Abstract : With the development of science and technology, Programmable Logic Controllers (PLCs) are widely used in industrial control. Their reliability is directly related to the safe production and economic operation of industrial enterprises. The ability of a PLC control system to resist interference is crucial for the reliable operation of the entire production system. Currently, various types of PLCs are generally installed in central control rooms or production sites, where they are mostly located in harsh electromagnetic environments formed by high-voltage circuits and equipment. Therefore, to improve the reliability of PLC control systems, firstly, PLC manufacturers need to improve the anti-interference capabilities of PLC hardware; secondly, engineering designers need to fully utilize PLC configuration software to eliminate interference, thus effectively enhancing the system's anti-interference performance. Keywords : Anti-interference methods, PLC Introduction: PLC control systems are widely used in industrial enterprises due to their powerful functions, simple programming, good expandability, convenient maintenance, high reliability, and adaptability to harsh industrial environments. However, due to harsh industrial environmental conditions and various industrial electromagnetic and radiation interferences, the normal operation of PLC control systems is affected. Therefore, it is essential to pay attention to the anti-interference design of PLC control systems. To prevent interference, a combination of hardware and software anti-interference methods can be adopted. Methods to prevent hardware interference include: 1. Using high-performance power supplies to suppress interference introduced by the power grid; 2. Selecting and laying cables to reduce electromagnetic interference; 3. Improving the grounding system; 4. Using opto-isolation to suppress interference introduced by input and output circuits. Using PLC software to reduce interference is a crucial aspect of the normal and stable operation of the PLC control system. The following mainly analyzes methods for reducing interference using PLC configuration software applied in production practice: I. Methods to reduce digital input disturbances 1. Counter method [align=center] Figure 1[/align] CON—Counter NOT—Inverter RS—Reset priority trigger IN—Input OUT—Output N—Number of pulse samples Note: When there is an external signal input, the control system collects N consecutive pulses to make the RS trigger output "1". Only when the external input signal changes from "1" to "0" will the reset terminal of the RS trigger be "1", resetting the output of the RS trigger to "0". When there is a momentary interference pulse, the CON counter will not collect N consecutive pulses, and the CON counter cannot output, thus reducing interference. (N is generally taken as 2) Advantages: Fast response speed, and it plays a certain role in suppressing periodic instantaneous interference. Disadvantages: It cannot eliminate interference that exceeds the sampling time of the CON counter. 2. Delayed Input Method [align=center] Figure 2[/align] IN—Input OUT—Output TIME (ET)—Delay Time TON—Delay Output (its curve is shown in the figure below) [align=center] Figure 3[/align] Note: When the input IN=1, the counter is started until the timing time (PT) = delay time, OUT=1. When the counter timing time < delay time, OUT=0. The delay time is best taken to be within 1 second. Advantages: Eliminates short-term periodic interference. Disadvantages: Slow response speed, which is not conducive to the rapid transmission of signals. II. Methods to Reduce Analog Input Disturbances 1. Limiting Method [align=center] Figure 4[/align] MOVE—Move and hold command (Enable terminal EN=1, OUT=IN. EN=0, OUT holds the previous value) GE—Greater than or equal to command (OUT=1, IF IN1≥IN2) LE—Less than or equal to command (OUT=1, IF IN1≤IN2) HL—Upper limit setting value LL—Lower limit setting value Note: When the analog input signal is between HL and LL, OUT=IN. When the IN-AI signal exceeds or equals HL or LL, GE or LE judges the IN-AI signal and makes OUT1 or OUT2 output "1" to block MOVE, thereby keeping the output of MOVE at the setting value of HL or LL. This achieves the effect of limiting. Advantages: Effectively overcomes pulse interference caused by accidental factors. Disadvantages: Poor smoothness. 2. Delayed Filtering Limiting Method [align=center] Figure 5 [/align] MOVE—Move-hold instruction (Enable terminal EN=1, OUT=IN. EN=0, OUT holds the previous value) GE—Greater than or equal to instruction (OUT=1, IF IN1≥IN2) LE—Less than or equal to instruction (OUT=1, IF IN1≤IN2) HL—Upper limit setting value LL—Lower limit setting value LG—Delayed filtering instruction (its curve is shown in the figure below) TIME—Delayed filtering time [align=center] Figure 6 [/align] Note: The function is basically the same as the limiting method, except that a delay filter is added at the input end, which acts as a delay buffer for the input signal. Advantages: Effectively suppresses periodic pulse interference. Smoother signal than the limiting method. Disadvantages: Slower signal response speed. 3. Delay Filtering Comparison Method [align=center] Figure 7 [/align] LG—Delay Filter SUB—Subtraction Command ABS—Absolute Value Command GE—Greater Than or Equal To Command HL—Maximum Deviation Value TIME—Delay Filtering Time Note: Under normal circumstances, the input signal IN-AI is directly output after first-order delay filtering, and OUT = the value of IN-AI; when there is a sudden change in signal, the input signal IN-AI is subtracted from the input signal IN-AI containing the sudden change after first-order delay filtering, and the absolute value is taken (regardless of whether there is a positive or negative deviation). This is compared with the HL value. If it is greater than or equal to the preset value of HL, OUT1 = 1, and the LG—delay filter is switched to tracking state. At this time, OUT maintains the value of the input signal IN-AI before the sudden change. Until the sudden change weakens, OUT1 = 0, and OUT = IN-AI. Advantages: Good suppression effect on periodic interference. High smoothness. Disadvantages: Sensitivity depends on the size of TIME—delay filtering time. 4. Integral Debouncing Filtering Method [align=center] Figure 8 [/align] LG—Delay Filter SUB—Subtraction Instruction GE—Greater Than or Equal To Instruction LE—Less Than or Equal To Instruction OR—OR Gate (Custom DFB Function Block) NOT—NOT Gate TON—Delay Output EOR—XOR Gate MOV—Move-Hold Instruction PI—Proportional-Integral Controller HL—Maximum Positive Deviation Value LL—Maximum Negative Deviation Value TIME—Delay Filtering Time TIME1—Delay Output Time TIME2—Delay Filtering Time Note: Parameter settings: LG (TIME=1S), TON (TIME1=10S), LG1 (TIME=30S), HL=0.2, LL=-0.2, PI (TI=10S, unlock P to become a pure integral controller) I. When a small signal changes within a certain amplitude 1. Final State: At this point, the input and output are close. OR output is "0", NOT=1, TON time has exceeded 10s, EOR=0, MOV is not held, PI does not integrate, SUB=0, the signal follows the PI tracking loop, and is output after filtering by LG1. Normal signal flow: IN→LG→PI tracking→LG1 (filtering for 30s)→output. 2. Transient changes of small signals: (before TON=10s) OR=0, NOT=1, TON has not reached 10s, EOR=1, MOV is held, PI integrates, LG1 does not function, the output skips LG1 (TIME=30s) and goes directly to the output terminal; this is the linear tracking filtering state. II. When the signal changes significantly (≥HL, ≤LL): OR=1, NOT=0, TON does not function, EOR=0, so LG1 (TIME=30s) does not function, PI does not function and follows the tracking loop. Normal signal flow: IN → LG → PI tracking → LG1 tracking → Output. III. Summary: 1. For small signals within 10 seconds, after LG (TIME=1S) and the integral action of PI, LG1 (TIME=30S) is skipped, and the signal is directly output, achieving input signal filtering and tracking. 2. For small signals after 10 seconds, after LG (TIME=1S) and the tracking of PI and LG1 (TIME=30S) respectively, the input is tracked. 3. When large signals change, LG (TIME=1S) is active, while LG1 (TIME=30S) is inactive, resulting in rapid output tracking. Advantages: Good filtering effect for the measured parameter; good suppression of periodic interference; high smoothness. Disadvantages: Slow response to slowly changing input signals. Conclusion The methods analyzed above have all been tested in actual production and have achieved certain results. With the needs of actual production and the accumulation of experience, the software processing methods for interference will be continuously improved.