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, the harsh industrial environment and various industrial electromagnetic and radiation interferences affect the normal operation of PLC control systems, so 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 used. Methods to prevent hardware interference include: 1. Using a high-performance power supply 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, etc. Utilizing PLC software to reduce interference is a crucial aspect of the normal and stable operation of a PLC control system. The following mainly analyzes the methods used in production practice to reduce interference using PLC configuration software: I. Methods to reduce digital input disturbances 1. Counter Method: CON—Counter NOT—Inverter RS—Reset Priority Flip-Flop IN—Input OUT—Output N—Number of Pulse Samples Note: When an external signal is input, the control system samples N consecutive pulses to make the RS flip-flop output "1". Only when the external input signal changes from "1" to "0" will the reset terminal of the RS flip-flop be "1", resetting the output of the RS flip-flop to "0". When there is a momentary interference pulse, the CON counter will not be able to sample N consecutive pulses, and the CON counter will not output, thus reducing interference. (N is generally taken as 2) Advantages: Fast response speed, and it plays a certain role in suppressing periodic momentary interference. Disadvantages: It cannot eliminate interference exceeding the sampling time of the CON counter. 2. Delayed Input Method: IN—Input OUT—Output TIME (ET)—Delay Time TON—Delayed Output (its curve is shown in the figure below) 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, not conducive to fast signal transmission. II. Methods to Reduce Analog Input Disturbances 1. Limiting Method MOVE—Move-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, causing OUT1 or OUT2 to output "1" to block MOVE, thus maintaining the MOVE output at the set 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 and Limiting Method: MOVE—Move-hold instruction (EN=1, OUT=IN. EN=0, OUT holds 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. 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 Filter Comparison Method: 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 a first-order delay filter, 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 the first-order delay filter, and the absolute value 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 mode. At this time, OUT maintains the value of the input signal IN-AI before the sudden change. This continues until the sudden change weakens, OUT1 = 0, and OUT = IN-AI. Advantages: Good suppression of periodic interference. High smoothness. Disadvantages: Sensitivity depends on the magnitude of the TIME—delay filter time. 4. Integral Debouncing Filtering Method: 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—Movement 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, P is unlocked to become a pure integral controller) . III. When a Small Signal Changes in Amplitude: 1. Final State: This is the steady state, with input and output close. OR output is "0", NOT=1, TON time has exceeded 10S, EOR=0, MOV does not hold, 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 (30S filtering) → Output. 2. Transient changes of small signals: (before TON=10S) OR=0, NOT=1, TON before 10S, EOR=1, MOV holds, PI integration, LG1 does not function, output skips LG1 (TIME=30S) and goes directly to the output terminal; this is the linear tracking filtering state. IV. 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 tracking occurs. Normal signal flow: IN → LG → PI tracking → LG1 tracking → Output . V. Summary : 1. Small signals within 10 seconds, after LG (TIME=1S), PI integration, skip LG1 (TIME=30S), and are directly output, achieving the filtering and tracking state of the input signal. 2. Small signals are tracked by LG (TIME=1S), PI, and LG1 (TIME=30S) after 10 seconds. 3. When large signals change, LG (TIME=1S) is active, while LG1 (TIME=30S) is inactive; this results 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, these software methods for handling interference will be continuously improved.