The various types of plc used in the automation system are concentrated in the control room, and some are distributed on the stand-alone equipment at the production site, although they are mostly in the harsh electromagnetic environment formed by strong electric circuits and strong electric equipment. However, PLC is a control device specially designed for industrial production environment. It adopts multi-level anti-interference and selected component measures in the design and manufacturing process, so it has strong ability to adapt to harsh industrial environment, operational stability and high. It can be used directly in an industrial environment without special measures, but since it is directly connected to the I/O equipment in the field, external interference can easily be intruded through the power line or I/O transmission line, causing Control system malfunction. The interference received by the PLC can be divided into external interference and internal interference. In the actual production environment, the external interference is random, independent of the system structure, and the interference source can not be eliminated, and can only be limited according to the specific situation; the internal interference is related to the system structure, mainly through the internal AC main circuit, simulation Caused by the input signal, etc., the system line can be reasonably designed to weaken and suppress internal interference and prevent external interference. In order to improve the reliability of the PLC control system, it is necessary to improve the anti-interference ability of the system from various aspects.
Analyze hardware circuits and propose hardware anti-jamming measures
1. Installation and use environment of PLC control system
PLCs are designed for industrial control and generally do not require any special measures to be used directly in industrial environments. However, in the PLC control system, if the environment is too harsh, or improper installation, it will reduce the system's reliability. The ambient temperature of the PLC is usually in the range of 0 °C to 55 °C. The direct sunlight should be avoided. The installation position should be away from the device with large heat generation, and the space for ventilation and ventilation should be sufficient. The ambient humidity should generally be less than 85% to ensure good insulation of the PLC. In the case of corrosive gases, dense fog or dust, the PLC must be closed. In addition, if there is a strong vibration source in the PLC installation position, the system's reliability will also be reduced, so the corresponding vibration reduction measures should be taken.
2, PLC power and ground
The anti-interference ability of the PLC itself is generally very strong. Generally, the power supply of the PLC can only be separately wired from the power supply of the power supply of the system. Generally, the interference from the power supply line is sufficiently strong. However, if there is a special situation, the power supply interference is particularly serious. An isolation transformer with shielding layer can be added to reduce the interference between the equipment and the ground, and improve the system's reliability. If a system contains expansion units, their power supply must share a switch control with the base unit, that is, their power-up and power-off must be performed simultaneously. Good grounding is an important condition to ensure that the PLC can be operated safely.
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-interference of the PLC control system. Grounding plays a large role in eliminating interference. Grounding here refers to the ground that determines the potential of the system, not the ground where the signal system returns. There are many suspended metal frames in the PLC control system. They are air lines that are afraid of air interference and need to have a ground line that determines the potential. The AC ground is required for the power supply of the PLC control system. It forms one of the two circuits for power supply through the center point of the transformer. The current on this circuit, various harmonic currents, etc. are a serious source of interference. Therefore, AC ground, DC ground, analog ground and digital ground must be separated. The co-point of the digital ground and the simulated ground is preferably suspended. The potential difference between the grounding points should be as small as possible, and the grounding wire should be thickened as much as possible. The system ground terminal (LG) is a neutral terminal that is resistant to interference and usually does not require grounding. However, when electromagnetic interference is severe, this terminal needs to be connected to the ground terminal (GR).
3. PLC input and output devices
The input circuit is a port for the PLC to accept input signals such as switching quantity and analog quantity. The quality of the components, the wiring method and whether it is strong* are also important factors affecting the control system. Taking the digital input as an example, the contact contact of the button and the travel switch should be kept in good condition, and the wiring should be firm*. The mechanical limit switch is a component that is prone to failure. When designing, the high-precision proximity switch should be used instead of the mechanical limit switch. In addition, the choice of button contacts also affects the system's reliability. When designing the circuit, it should try to use components with high reliability. For analog input signals, 4~20mA, 0~20mA DC current signals are commonly used; 0~5V, 0~10V DC voltage signals, power supply It is 24V DC.
For the digital output, the output of the PLC has three forms: relay output, thyristor output, and transistor output. The specific output should be determined according to the load requirements. If the selection is improper, the system can be reduced. The system is not working properly. If the thyristor output can only be used for AC loads, the transistor output can only be used for DC loads. In addition, the output terminal of the PLC has limited load capacity. If the specified maximum limit is exceeded, an external relay or contactor must be connected to work properly. The quality of external relays, contactors, solenoid valves and other actuators is an important factor affecting the system's reliability. Common faults are coil short circuit, mechanical faults that cause the contacts to move or poor contact. In this respect, high-quality components can be used to improve the usability. On the other hand, in the case where the system can be highly flexible and intelligent, the focus of the output unit can be based on the abnormal current in the circuit. The part is diagnosed. When an abnormal signal is detected, the system automatically transfers to the fault processing according to the program, thereby improving the reliability of the system work. If the PLC output terminal is connected with an inductive component, corresponding protective measures should be taken to protect the output contact of the PLC.
In order to prevent or reduce the interference of external wiring, AC input and output signals and DC input and output should use separate cables; for input and output signal lines of integrated circuits or transistor devices, shielded cables must be used, shielded cables are input, The output side is floating and grounded on the control side.
Software anti-jamming measures
The purpose of the hardware anti-interference measures is to cut off the interference into the control system as much as possible, but due to the randomness of the interference, especially in the industrial production environment, the hardware anti-interference measures can not completely reject the various interferences. The flexibility of the software and the hardware measures can be combined to improve the anti-interference ability of the system.
1. Use the "watchdog" method to monitor the motion state of the system
The PLC has a wealth of devices, such as timers, counters, auxiliary relays, etc., which are used to design programs that can shield the input components from false signals and prevent output components from malfunctioning. When designing an application, you can use the "watchdog" method to monitor the operating status of the various components of the system. When using a PLC to control a moving part, a timer can be defined for "watchdog" during programming to monitor the working state of the moving part. The set value of the timer is the maximum possible time required for the moving part. When the action command of the 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 issued to clear the timer, indicating that the monitoring object is working normally; otherwise, the monitoring object is not working properly, and an alarm or stop working signal is issued.
2, debounce
In the vibration environment, the travel switch or button often sends a false signal due to the jitter, and the general jitter time is relatively short. For the short jitter time, the PLC internal timer can be used for a certain time delay to obtain the jitter-eliminated * Effective signal to achieve anti-interference.
3. Improve the signal-to-noise ratio of the input signal by software digital filtering
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 adopted, that is, continuous sampling is five times. If the sampling value is much larger than the amplitude of other samplings, then it is discarded. For parameters such as flow, pressure, liquid level, displacement, etc., which tend to fluctuate frequently within a certain range, the arithmetic average method is used. That is, the average value of n samples is used instead of the current value. It is generally considered that the flow rate n=12 and the pressure n=4 are most suitable. For slowly changing signals such as temperature parameters, three consecutive samples can be taken, and the centered sample value is selected as the effective signal. For integrator A/D conversion, the sampling time should be an integer multiple of the power frequency period (20ms). Practice has proved that its ability to suppress power frequency interference exceeds the effect of a simple integrator.
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