Anti-interference and Fault Analysis of Control System

Abstract

There are various reasons for interference. The most important principle is that the wiring of strong current and weak point should be separated. There are many other requirements. It is impossible for us to list them here. Here are some common anti-interference methods.

13.1 Anti-interference Measures

There are various reasons for interference. The most important principle is that the wiring of strong current and weak point should be separated. There are many other requirements. It is impossible for us to list them here. Here are some common anti-interference methods.

13.1.1 Common Mode Interference

In the practical application of automation engineering, it often occurs that the sensor signal needs to be sent to several places. For example, the local display instrument needs to display important pressure signals on site, and the PLC also needs to collect the pressure signal, so that there will be sharing question of sensor signals. When the 4–20 mA, 0–10 mA, 1–5 V measurement signals of more than one channel in the field need to be sent to multiple controllers at the same time. If no appropriate measures are taken, the controller may not be able to detect the correct signal due to common mode interference. Input signal, signal overflow or abnormal phenomenon. In Fig. 13.1, two 4–20 mA sensor signals are sent to PLC and RTU at the same time.

Fig. 13.1

The sensor signal is sent to PLC and RTU at the same time

For the current signal output by the sensor, as long as the sum of the input impedance of PLC1 and RTU1 is not greater than the output load impedance required by the pressure sensor. The signal of the pressure sensor P1 can be input to PLC1 and RTU1 in series at the same time. If there is only P1 signal, PLC1 and RTU1 It can also receive the signal of P1 normally. When the 4–20 mA signal of the liquid level sensor is also sent to PLC1 and RTU1 at the same time, the problem may come, because the (−) end of the signal received by PLC1 is at the (+) end of RTU1 terminal. When the signal currents of P1 and L1 are different, it is likely to cause a large difference in the potential of the two (−) terminals of PLC1, which forms a common-mode interference voltage. If the input terminals of PLC1 are not isolated from each other, it will cause the analog signal input terminal of PLC1 to receive abnormally. The two receiving signals of RTU1 are behind PLC1, and the potential of its (−) terminal can be kept at the state set by the internal circuit of RTU1, which is not a big problem.

The example shown in Fig. 13.1 can be solved by installing an isolation transmission module on the PLC1 side, and the isolation transmission module is connected as shown in Fig. 13.2. The potential between the input and output of the isolated transmission module is isolated, and the input, output, and power supply of some isolated modules are all isolated.

Fig. 13.2

Isolation and transmission module for one side

After the isolation transmission module is connected, one of the two (−) terminals of PLC1 is in an isolated state, so there will be no two (−) terminals with one high and one low. Which will cause the reference potential of the PLC internal circuit to be disordered. The power supply voltage of the isolation module is mostly DC24V, and some are AC220V.

13.1.2 Signal Transmission Interference by Other Means

Since the input and output of the isolation module are isolated, the isolation methods include photoelectric methods, and isolation transformers are also used to isolate the metal signal lines from the field. Due to electromagnetic induction or coupling between lines, the lines carry electromagnetic waves. For interference, we can also use isolation methods to keep these interference signals away from the control system. However, the photoelectric isolation method has a limited effect on limiting the interference that has already generated current reflection on the line. This is because the current generated by the interference on the input side has become part of the signal, and the isolation module cannot distinguish whether it is an interference signal or a useful signal. High-frequency interference can be solved by filtering, but for low-frequency interference, the isolation module is not easy to deal with.

If funds permit, an isolated transmission module can be connected to each signal input terminal. The isolation module has various forms such as 1–5 V input 4–20 mA output, 4–20 mA input 1–5 V output, 1–5 V input 1–5 V output, 4–20 mA input 4–20 mA output, etc.

There are also many signal transmission modules themselves with photoelectric isolation function, it is no longer necessary to add isolation modules for such lines. The connection method of the single-channel isolation module is shown in Fig. 13.3. The power supply of the isolation module has specifications such as DC12V, DC24V and AC220V. In actual use, generally for a set of control systems, it is more convenient to use only one isolation module in the form of power supply. The output signal of the isolation module is sent to the PLC and generally converted into the same signal.

Fig. 13.3

Multiple use of isolation modules

13.1.3 Communication Interference

• 1. Communication ports RS232, RS485, etc. between controllers such as PLCs and touch screens, etc., are isolated and then connected by photoelectric isolation modules to avoid system signals not normally transmitted due to ground potential differences between multiple devices.

• 2. Ground the 0 V of each device directly to the nearest ground, which can solve the problem that the 0 V of each device has a large voltage to the ground caused by induction. However, the application of this method requires that all analog signal types of the system allow 0 V to be grounded, otherwise it needs to be grounded. After signal isolation, 0 V is grounded, and the 0 V of each power supply can also be connected to the ground terminal through a 0.47 μF and a 100 μF in parallel to reduce the excessively high voltage floating between 0 V.

• 3. When multiple PLCs, touch screens, computers, synchronizers and other devices communicate with each other, it is often easy to cause unreliable communication. Using products from the same manufacturer can improve reliability. Due to different 0 V, poor shielding or Due to the interference caused by the communication design of the manufacturer’s product itself. When the transmitted data is disturbed and defective, resulting in unreliable data and random data, you can put a number in two addresses, and the numbers in the two addresses are equal. The method used removes unreliable data.

• 4. In intermittent communication such as wireless communication, the same number is transmitted twice, and each time the number is placed in two addresses. In this way, only when the four numbers are equal can the control command be used to execute the action, which can avoid misoperation.

• 5. The same data can only be used within a credible range. When it is greater than the possible maximum value allowed by the process, or less than the possible minimum value allowed by the process, use the value passed last time.

• 6. Strictly restrict the execution of the control function. Only when the conditions are accurately met, the program will be executed. Some irrelevant conditions may affect the output control. When the conditions are not satisfied, the control will not be executed, even if there is interference. There will be no false action to reduce the probability of false action.

• 7. Many buses with RS485 hardware protocol as the bus structure, such as MPI, Profibus, CAN, etc., cannot fork too long. For analysis and solutions, see the previous bus part.

• 8. For the bus structure, if there are repeaters, the bifurcated lines of two adjacent repeaters are not allowed to be too long, otherwise there will be abnormal communication problems. For the analysis and solution, see the previous bus part.

13.1.4 Signal Connection and Conversion Between 4-Wire Sensor and 2-Wire Sensor

The four-wire sensor means that the 2 power lines and the 2 signal lines of the sensor are separated. If the (−) end of the power line and the (−) end of the signal line share (such as 0 V), the four-wire sensor can also be used. There are 3 wires, as shown in Fig. 13.4.

Fig. 13.4

Four-wire sensor

The two-wire sensor means that the two wires on the sensor provide power to the sensor, and at the same time, the two wires provide a 4–20 mA signal output. The wiring method of the two-wire sensor is shown in Fig. 13.5.

Fig. 13.5

Two-wire sensor

If the 2-wire pressure sensor is connected to the analog input module of the PLC, the signal input selection block on the analog input module must be pried up and turned to the 2-wire position. If the 4-wire pressure sensor is connected to the analog input module of S7-300, the signal input selection block on the analog input module must be turned to the 4-wire position.

For two-wire sensors, the (−) terminal is also the signal output terminal, and the signal current 4–20 mA is output through this terminal. The (+) terminal of the two-wire sensor is equivalent to the (+) terminal of the power supply, so where are the (−) terminal of the power supply terminal and the (−) terminal of the signal? In fact, the (−) end of the power supply and the (−) end of the signal are moved to the PLC on the signal receiving side.

Take the connection between the two-wire sensor and the S7-300 analog input block as an example, as shown in Fig. 13.6.

Fig. 13.6

Connection of two-wire sensor signals

In Fig. 13.6, the (+) terminal of the two-wire sensor is connected to the (+) of the SM331 block on the S7-300, the voltage of the (+) terminal is close to the internal power supply +24 V. The (−) terminal of the SM331 passes through a resistor R is connected to the internal power supply 0 V. The current signal output by the pressure sensor flows through the resistor R, the signal is collected and measured on the resistor R, and the 0 V of the power supply and the (−) terminal of the signal are moved to the PLC.

If the isolation module is used to receive 2-wire signals, the sensor and the isolation module share the same power supply, and the SM331 module of the S7-300 is set to the 4-wire receiving position. The wiring diagram is shown in Fig. 13.7.

Fig. 13.7

Connection of two-wire sensor and isolation module

If the isolation module is used to receive 2-wire signals, the sensor and the isolation module do not share the same power supply, and the SM331 module of the S7-300 is set to the 4-wire receiving position. The wiring diagram is shown in Fig. 13.8.

Fig. 13.8

Connection of two-wire sensor and isolation module (dual power supply)

13.1.5 Power Isolation and Sharing of Isolation Modules

After adding the isolation module, the power supply on the sensor can no longer be connected casually, otherwise, the isolation module will not work at all, why? Because the signal input and signal output of the isolation module must be isolated, but there are three possibilities for the power supply on the isolation module. Depending on the products of different manufacturers, the power supply is isolated from the input signal, the power supply is isolated from the output signal, and the power supply is isolated from the input signal and the output signals.

• 1. If the power supply of the sensor and the power supply on the isolation module share the same power supply, after the signal is input to the isolation module. If the signal input of the isolation module is connected to the public power supply, the input signal of the isolation module is no longer isolated from its own power supply.

  • (1) If the isolation module is selected as the one that does not isolate the output signal and power supply, the use of such an isolation module will have no effect, but only adds a possible failure point;

  • (2) If the isolation module is selected If the signal input and power supply are not isolated, then it is no problem to use such an isolation module;

  • (3) If the isolation module is the one that isolates the input, output and power supply, then such an isolation module is even more problematic to use, It’s just that the input and power supply are no longer isolated.

• 2. If the power supply of the sensor and the power supply on the PLC share the same power supply, the isolation module has no isolation meaning, because the signal of the field sensor has been connected to the analog input module of the PLC on the signal receiving side through the power supply of the PLC. This is the most important thing to be noticed. Some enterprise technicians have encountered such problems and found that adding an isolation module has no effect. In places where the interference is not strong and isolation is not required, the isolation module may not be used.

• 3. If an isolation module is used, if possible, the power supplies of all isolation modules can share one power supply. All on-site sensors can share one or more power supplies (depending on the distance of the sensor, the strength of on-site interference, etc.). One or more power supplies are used on the PLC side (depending on the safety and current load conditions), and the three-party power supplies are best not to be electrically connected.

13.1.6 Inverter Interference

The sine wave output by the frequency converter is superimposed by many high-frequency square waves, which contains a large number of harmonic components. The equipment cannot work normally, and sometimes the frequency converter itself often has ground faults, resulting in failure to work normally. The high-frequency harmonic interference of the frequency converter has become a public nuisance.

The line from the frequency converter to the controlled motor is long, the insulation of the cable is good. Since the metal wire in the cable and the ground at the cable form a distributed capacitance, the high-frequency harmonics of the frequency converter will still form a displacement current and enter the ground. No matter how well insulated the cable is, it will not help, because this is not a problem caused by the insulation strength of the cable, which will cause the sum of the three-phase current vectors entering the inverter to be non-zero, as shown in Fig. 13.9, which may cause the leakage protection switch to trip. Malfunction or cause the inverter to stop due to a motor ground fault, and other electrical equipment connected to the line will also introduce interference through the ground terminal.

Fig. 13.9

Inverter output interference

To reduce the frequency converter interference, the measures that can be taken are:

• 1. The inverter must be grounded correctly and reliably according to the instructions.

• 2. Set the carrier frequency of the inverter as low as possible to reduce the harmonic radiation intensity, reduce the displacement current formed between the inverter output and the earth, avoid tripping of the leakage switch. Reduce the potential formed by the inverter output to other ground terminals through radiation Interference and the interference introduced into the system, and also reduce the frequency converter’s interference to video signals (such as closed-circuit television).

• 3. The output side of the inverter is connected to an output reactor to reduce the electromagnetic radiation of the cable, reduce the displacement current formed between the inverter output and the earth, avoid tripping of the leakage switch, and reduce the potential formed by the inverter output to other ground terminals through radiation Interference and the interference introduced into the system, and also reduce the interference of the frequency converter to the video signal, see the chapter of the frequency converter.

• 4. The analog input and output signals connected to the frequency converter are isolated with an isolation module, and the switch signals are isolated with an intermediate relay.

• 5. Add an input reactor and a radio interference suppressor to the power input side of the frequency converter, see the chapter on the frequency converter, to reduce the harmonic pollution of the frequency converter to the grid side. When multiple frequency converters share a power grid, the input reactor can also reduce mutual influence and malfunction.

• 6. The power cable on the input side of the inverter is armored cable, and the metal armored inverter side is grounded to reduce electromagnetic radiation and also reduce the interference of the inverter to video signals.

• 7. The power cable on the output side of the inverter is armored cable, and the metal armored inverter side is grounded to reduce electromagnetic radiation and also reduce the interference of the inverter to the video signal.

• 8. If the frequency converter is used, the interference to the closed-circuit television monitoring signal is very serious. The technical personnel responsible for the closed-circuit television project should consider whether to add an anti-interference frequency conversion device, and first modulate the on-site video signal to the interference band of the frequency converter. In addition, make it much higher than the frequency of the interference signal, and then add a demodulator at the receiving end to restore the high-frequency signal back to the normal video signal.

13.1.7 Power Interference

Many interference signals are propagated through power lines. For control lines and control devices, the following measures can be used to reduce the impact of interference on the system.

• 1. Its power supply can be powered by a 1:1 isolation transformer and the shield of the isolation transformer is grounded reliably. Note that the AC output after isolation has no ground terminal, so there is no fire or zero, so touching any one of the wires will not cause electric shock, as shown in Fig. 13.10.

  Fig. 13.10

  

  Isolation transformer

• 2. For a more complex system, if there are several controllers such as PLC or many signals from different sites inside, in order to increase the anti-interference ability, it is best to use its own independent power supply for each part. Try to avoid each part make an electrical connection, because any electrical connection, whether it is 0 V or +24 V, will cause interference signals to enter each other! If you need to use five 1A power supplies to separate the electrical connections of each part, you cannot use one 5A power supply to make the controllers of each part public to replace.

  • 3. Use a single-phase or three-phase power filter to block the interference from the power system.

13.1.8 Anti-interference of Sensor Output Signal

The weak current signal from the sensor to the PLC (or other controllers) can be filtered by resistance–capacitance to reduce the influence of interference (signal noise), as shown in Fig. 13.11.

Fig. 13.11

RC filter

The output signal of the sensor, whether it is voltage or current, is filtered by R and C resistors, the high-frequency interference signal in the signal is filtered out, and the output signal is smooth. If the sensor is a voltage signal, the resistance R can be larger, ranging from 1 K to hundreds of K, and the capacitance C is from 0.1 to 10 μF. If the sensor outputs a current signal, the sum of the resistance R and the input resistance of the PLC side cannot be greater than the sensor the maximum load resistance value. In most cases R ≤ 500Ω, the value of capacitor C is between 0.10 and 10 Μf. After adding resistance–capacitance filtering, the signal reflection speed will slow down, for occasions with high reflection speed requirements (such as fast precision transmission), cannot be handled in this way.

13.1.9 Digital Input of the Controller

Sometimes the switching value input of PLC or other controllers is affected by external interference, and the input error occurs instantaneously, causing the PLC to malfunction. Eliminate interference such as electrostatic induction or electric field coupling, as shown in Fig. 13.12. For an effective input connected to +24 V, the capacitor and resistor are grounded, and for an effective input connected to 0 V, connect the resistor and capacitor in parallel and then connect to +24 V.

Fig. 13.12

Eliminate the interference of digital input

13.1.10 Electrical Circuit Control Failure

When using buttons and switches to start and stop an AC motor (or equipment) far away, sometimes you want to turn it off but you can’t. The circuit is shown in Fig. 13.13.

Fig. 13.13

Remote AC contactor control failure

Since the switch K is far away from the AC contactor KM, the two wires are very long and the distributed capacitance C will become larger. In the AC circuit, there will be a displacement current flowing in this distributed capacitance, even if the switch K is turned off. It may be that the KM cannot be released because the energy maintained by the KM does not need to be too large, and the equipment cannot stop. In this case, the following solutions can be adopted:

• 1. It can be remotely controlled by DC signal.

• 2. Add a resistor to the coil of the AC contactor KM, so that the current flowing through the distributed capacitance is not enough to maintain the pull-in of the coil.

13.2 Selection of Signal Lines and Shielding Grounding Issues

For the transmission of weak signals, if a pair of loops with equal currents and opposite directions can be formed, it is best to use twisted-pair wires, so that the wire itself has a certain anti-interference ability. Because the induced voltage formed by two similar paired wires is just opposite, itself offsets the interference signal coupled in from the outside world, and if coupled with good shielding, its anti-interference ability will be stronger.

The shielding layer of most weak current signal lines can be grounded at one point on the receiving signal side (such as the PLC side), or not grounded on both sides, depending on the anti-interference effect of the actual site. Most books and materials introducing anti-interference emphasize shielding A little grounding principle, but we have also found cases where the grounding effect is better on both sides of the shielded wire in practice. If one end cannot be grounded, the other end of the shield can also be grounded with a 0.1 μ capacitor.

13.3 Failure Analysis

The sequence of failure analysis is generally as follows:

• 1. The first is to analyze the power supply part: measure the power supply to see if there is electricity or lack of phase. If the power supply is abnormal, check whether the circuit breaker of the power supply is tripped, whether the fuse or fuse of the secondary control line is blown, and whether the power switch Whether the contacts are good. In actual work, many people often ignore this step. The equipment that has not been repaired for several days may be that a fuse is broken or the contacts of the power switch are in poor contact, or there is no power at all. If the power supply of the device is normal, this step can be skipped.

• 2. Check the input part of the equipment: In the closed-loop automatic control system, if there is no input signal or is abnormal, the system cannot work normally, just like a normal person walking. If his eyes have problems, he will not be able to walk naturally. Normally, check whether the input sensor is faulty or disconnected. In electrical control, if the contact of the function input button is abnormal or the self-protection contact of the relay is not in good contact, the electrical control system cannot work normally. This step can be skipped if all the input signals are normal or the function control buttons and self-protection contacts are normal.

• 3. Check the output part of the equipment: If the output signal of the controller is available, but the actuator (such as a frequency converter) does not act. It means that there is a problem with the execution part or the connection to the actuator. In electrical control, check whether the power supply to the motor and other electrical equipment is normal, and whether there is a phase loss problem. If it is normal, it means that there is a problem with the electrical equipment (or motor). If the output signal of the controller is normal, skip this step.

• 4. Check the intermediate circuit and the main controller: start from the power supply and start from top to bottom, check the intermediate circuit to see which component has the power failure or phase loss, and then solve it. For the main controller (such as PLC), first check whether the action of the output port and the output signal are normal, and if it is normal, then focus on checking the program to see where there is a problem.

• 5. After a fault occurs, unless there is danger, do not rush to reset, carefully check whether the signals of each link are normal or not, and use the detection function of the programming software on the computer to diagnose the cause of the fault.

13.4 Lightning Protection Measures

Lightning sometimes causes devastating damage to the automatic control system. Many lightning strikes are transmitted to the system through outdoor sensors, especially cables that transmit signals through high overhead lines are more likely to be struck by lightning. The sensors are located near the liquid surface in a large area. The sensor is easy to damage the sensor because the charge induced by the liquid level does not have a good discharge channel, but more lightning strikes are transmitted through the power system.

The measures for power and signal lightning protection of the automatic control system are shown in Fig. 13.14. The ground terminal of the arrester must have a good ground connection. In the figure, arrester 1 is a three-phase power arrester, and arrester 2 is a signal lightning protection module.

Fig. 13.14

Lightning protection measures

13.5 Communication Port Crash Problem

When using a custom communication port, sometimes the communication port crash problem occurs, and resetting the communication port does not work, it can only be restored by disconnecting the power supply, such as the RS232/485 communication module of Siemens SMART and the CP340 equipped with S-300PLC series, this problem has occurred. The author finally found a solution, that is, once the communication port is found to be down, modify the parameters of the communication port to other parameters, and then modify the parameters of the communication port back to solve this problem.