Anti-interference design of PLC control systems is an important component of the system. This article discusses this from the perspective of hardware. I. Introduction Programmable Logic Controller (PLC) is a special type of control computer with advantages such as simple programming, good versatility, and powerful functions. It is widely used in industrial control and large medical equipment, and it is directly connected to the electronic equipment of the controlled equipment. Therefore, it is easily interfered with by surrounding interference sources, which can cause the control system to malfunction. In order to ensure the stable operation of the control system and improve its reliability, certain anti-interference measures should be taken during system design and installation. II. Common Interference Factors in PLC Control Systems (I) Spatial Radiation Interference The high-density use of electrical and electronic equipment has made spatial electromagnetic wave pollution increasingly serious. The radiation waves generated by these interference sources have a wide frequency range and are irregular. Spatial radiation interference forms a receiving circuit through the casing, wires, etc. of the detection system by electromagnetic induction, causing interference to the system. (II) Signal Channel Interference 1. Common Mode Interference Common mode interference causes significant interference to the system's amplifier circuit. Various signal currents of the system will generate interference voltage through the internal resistance of the power supply and the impedance of the common ground line, forming common mode interference. 2. The parasitic capacitance between electrostatic discharge circuits causes changes in the signal of one circuit within the system to affect other circuits, forming electrostatic coupling interference. Electrostatic coupling interference exists as long as there are high-frequency signals such as spike signals and pulse signals in the circuit. 3. Conducted Coupling Interference The signal in the system is prone to delay and distortion during transmission and receives interference signals, forming conducted coupling interference. (III) Power Supply Interference PLC systems are generally powered by industrial power networks. The start-up and shutdown of some large equipment in industrial systems may cause power supply overvoltage, undervoltage, surge, sag and spike interference. These voltage noises will be coupled to the PLC system circuit through the internal resistance of the power supply, causing great harm to the system. (IV) Interference Caused by Digital Circuits Although the DC current drawn from digital integrated circuits is only in the mA range, it will form a large interference when the circuit is in high-speed switching. For example, a TTL gate circuit draws about 5mA of current from a DC power supply when it is on and 1mA when it is off. Within 5ns, the current changes to 4mA. If there is an inductance of 0.5μH on the power distribution line, the noise voltage generated on the power distribution line when this gate circuit changes state is: U=L x di/dt=0.5x 10⁻⁶ x 4x10⁻³ /5x10⁻⁹=0.4V. If this value is multiplied by the values ​​of a large number of gates in a typical system, even though the supply voltage of this gate circuit is only 5V, the resulting interference noise will be very serious. When processing pulse digital circuits, a rough concept of the spectrum contained in the pulse should be obtained. If the pulse rise time t is known, its equivalent highest frequency can be calculated using an approximate formula: f[sub]max[/sub]=1/2πt. III. Anti-interference design in PLC systems (I) Anti-interference design of the power supply section The power transformer is the main component of the power supply section. In order to suppress interference in the power grid, an isolation transformer is generally selected, and the transformer capacity should be about 1.2 to 1.5 times larger than the actual needs. In use, the shielding layer of the transformer should be well grounded, and the secondary coil connection line should use twisted pair wire to reduce interference between power lines. For the PLC controller power supply, if conditions permit, a filter can also be added before the isolation transformer. At this time, both the primary and secondary connection lines of the transformer should use twisted pair wire, as shown in Figure 1. In this way, the interference signal can be greatly reduced after filtering and isolation, which enhances the reliability of the system. [align=center] Figure 1 Connection of filter and isolation transformer[/align] The PLC power supply system can be adopted in the following way: the controller and I/O system are powered by their respective isolation transformers and are separated from the main circuit power supply. When a part of the power supply fails, it will not affect other parts. For example, when the input and output power supply is interrupted, the controller can still continue to supply power, which improves the reliability of the system, as shown in Figure 2. When the power supply quality is not guaranteed (not a long-term power outage), the controller can be powered by a UPS uninterruptible power supply, that is, the shielded transformer in front of the controller is replaced by a UPS uninterruptible voltage regulator. For some important equipment (medical equipment, etc.), in order to improve the reliability of the system, the AC power supply circuit can adopt a dual power supply system. [align=center] Figure 2 Power supply system using isolation transformer[/align] (II) Anti-interference design of input and output signals In order to prevent the input and output signals from being interfered with, the isolated I/O module should be selected. 1. Anti-interference design of input signals Differential mode interference between the input lines of the input signal can be reduced by using the input module filter, while common mode interference between the input line and the ground can be suppressed by the grounding of the controller. When there is an inductive load at the input terminal, in order to prevent the influence of induced electromotive force caused by sudden changes in circuit signals, hardware reliability, fault tolerance, and tolerance design techniques can be adopted. That is, a capacitor C and a resistor R are connected in parallel across the load terminals. For DC input signals, a freewheeling diode D can be connected in parallel. The specific circuit is shown in Figure 3. [align=center] (a) Anti-interference of AC input signals (b) Anti-interference of DC input signals Figure 3 Anti-interference design of input signals[/align] The selection of CR in Figure 3 should be appropriate. Generally, when the load capacity is below 10VA, C should be 0.1μF and R should be 120Ω. When the load capacity is above 10VA, C should be 0.47μF and R should be 47Ω. 2. Anti-interference design of output circuit For PLC systems with switching output, there are three forms: relay output, thyristor output, and transistor output. The specific selection depends on the load requirements. If the load exceeds the output capacity of the PLC, an external relay or contactor should be connected for normal operation. If an inductive load is connected to the PLC output terminal, sudden changes in electrical quantity when the output signal changes from OFF to ON or vice versa may cause interference. Appropriate protection measures should be taken during the design to protect the PLC output contacts, as shown in Figure 4. For DC loads, a freewheeling diode D is typically connected in parallel across the coil. The diode should be as close to the load as possible, and can be a 1A diode. For AC loads, an RC absorption circuit should be connected in parallel across the coil. Depending on the load capacity, the capacitor can be 0.1μF to 0.47μF, and the resistor can be 47Ω to 120Ω, with the RC circuit as close to the load as possible. [align=center] DC Load AC Load Figure 4 Protection of PLC Output Contacts[/align] For high-capacity load circuits, since relays or contactors generate arcing interference when switching on and off, an RC surge absorber must be connected across the main contacts, as shown in Figure 5(A). If there is interference from the motor or transformer switch, an RC surge absorber can be used between the lines, as shown in Figure 5(B). [align=center]Figure 5 Anti-interference design for high-capacity loads[/align] (III) Anti-interference design for external wiring Mutual inductance and distributed capacitance exist between external wirings, which can cause crosstalk during signal transmission. To prevent or reduce interference from external wiring, AC input/output signals and DC input/output signals should use separate cables. Shielded cables should be used for the input/output signal lines of integrated circuits or transistor devices. The shielded cables should be left floating on the input/output side and grounded on the controller side. The specific circuit design is shown in Figure 6. For short distances of less than 30 meters, DC and AC input/output signal lines should preferably not use the same cable. If they must run through the same conduit, the input signal should use a shielded cable. For wiring distances of 30 to 300 meters, DC and AC input/output signal lines should use separate cables, and the input signal line must be shielded. For long distances of more than 300 meters, intermediate relays can be used to switch signals, or remote I/O channels can be used. The grounding wire of the controller should be separated from the power supply line or power line, and the input and output signal lines should be wired separately from the high voltage and high current power lines. [align=center] Figure 6 Shielded cable handling method[/align] (IV) Grounding design of PLC device Good grounding is one of the important conditions for ensuring the reliable operation of PLC and can avoid the damage caused by accidental voltage surges. The grounding wire is connected to the grounding terminal of the machine. The basic unit must be grounded. If an extension unit is selected, its grounding point is connected to the grounding point of the basic unit. In order to suppress the interference attached to the power supply and input and output terminals, the PLC should be connected to a dedicated grounding wire. The grounding wire should be separated from the grounding point of the power equipment (such as the motor). If this requirement cannot be met, it can be grounded together with other equipment. It is strictly forbidden to connect it in series with other equipment for grounding. The specific design is shown in Figure 7. In addition, the grounding resistance should be less than 100 ft, the grounding wire should be thick, the grounding area should be greater than 2 square millimeters, and the grounding point should preferably be close to the PLC device, with a distance of less than 50 meters. The grounding wire should avoid high voltage circuits. If it is not possible to avoid them, they should intersect perpendicularly and shorten the length of parallel lines. [align=center] (a) Best (b) Possible (c) Not allowed Figure 7 PLC System Grounding Design[/align] IV. Conclusion The anti-interference design of the PLC control system is an important component of the system. This paper only discusses the hardware part. In software programming, redundancy design techniques can also be used to design programs to shield erroneous signals from input components and prevent malfunctions of output components. In practical applications, both hardware and software anti-interference techniques can be used simultaneously to ensure that the PLC system meets the requirements and achieves an ideal working state.