Anti-interference Countermeasures for PLC Control Systems

Overview
Various types of PLCs used in automation systems, some are installed centrally in the control room, and some are installed dispersedly on individual equipment at the production site. Although most of them are in the harsh electromagnetic environment formed by strong current circuits and strong current equipment, However, PLC is a control device specially designed for industrial production environments. In the design and manufacturing process, multi-level anti-interference and selected component measures are adopted, so it has strong ability to adapt to harsh industrial environments, operational stability and high Therefore, it can be used directly in an industrial environment without taking any special measures. However, because it is directly connected to the on-site I/O equipment, external interference can easily invade through the power line or I/O transmission line, causing control System malfunction. The interference suffered by PLC can be divided into external interference and internal interference. In the actual production environment, external interference is random and has nothing to do with the system structure, and the interference source cannot be eliminated and can only be restricted according to specific circumstances; internal interference is related to the system structure, mainly through the AC main circuit in the system, simulating Caused by input signals, etc., the system circuit can be reasonably designed to weaken and suppress internal interference and prevent external interference. To improve the reliability of the PLC control system, it is necessary to improve the anti-interference ability of the system from many aspects.
Analyze hardware circuits and propose hardware anti-interference measures
1. Installation and usage environment of PLC control system
PLC is specially designed for industrial control. Generally, it can be used directly in industrial environments without taking any special measures. However, in the PLC control system, if the environment is too harsh or the installation and use are improper, the reliability of the system will be reduced. The ambient temperature of PLC is usually in the range of 0℃ ~ 55℃. Direct sunlight should be avoided. The installation location should be away from devices that generate large amounts of heat. At the same time, sufficient heat dissipation space and ventilation conditions should be ensured. The ambient humidity should generally be less than 85% to ensure good insulation of the PLC. In places containing corrosive gas, dense fog or dust, the PLC needs to be installed in a closed enclosure. In addition, if there is a strong vibration source at the PLC installation location, the reliability of the system will also be reduced, so corresponding vibration reduction measures should be taken.
2. PLC power supply and grounding
The anti-interference ability of PLC itself is generally very strong. Usually, the power supply of the PLC can only be wired separately from the power supply of the system’s power equipment. Generally, there is strong enough suppression capability for interference from the power line. However, if you encounter special circumstances and the power supply interference is particularly serious, you can add an isolation transformer with a shielding layer to reduce the interference between the equipment and the ground and improve the reliability of the system. If a system contains expansion units, their power supplies must share a switch control with the basic unit, that is, their power on and off must be done at the same time. Good grounding is an important condition for ensuring the safe and reliable operation of PLC. In order to suppress the interference attached to the power supply, input terminal, and output terminal, a dedicated ground wire should be connected to the PLC, and the ground point should be separated from other equipment, as shown in Figure 1(a). If this requirement cannot be met, a public grounding method can also be used, as shown in Figure 1(b). However, it is forbidden to use the series grounding method, as shown in Figure 1(c), because it will cause potential differences between various devices and introduce interference. In addition, the grounding wire should be thick enough, the grounding resistance should be small, and the grounding point should be as close to the PLC as possible.

Grounding usually has two purposes, one is for safety, and the other is to suppress interference. A complete grounding system is one of the important measures for anti-interference in PLC control systems. Grounding plays a big role in eliminating interference. The grounding here refers to the ground that determines the potential of the system, not the grounding of the return path of the signal system. There are many suspended metal frames in the PLC control system. They are aerial wires that are afraid of aerial interference and require a ground wire that determines the potential. The AC ground is necessary for the power supply of the PLC control system. It forms one of the two power supply circuits through the center point of the transformer. The current and various harmonic currents on this loop are a serious source of interference. Therefore, AC ground, DC ground, analog ground and digital ground must be separated. The common ground between digital ground and analog ground is best placed in floating mode. The potential difference between each point of the ground wire should be as small as possible, and the ground swire cold chain warehouse model should be as thick as possible. If possible, a ring-shaped ground wire can be used. The system ground terminal (LG) is an anti-interference neutral terminal and usually does not need to be grounded. However, when electromagnetic interference is severe, this terminal needs to be connected to the ground terminal (GR).
3. PLC input and output equipment
The input circuit is the port through which the PLC accepts switching, analog and other input signals. The quality of its components, wiring methods and reliability are also important factors that affect the reliability of the control system. Taking switch input as an example, the contacts of buttons and travel switches must be kept in good condition, and the wiring must be firm and reliable. Mechanical limit switches are components that are prone to failure. When designing, try to use highly reliable proximity switches to replace mechanical limit switches. In addition, the choice of button contacts also affects the reliability of the system. When designing a circuit, you should try to use components with high reliability. For analog input signals, the commonly used ones are 4~20mA, 0~20mA DC current signals; 0~5V, 0~10V DC voltage signals. The power supply is DC 24V.
For switching output, PLC output has three forms: relay output, thyristor output, and transistor output. The specific output form to be selected should be determined according to the load requirements. Improper selection will reduce the reliability of the system, and in serious cases, the system will be damaged. Can not work normally. For example, thyristor output can only be used for AC loads, and transistor output can only be used for DC loads. In addition, the load capacity of the PLC’s output terminal is limited. If it exceeds the specified maximum limit, an external relay or contactor must be connected to operate normally. The quality of external relays, contactors, solenoid valves and other actuators is an important factor affecting system reliability. Common faults include coil short circuit, mechanical failure causing contacts to move or poor contact. On the one hand, reliability can be improved by selecting high-quality components. On the other hand, in situations where system reliability and intelligence are required, some key parts of the output unit can be inspected based on abnormal current conditions in the circuit. Diagnosis: when an abnormal signal is detected, the system automatically switches to fault processing according to the program, thereby improving the reliability of the system. If the PLC output terminals are connected to inductive components, corresponding protective measures should be taken to protect the PLC output contacts.
In order to prevent or reduce interference from external wiring, AC input and output signals and DC input and output should use separate cables respectively; for input and output signal lines of integrated circuits or transistor equipment, shielded cables must be used, and shielded cables must be used between input and The output side is suspended and the control side is grounded. The processing method is shown in Figure 2.

Software anti-interference measures
The purpose of hardware anti-interference measures is to cut off interference from entering the control system as much as possible. However, due to the randomness of interference, especially in industrial production environments, hardware anti-interference measures cannot completely shut out all kinds of interference. At this time, , the flexibility of software can be combined with hardware measures to improve the anti-interference ability of the system.
1. Use the “watchdog” method to monitor the motion status of the system

PLC has a wealth of soft components inside, such as timers, counters, auxiliary relays, etc. Using them to design some programs can shield false signals from input components and prevent malfunctions of output components. When designing an application, you can use the “watchdog” method to monitor the operating status of each component of the system. For example, when using a PLC to control a certain moving part, a timer can be defined as a “watchdog” during programming to monitor the working status of the moving part. The set value of the timer is the maximum possible time required by the moving bike parts. When the action command of this component is issued, the “watchdog” timer is started at the same time. If the moving part reaches the specified position within the specified time, an action completion signal is sent to clear the timer, indicating that the monitored object is working normally; otherwise, it indicates that the monitored object is not working properly, and an alarm or stop working signal is issued.
2. Eliminate jitter
In a vibrating environment, travel switches or buttons often send out false signals due to jitter. Generally, the jitter time is relatively short. In view of the short jitter time, the PLC internal timer can be used to delay the jitter for a certain period of time to obtain the result after the jitter is eliminated. Reliable and effective signals to achieve the purpose of anti-interference.
3. Use software digital filtering to improve the signal-to-noise ratio of the input signal

In order to improve the signal-to-noise ratio of the input signal, software digital filtering is often used to improve the authenticity of the useful signal. For systems with large random interference, the program limiting method is used, that is, five consecutive samples are taken. If a certain sample value is much larger than the amplitude of other samples, then it is discarded. For parameters such as flow, pressure, liquid level, and displacement, which often fluctuate frequently within a certain range, the arithmetic mean method is used. That is, the current value is replaced by the average of n samples. It is generally believed that flow n=12 and pressure n=4 are most suitable. For slowly changing signals such as temperature parameters, three consecutive samples can be taken, and the middle sample value is selected as the effective signal. For A/D conversion with an integrator, the sampling time should be an integer multiple of the power frequency cycle (20ms). Practice has proven that its ability to suppress power frequency interference exceeds the effect of a simple integrator.