of the most common and most likely factors affecting system reliability in PLC control systems. The so-called palliative treatment of the first problem, to find out the problem, can propose a solution to the problem. It is therefore necessary to know the source of the on-site interference. (1) Interference sources and general classification
The interference sources that affect the PLC control system are mostly generated in places where the current or voltage changes drastically. The reason is that the current changes to generate a magnetic field, which generates electromagnetic radiation to the device; the magnetic field changes to generate current, and the electromagnetic high-speed generates electromagnetic waves. Generally, electromagnetic interference is classified into common mode interference and differential mode interference according to different interference modes. Common mode interference is the potential difference of the signal to the ground, which is mainly formed by the superposition of the common state (same direction) voltage induced by the power grid, the ground potential difference and the space electromagnetic radiation on the signal line. 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 modules). This common mode interference can be DC. It can also be an exchange. Differential mode interference refers to the interference voltage acting between the two poles of the signal. It is mainly caused by the coupling of the space electromagnetic field between the signals and the common mode interference caused by the unbalanced circuit. This interference is superimposed on the signal and directly affects the measurement and control. Precision.
(2) Main sources and ways of interference in PLC system
Strong electrical interference
The normal power supply of the PLC system is powered by the grid. Due to the wide coverage of the grid, it will be subject to electromagnetic interference from all spaces and induce voltage on the line. In particular, internal changes in the power grid, surges in knife switch operation, start and stop of large-scale power equipment, harmonics caused by AC and DC transmissions, and short-circuit transients of power grids are transmitted to the primary side of the power supply through transmission lines.
Interference in the cabinet
High-voltage electrical appliances in the control cabinet, large inductive loads, and chaotic wiring are all likely to cause a certain degree of interference to the PLC.
Interference from signal line introduction
In addition to transmitting various types of information, various types of signal transmission lines connected to the PLC control system always have external interference signals intruded. There are two main ways of this interference: one is the power grid interference through the power supply of the transmitter or the power supply of the shared signal meter, which is often neglected; the second is that the signal line is interfered by the space electromagnetic radiation, that is, the signal line The external inductive interference, which is very serious. The introduction of interference from the signal will cause the I/O signal to work abnormally and the measurement accuracy will be greatly reduced. In severe cases, the component will be damaged.
Interference from grounded system chaos
Grounding is one of the effective means to improve electromagnetic compatibility (EMC) of electronic equipment. The correct grounding can not only suppress the influence of electromagnetic interference, but also suppress the external interference of the equipment; and the wrong grounding will introduce serious interference signals, so that the PLC system will not work normally.
Interference from the PLC system
It is mainly generated by mutual electromagnetic radiation between internal components and circuits of the system, such as the mutual radiation of logic circuits and their influence on analog circuits, the interaction between analog ground and logic ground, and the mismatch between components.
Inverter interference
First, the harmonics generated during the startup and operation of the inverter cause conduction interference to the power grid, causing voltage distortion of the power grid and affecting the power supply quality of the power grid. Second, the output of the frequency converter will generate strong electromagnetic radiation interference, affecting the normal operation of peripheral equipment. .
3. Main anti-jamming measures
(1) Reasonable handling of power supply to suppress interference introduced by power grid
For the grid interference introduced by the power supply, an isolation transformer with a shielding ratio of 1:1 can be installed to reduce the interference between the equipment and the ground, and the LC filter circuit can be connected in series at the input end of the power supply. As shown in Figure 1
(2) Installation and wiring
â— The power line, control line, and PLC power line and I/O line should be separately wired. The double-glue line should be used between the isolation transformer and the PLC and I/O. Separate the IO line and the high power line of the PLC separately. If it is necessary to bundle the AC line and the DC line separately in the same line slot, if the conditions permit, the slot line is the best, which not only makes it as large as possible. Space distance and minimize interference.
â— The PLC should be kept away from strong interference sources such as electric welders, high-power silicon rectifiers and large-scale power equipment. It cannot be installed in the same switch cabinet as high-voltage electrical appliances. The PLC in the cabinet should be away from the power line (the distance between the two should be greater than 200mm). Inductive loads installed in the same cabinet as the PLC, such as relays with larger power and coils of contactors, should be connected in parallel with the RC arc-extinguishing circuit.
â— It is best to separate the input and output of the PLC separately. The switch quantity and analog quantity should also be laid separately. The transmission of analog signals should be shielded. The shield should be grounded at one or both ends. The grounding resistance should be less than 1/10 of the resistance of the shield.
â— Do not use the same cable for the AC output line and DC output line. The output line should be as far away as possible from the high voltage line and the power line to avoid parallelism.
(3) Wiring at the I/O side
Input wiring
â— The input wiring should not be too long. However, if the environmental interference is small and the voltage drop is not large, the input wiring can be longer.
â— The input/output lines cannot use the same cable, and the input/output lines should be separated.
â— Connect to the input terminal as often as possible with the normally open contact, so that the programmed ladder diagram is consistent with the relay schematic and is easy to read.
Output connection
â— The output wiring is divided into independent output and common output. Different types and voltage levels of output voltage can be used in different groups. However, the output in the same group can only use the same type of power supply of the same voltage level.
â— Since the output component of the PLC is packaged on the printed circuit board and connected to the terminal block, if the load connected to the output component is short-circuited, the printed circuit board will be burnt.
â— When the relay output is used, the inductive load will affect the service life of the relay. Therefore, when using the inductive load, it should be reasonably selected or an isolation relay.
â— The output load of the PLC may cause interference, so measures should be taken to control it, such as the freewheeling protection of the DC output, the RC absorption circuit of the AC output, and the bypass resistor protection of the transistor and the triac output.
(4) Correctly select the grounding point and perfect the grounding system
Good grounding is an important condition to ensure reliable operation of the PLC, which can avoid accidental voltage shock hazards. There are usually two purposes for grounding, one for safety and the other for interference suppression. The perfect grounding system is one of the important measures for the anti-electromagnetic interference of the PLC control system.
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 distribution of potential at each grounding point. There is a ground potential difference between different grounding points, causing ground loop current and affecting the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends A and B of the cable shielding layer are grounded, there is a ground potential difference, and a current flows through the shielding layer. When an abnormal state such as a lightning strike occurs, the ground current will be larger.
In addition, the shielding layer, the grounding wire and the earth may form a closed loop. Under the action of the changing magnetic field, an induced current will appear in the shielding layer, and the signal loop is interfered by the coupling between the shielding layer and the core wire. If the system is confused with other grounding, the generated ground loop may generate unequal potential distribution on the ground, which affects the normal operation of the logic circuit and analog circuit in the PLC. The logic voltage interference tolerance of PLC operation is low, and the distribution interference of logic ground potential easily affects the logic operation and data storage of PLC, resulting in data confusion, program running or crash. The distribution of the analog ground potential will result in a decrease in measurement accuracy, causing severe distortion and malfunction of the signal measurement and control.
â— Safely or power grounded
Ground the power cord grounding terminal and the cabinet wiring to a safety ground. If the power supply is leaking or the cabinet is charged, it can be introduced into the ground from the safety ground without causing harm to people.
â— System grounding
The PLC controller is grounded in order to be at the same potential as the controlled devices, and the system is grounded. The grounding resistance value shall not be greater than 4Ω. Generally, the PLC equipment system ground and the negative terminal of the switching power supply in the control cabinet shall be connected together as the control system ground.
â— Signal and shield grounding
Generally, the signal line must have a unique reference ground. When the shielded cable encounters a possibility of conductive interference, it must be grounded locally or in the control room to prevent the formation of a "ground loop." 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 joint 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 shielded cable, the shield layers should be connected to each other and insulated, and a single point contact at the appropriate grounding point should be selected.
(5) Suppression of inverter interference
The interference handling of the inverter is generally as follows:
The isolation transformer is mainly used for conducting interference from the power supply, and can block most of the conducted interference before the isolation transformer.
The filter is used, the filter has strong anti-interference ability, and also prevents the interference of the device itself from being transmitted to the power supply, and some also have the function of peak voltage absorption.
The use of output reactors, adding AC reactors between the inverter and the motor is mainly to reduce the electromagnetic radiation generated by the inverter output during the energy transmission process, affecting the normal operation of other equipment.
V. Conclusion
The interference in the PLC control system is a very complicated problem. Therefore, in the anti-interference design, all aspects should be considered comprehensively, and the anti-interference should be effectively and effectively suppressed to enable the PLC control system to work normally. With the continuous expansion of PLC application fields, how to use PLC efficiently and reliably has become an important factor in its development. In the 21st century, PLC will have greater development, the product variety will be more abundant, the specifications are more complete, through the perfect human-machine interface, complete communication equipment will better adapt to the needs of various industrial control occasions, PLC as automatic control An important part of the network and the international common network will play an increasingly important role in the field of industrial control.
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