In order to improve the anti-interference ability of the control system, besides the anti-jamming measures that should be considered when designing the electronic circuit, structure, and software of the control system body, it is more important how to improve the anti-jamming technology of the control system in engineering applications because We still cannot fully implement the control system's anti-jamming performance on the control system itself.
The application of the control system in the project will inevitably encounter various kinds of noise, and the noise will interfere with the normal operation of the control system through various misfortunes. How to effectively suppress the generation of noise and the effect of noise in the transmission route is the entire content of the anti-jamming technology of the control system in engineering applications.
First, the three elements that make up the noise problem All electronic signals other than the wanted signal are collectively referred to as noise. When the noise voltage is large enough to cause disturbances in the reception, a circuit or system will malfunction. This is interference.
The typical noise path is shown / shown. The noise problem, including the three sources of noise source, noise propagation path, and susceptor, must first define the following three issues when dealing with the control system's anti-jamming problem:
(1) What is the source of noise?
(2) Which are sensitive to noise?
(3) What is the coupling approach to transfer noise from source to susceptor?
After answering these three questions, we can begin to solve the noise problems encountered. In general, suppressing the intensity of an objectively existing noise source is the most effective method. But not all noise sources can be suppressed, such as lightning strikes, radio antenna emissions, and so on. To increase the susceptibility of a susceptor to interference, it depends on the electromagnetic compatibility (EMC) of the control system body. Reducing or intercepting the noise transmitted through the coupling path, that is, reducing the transmission of noise on the propagation path, is the measure that the control system should take in engineering applications.
In physical terms, there are roughly 5 types of noise transmission:
(1) Direct conductive coupling of wires. Refers to the noise through the signal line and AC and DC power lines and communication lines, etc., to directly transmit noise from the signal source or power supply to the system.
(2) Common impedance dissipation. There is a common impedance between the noise source circuit and the victim circuit. The noise current generated by this common impedance is conducted to the victim circuit.
(3) Capacitive damage. Also known as electrostatic coupling or electrostatic induction. The main reason for this is that the circuit has distributed capacitance.
(4) Inductive depletion. Also known as electromagnetic coupling or electromagnetic induction. The main reason for this coupling is the mutual inductance between circuits.
(5) electromagnetic field radiation. Also known as radiation dissipating or far-field radiation, it is a combination of electric and magnetic fields, and it interferes with the circuit through energy radiation.
The first four types of combinations are called conductive couplings. Among them, capacitive coupling and inductive dissipation are also called near-field radiation.
Strictly speaking, the solution to the noise problem can only be obtained through Maxwell's equations, but it is very complicated. In engineering, the "circuit" theory is also used to approximate the solution by lumping parameters. So we took the following assumptions:
(1) Using a capacitor connected between two conductors to represent a time-varying electric field between two conductors;
(2) Using a mutual inductance connected between two conductors to represent a time-varying magnetic field coupled between two conductors.
Second, the control system in the engineering application of the most common classification of electromagnetic interference In order to effectively assess the interference effects and hazards, "IEC6l000-4 electromagnetic compatibility test and measurement technology," the control system in engineering applications common electromagnetic interference Their properties are classified, and the interference test models and test levels are regulated accordingly. Different test levels indicate different levels of immunity. The electromagnetic interference classified in “IEC6l000-4†is briefly described below.
(1) The control system of the short-term voltage interruption or temporary dips connected to the low-voltage power grid, due to the failure of the power supply network and substation equipment, or sudden changes in load and even continuous changes in load, until the control system can switch standby power. Causes voltage dips and short interruptions.
For DCS, PLC and other control systems, they have permissible voltage sags and voltage interruptions (such as 10ms or 20ms). The design of the control project lies in how to ensure that the actual voltage sag value and the voltage interruption time should be less than the allowable value of the normal operation of the control system.
(2) Electrical fast transients Pulse group electrical fast transient bursts (referred to as "group pulses") originate from the transient process of switching. Its spectral range is (l-l0O) MHz, sometimes up to 30OMHz.
“lEC6l000-4†stipulates the test waveforms of electrical fast transients, characterized by fast rise time (50ns), short duration (5Ons), low energy, but high repetition frequency.
Group pulses always act on various ports of the control system through a variety of dissipative paths. Its severity can be expressed in terms of voltage peaks and repetition rates.
(3) The main sources of surge surges are lightning (including direct lightning strikes and lightning electromagnetic impulses, voltage surges and current surges caused by electrostatic induction and electromagnetic induction at power ports, I/O ports and communication ports, etc.) and including power supplies. System switching transients and interference caused by various system failures.
Surge is characterized by rapid changes in the rising edge (edge ​​time μs), high transient power, high peak, so the greatest threat.
(4) Static electricity Static electricity is generated by the contact and separation of any object. The human body is the main electrostatic discharge head.
Electrostatic charge is a very low energy accumulation, stored in capacitive mode on the surface of the human body or equipment, and triggered by burst contact to make its energy storage discharge at an extremely high rate of collapse. Its frequency bandwidth can range from hundreds of MHz to several GHz. The discharge process is shown in Figure 2. As can be seen from the figure, since the rising edge of the ESD waveform is very short, the minimum is up to about 1 ns, the rate of change of the current is large, and the peak current can reach tens of amps. The electromagnetic radiation field can induce hundreds of nearby wires. Volts are even thousands of volts, so the impact on the circuit is great, especially for CMOS devices.
(5) Power frequency magnetic field Power frequency magnetic field is the most common source of interference.
The power frequency magnetic field can be divided into two situations: (a) The stable magnetic field produced by the current under normal operating conditions has a small amplitude of about (1-l0O) A/m. (b) The current in the event of a fault can generate a magnetic field with a higher amplitude but a shorter duration until the protection device operates (300-1000 A)/m.
(6) Pulsed Magnetic Fields Pulsed magnetic fields are generated by lightning strikes on buildings and other metal structures, as well as initial transients in faults in low, medium, and high voltage power systems. For the control system, the greatest threat is the pulsed magnetic field generated in space during a lightning strike.
(7) Radio frequency electromagnetic field radiation Radio frequency electromagnetic field radiation originates from the following conditions:
(a) The operation, maintenance and inspection personnel of the system are using mobile phones or walkie-talkies;
(b) Including the role and influence of radio, television transmitters, transmitters and various sources of industrial electromagnetic radiation.
(8) Conducted disturbance of RF field induction The main source of induced disturbance of RF field induction is in the electromagnetic field of radio frequency transmission equipment such as switching power supply and frequency converter. It is generated on the power line, communication line and interface cable of the control system. Conducted harassment. Its frequency range is (9k~80M)Hz.
Third, the basic approach to control engineering anti-jamming In engineering, in addition to simple circumstances, anti-jamming solutions often require comprehensive measures. It includes: grounding and equipotential bonding of control systems; electrostatic shielding and electromagnetic shielding of cables; grid shielding of control rooms; surge protection of control systems; signal processing, filtering and isolation; electrostatic protection of control systems; System's anti-jamming design; control room's working environment design, etc.
This article only briefly describes some of these measures.
1. Grounding and equipotential bonding of the control system Functionally speaking, there are two functions of grounding:
(1) Protection of equipment and personal safety, such as protection of land, lightning protection, on-site safety, anti-static ground, etc.;
(2) Suppression of interference provides a stable potential reference point for the signal voltage or system voltage.
The same grounding device often has multiple uses.
Grounding technology is passed as a basic safety technology. But in the past ten years, the grounding system has undergone great changes in both concept and technology. The most important change is whether the previous grounding system passed the grounding resistance value, but now it focuses on the grounding structure taking into account the grounding resistance value. In particular, the control system has changed from the conventionally used independent grounding to the use of a common grounding net to achieve equipotential bonding.
2. Electrostatic Shielding and Electromagnetic Shielding of Cables It is a well-known fact that twisted-pair shielded cables are used in the project to suppress the capacitive coupling and inductive coupling of noise during signal transmission. However, in the corresponding national standards and industry standards, there is no provision for the selection of the twist length of the twisted-pair cable.
The table compares the effects of several different twisted pairs. The degree of noise attenuation in the table refers to the ratio of the disturbing magnetic field value when the parallel conductors are used and the disturbing magnetic field value after the twisted pair is used. As can be seen from the table, the shielding effect of the twisted pair increases with the increase in the number of strands per unit length. However, the shorter the pitch, the higher the cost of the cable. According to the data in the table and the standards of some foreign companies, it is advisable to use a twisted pair cable with a twist pitch of about 5Omm.
For the grounding of cable shields, many industry regulations state that one end should be grounded and the other end should be left unconnected. However, the single-end grounding can only prevent static induction, and it cannot suppress the intrusion of lightning waves during a lightning strike.
For this reason, in addition to the grounding of one end of the inner shield layer, an outer shield layer with insulation and insulation should also be added, and the outer shield layer should be grounded at equal potentials at least at both ends. During the lightning strike, the outer shield layer forms a loop with the ground and induces a current. The magnetic flux generated by the current cancels or partially cancels the magnetic flux of the source magnetic field during the lightning strike, thereby suppressing or partially suppressing the voltage induced when there is no outer shield.
Usually, metal wire troughs or metal tubes can be used as the outer shield, but it must be ensured that the groove and groove or between the metal tube and the metal tube is well connected and the two ends are grounded. A grounding point).
It has been proved that the outer shield is directly buried (or reinforced concrete cable trenches shielded with metal mesh), especially at a distance L from the control room
Inside, it will receive very good lightning protection. L should meet the requirements of the following expression, but should not be less than l5m.
Where: p - soil resistivity (Ω.m) at the buried cable.
3. Metal mesh shielding in the control zone The control system in the control room is often affected by various magnetic fields, in which the pulsed magnetic field of lightning electromagnetic radiation is the most dangerous.
The shielding methods of the control room include several types, such as the shielding of the building itself, the shielding of the metal mesh, and the shielding of the housing surrounded by sheet metal. The effect of shielding the shell is good, but the investment is also large, suitable for laboratory devices. The building itself has a certain function of shielding, but the effect is not ideal. The grid shield can be used to meet the needs of the control system through the choice of grid width, and is most practical.
The grid size can be calculated based on the magnitude of the pulsed magnetic field immunity of the control system. However, there is a problem that is often overlooked. Once grid shielding is used, the control system's cabinet should maintain a safe distance from the side walls of the grid. The size of the safety distance depends on the size of the grid and can be calculated by calculation.
4. Surge protection of the control system The main cause of surges is lightning electromagnetic pulses. The lightning protection of the control system needs to be considered in many aspects such as grounding, equipotential bonding, shielding, and reasonable wiring.
In terms of engineering, the use of surge protectors (SPDs) has become the main method of lightning protection for retrofit projects due to the difficulty in changing the selection of grounding systems and cable selection and laying. For newly-built projects, the SPD settings should be determined based on factors such as the degree of protection of the control system, the importance of protected ports (parameters), and the length of cables laid outside. Can not be designed in accordance with the requirements of "failsafe", we must consider "economic reasonable."
5. Electrostatic Protection of the Control System According to data, 15% of electronic device hardware is damaged by static electricity. Of these 15%, another 90% is due to the potential failure caused by static electricity. This type of failure is difficult to detect in advance and can only be allowed to occur and accumulate in the life cycle of the components, which greatly reduces the reliability of components, components, or the entire machine. The danger is greater than the sudden failure. Therefore, ESD protection technology plays an important role in the research field of electromagnetic compatibility (EMC) or the subject of electrical overstress (EOS).
In addition to the consideration of the electrostatic protection of the control system during the development and design phase of the system, the following two points should be noted in engineering applications.
(1) Keep the relative humidity in the control room between 40-60%. Some sources believe that if the relative humidity is controlled at about 65%, because the surface of the object has a thin layer of water film, the static charge can be slowly released through the water film layer, and static electricity cannot be formed.
(2) If the object is already carrying charge, the discharge of the object should be performed slowly to limit the rate of change of the discharge current. All objects that come in contact with humans, including floors, workbenches, etc., should be considered with regard to their choice of surface resistivity.
Electrostatic conductor materials with a surface resistivity of less than 10 4 Ω/m 2 dissipate the charge most quickly. It is easy to release the charge carried on the surface by using a grounding method. However, due to rapid discharge, the peak discharge current is very large. Once the discharge channel is close to the charged control system, some damage may occur. Therefore, it should not be used as an antistatic material. The surface resistivity of the static insulating material is greater than 10 4 Ω/m 2 , and it does not dissipate charge. It is strictly prohibited to use it in an electrostatic sensitive environment.
Static dissipative materials have a slower and safer dissipative charge than electrostatic conductors. Grounded static dissipative materials can also be used to prevent the accumulation of static electricity, which can be safely discharged once the object is charged. It is generally believed that the surface resistivity of anti-static materials should not exceed 104Ω/m2.
IV. Concluding remarks In the past few years, the author through the investigation and research of the anti-jamming technology in the control engineering, quite experienced:
(1) Most of the interference problems that control systems actually encounter in engineering tend to originate from those basic sources of noise and the basic coupling pathways. Any interference phenomenon can be explained by some basic physical concepts.
(2) Noise usually cannot be eliminated. It is only minimized to the extent that no interference is formed. A single solution to reduce noise problems often does not exist and usually requires comprehensive measures.
(3) Considering the problem of interference suppression at the engineering design stage, the technical methods used are many and simple, and the cost is low. If problems are found in the process of being put into operation and then resolved, it will require higher costs and efforts, and it may not even be completely resolved.
The application of the control system in the project will inevitably encounter various kinds of noise, and the noise will interfere with the normal operation of the control system through various misfortunes. How to effectively suppress the generation of noise and the effect of noise in the transmission route is the entire content of the anti-jamming technology of the control system in engineering applications.
First, the three elements that make up the noise problem All electronic signals other than the wanted signal are collectively referred to as noise. When the noise voltage is large enough to cause disturbances in the reception, a circuit or system will malfunction. This is interference.
The typical noise path is shown / shown. The noise problem, including the three sources of noise source, noise propagation path, and susceptor, must first define the following three issues when dealing with the control system's anti-jamming problem:
(1) What is the source of noise?
(2) Which are sensitive to noise?
(3) What is the coupling approach to transfer noise from source to susceptor?
After answering these three questions, we can begin to solve the noise problems encountered. In general, suppressing the intensity of an objectively existing noise source is the most effective method. But not all noise sources can be suppressed, such as lightning strikes, radio antenna emissions, and so on. To increase the susceptibility of a susceptor to interference, it depends on the electromagnetic compatibility (EMC) of the control system body. Reducing or intercepting the noise transmitted through the coupling path, that is, reducing the transmission of noise on the propagation path, is the measure that the control system should take in engineering applications.
In physical terms, there are roughly 5 types of noise transmission:
(1) Direct conductive coupling of wires. Refers to the noise through the signal line and AC and DC power lines and communication lines, etc., to directly transmit noise from the signal source or power supply to the system.
(2) Common impedance dissipation. There is a common impedance between the noise source circuit and the victim circuit. The noise current generated by this common impedance is conducted to the victim circuit.
(3) Capacitive damage. Also known as electrostatic coupling or electrostatic induction. The main reason for this is that the circuit has distributed capacitance.
(4) Inductive depletion. Also known as electromagnetic coupling or electromagnetic induction. The main reason for this coupling is the mutual inductance between circuits.
(5) electromagnetic field radiation. Also known as radiation dissipating or far-field radiation, it is a combination of electric and magnetic fields, and it interferes with the circuit through energy radiation.
The first four types of combinations are called conductive couplings. Among them, capacitive coupling and inductive dissipation are also called near-field radiation.
Strictly speaking, the solution to the noise problem can only be obtained through Maxwell's equations, but it is very complicated. In engineering, the "circuit" theory is also used to approximate the solution by lumping parameters. So we took the following assumptions:
(1) Using a capacitor connected between two conductors to represent a time-varying electric field between two conductors;
(2) Using a mutual inductance connected between two conductors to represent a time-varying magnetic field coupled between two conductors.
Second, the control system in the engineering application of the most common classification of electromagnetic interference In order to effectively assess the interference effects and hazards, "IEC6l000-4 electromagnetic compatibility test and measurement technology," the control system in engineering applications common electromagnetic interference Their properties are classified, and the interference test models and test levels are regulated accordingly. Different test levels indicate different levels of immunity. The electromagnetic interference classified in “IEC6l000-4†is briefly described below.
(1) The control system of the short-term voltage interruption or temporary dips connected to the low-voltage power grid, due to the failure of the power supply network and substation equipment, or sudden changes in load and even continuous changes in load, until the control system can switch standby power. Causes voltage dips and short interruptions.
For DCS, PLC and other control systems, they have permissible voltage sags and voltage interruptions (such as 10ms or 20ms). The design of the control project lies in how to ensure that the actual voltage sag value and the voltage interruption time should be less than the allowable value of the normal operation of the control system.
(2) Electrical fast transients Pulse group electrical fast transient bursts (referred to as "group pulses") originate from the transient process of switching. Its spectral range is (l-l0O) MHz, sometimes up to 30OMHz.
“lEC6l000-4†stipulates the test waveforms of electrical fast transients, characterized by fast rise time (50ns), short duration (5Ons), low energy, but high repetition frequency.
Group pulses always act on various ports of the control system through a variety of dissipative paths. Its severity can be expressed in terms of voltage peaks and repetition rates.
(3) The main sources of surge surges are lightning (including direct lightning strikes and lightning electromagnetic impulses, voltage surges and current surges caused by electrostatic induction and electromagnetic induction at power ports, I/O ports and communication ports, etc.) and including power supplies. System switching transients and interference caused by various system failures.
Surge is characterized by rapid changes in the rising edge (edge ​​time μs), high transient power, high peak, so the greatest threat.
(4) Static electricity Static electricity is generated by the contact and separation of any object. The human body is the main electrostatic discharge head.
Electrostatic charge is a very low energy accumulation, stored in capacitive mode on the surface of the human body or equipment, and triggered by burst contact to make its energy storage discharge at an extremely high rate of collapse. Its frequency bandwidth can range from hundreds of MHz to several GHz. The discharge process is shown in Figure 2. As can be seen from the figure, since the rising edge of the ESD waveform is very short, the minimum is up to about 1 ns, the rate of change of the current is large, and the peak current can reach tens of amps. The electromagnetic radiation field can induce hundreds of nearby wires. Volts are even thousands of volts, so the impact on the circuit is great, especially for CMOS devices.
(5) Power frequency magnetic field Power frequency magnetic field is the most common source of interference.
The power frequency magnetic field can be divided into two situations: (a) The stable magnetic field produced by the current under normal operating conditions has a small amplitude of about (1-l0O) A/m. (b) The current in the event of a fault can generate a magnetic field with a higher amplitude but a shorter duration until the protection device operates (300-1000 A)/m.
(6) Pulsed Magnetic Fields Pulsed magnetic fields are generated by lightning strikes on buildings and other metal structures, as well as initial transients in faults in low, medium, and high voltage power systems. For the control system, the greatest threat is the pulsed magnetic field generated in space during a lightning strike.
(7) Radio frequency electromagnetic field radiation Radio frequency electromagnetic field radiation originates from the following conditions:
(a) The operation, maintenance and inspection personnel of the system are using mobile phones or walkie-talkies;
(b) Including the role and influence of radio, television transmitters, transmitters and various sources of industrial electromagnetic radiation.
(8) Conducted disturbance of RF field induction The main source of induced disturbance of RF field induction is in the electromagnetic field of radio frequency transmission equipment such as switching power supply and frequency converter. It is generated on the power line, communication line and interface cable of the control system. Conducted harassment. Its frequency range is (9k~80M)Hz.
Third, the basic approach to control engineering anti-jamming In engineering, in addition to simple circumstances, anti-jamming solutions often require comprehensive measures. It includes: grounding and equipotential bonding of control systems; electrostatic shielding and electromagnetic shielding of cables; grid shielding of control rooms; surge protection of control systems; signal processing, filtering and isolation; electrostatic protection of control systems; System's anti-jamming design; control room's working environment design, etc.
This article only briefly describes some of these measures.
1. Grounding and equipotential bonding of the control system Functionally speaking, there are two functions of grounding:
(1) Protection of equipment and personal safety, such as protection of land, lightning protection, on-site safety, anti-static ground, etc.;
(2) Suppression of interference provides a stable potential reference point for the signal voltage or system voltage.
The same grounding device often has multiple uses.
Grounding technology is passed as a basic safety technology. But in the past ten years, the grounding system has undergone great changes in both concept and technology. The most important change is whether the previous grounding system passed the grounding resistance value, but now it focuses on the grounding structure taking into account the grounding resistance value. In particular, the control system has changed from the conventionally used independent grounding to the use of a common grounding net to achieve equipotential bonding.
2. Electrostatic Shielding and Electromagnetic Shielding of Cables It is a well-known fact that twisted-pair shielded cables are used in the project to suppress the capacitive coupling and inductive coupling of noise during signal transmission. However, in the corresponding national standards and industry standards, there is no provision for the selection of the twist length of the twisted-pair cable.
The table compares the effects of several different twisted pairs. The degree of noise attenuation in the table refers to the ratio of the disturbing magnetic field value when the parallel conductors are used and the disturbing magnetic field value after the twisted pair is used. As can be seen from the table, the shielding effect of the twisted pair increases with the increase in the number of strands per unit length. However, the shorter the pitch, the higher the cost of the cable. According to the data in the table and the standards of some foreign companies, it is advisable to use a twisted pair cable with a twist pitch of about 5Omm.
For the grounding of cable shields, many industry regulations state that one end should be grounded and the other end should be left unconnected. However, the single-end grounding can only prevent static induction, and it cannot suppress the intrusion of lightning waves during a lightning strike.
For this reason, in addition to the grounding of one end of the inner shield layer, an outer shield layer with insulation and insulation should also be added, and the outer shield layer should be grounded at equal potentials at least at both ends. During the lightning strike, the outer shield layer forms a loop with the ground and induces a current. The magnetic flux generated by the current cancels or partially cancels the magnetic flux of the source magnetic field during the lightning strike, thereby suppressing or partially suppressing the voltage induced when there is no outer shield.
Usually, metal wire troughs or metal tubes can be used as the outer shield, but it must be ensured that the groove and groove or between the metal tube and the metal tube is well connected and the two ends are grounded. A grounding point).
It has been proved that the outer shield is directly buried (or reinforced concrete cable trenches shielded with metal mesh), especially at a distance L from the control room
Inside, it will receive very good lightning protection. L should meet the requirements of the following expression, but should not be less than l5m.
Where: p - soil resistivity (Ω.m) at the buried cable.
3. Metal mesh shielding in the control zone The control system in the control room is often affected by various magnetic fields, in which the pulsed magnetic field of lightning electromagnetic radiation is the most dangerous.
The shielding methods of the control room include several types, such as the shielding of the building itself, the shielding of the metal mesh, and the shielding of the housing surrounded by sheet metal. The effect of shielding the shell is good, but the investment is also large, suitable for laboratory devices. The building itself has a certain function of shielding, but the effect is not ideal. The grid shield can be used to meet the needs of the control system through the choice of grid width, and is most practical.
The grid size can be calculated based on the magnitude of the pulsed magnetic field immunity of the control system. However, there is a problem that is often overlooked. Once grid shielding is used, the control system's cabinet should maintain a safe distance from the side walls of the grid. The size of the safety distance depends on the size of the grid and can be calculated by calculation.
4. Surge protection of the control system The main cause of surges is lightning electromagnetic pulses. The lightning protection of the control system needs to be considered in many aspects such as grounding, equipotential bonding, shielding, and reasonable wiring.
In terms of engineering, the use of surge protectors (SPDs) has become the main method of lightning protection for retrofit projects due to the difficulty in changing the selection of grounding systems and cable selection and laying. For newly-built projects, the SPD settings should be determined based on factors such as the degree of protection of the control system, the importance of protected ports (parameters), and the length of cables laid outside. Can not be designed in accordance with the requirements of "failsafe", we must consider "economic reasonable."
5. Electrostatic Protection of the Control System According to data, 15% of electronic device hardware is damaged by static electricity. Of these 15%, another 90% is due to the potential failure caused by static electricity. This type of failure is difficult to detect in advance and can only be allowed to occur and accumulate in the life cycle of the components, which greatly reduces the reliability of components, components, or the entire machine. The danger is greater than the sudden failure. Therefore, ESD protection technology plays an important role in the research field of electromagnetic compatibility (EMC) or the subject of electrical overstress (EOS).
In addition to the consideration of the electrostatic protection of the control system during the development and design phase of the system, the following two points should be noted in engineering applications.
(1) Keep the relative humidity in the control room between 40-60%. Some sources believe that if the relative humidity is controlled at about 65%, because the surface of the object has a thin layer of water film, the static charge can be slowly released through the water film layer, and static electricity cannot be formed.
(2) If the object is already carrying charge, the discharge of the object should be performed slowly to limit the rate of change of the discharge current. All objects that come in contact with humans, including floors, workbenches, etc., should be considered with regard to their choice of surface resistivity.
Electrostatic conductor materials with a surface resistivity of less than 10 4 Ω/m 2 dissipate the charge most quickly. It is easy to release the charge carried on the surface by using a grounding method. However, due to rapid discharge, the peak discharge current is very large. Once the discharge channel is close to the charged control system, some damage may occur. Therefore, it should not be used as an antistatic material. The surface resistivity of the static insulating material is greater than 10 4 Ω/m 2 , and it does not dissipate charge. It is strictly prohibited to use it in an electrostatic sensitive environment.
Static dissipative materials have a slower and safer dissipative charge than electrostatic conductors. Grounded static dissipative materials can also be used to prevent the accumulation of static electricity, which can be safely discharged once the object is charged. It is generally believed that the surface resistivity of anti-static materials should not exceed 104Ω/m2.
IV. Concluding remarks In the past few years, the author through the investigation and research of the anti-jamming technology in the control engineering, quite experienced:
(1) Most of the interference problems that control systems actually encounter in engineering tend to originate from those basic sources of noise and the basic coupling pathways. Any interference phenomenon can be explained by some basic physical concepts.
(2) Noise usually cannot be eliminated. It is only minimized to the extent that no interference is formed. A single solution to reduce noise problems often does not exist and usually requires comprehensive measures.
(3) Considering the problem of interference suppression at the engineering design stage, the technical methods used are many and simple, and the cost is low. If problems are found in the process of being put into operation and then resolved, it will require higher costs and efforts, and it may not even be completely resolved.
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