1 Introduction
PLCs, due to their powerful functions, simple programming, and convenient maintenance, especially their high reliability and strong adaptability to harsh industrial environments, have been widely used in the water supply industry. However, harsh on-site environmental conditions, high humidity, and various industrial electromagnetic and radiation interferences can affect the normal operation of the system. Therefore, it is essential to pay attention to the anti-interference design of the project.
The main sources of interference to PLCs used in water treatment plants fall into three categories: interference introduced by the power supply system, interference introduced by the grounding system, and interference introduced by the input/output circuits. If the interference problem of the PLC is not properly addressed, the system will not operate reliably, affecting the normal water supply. Therefore, it is necessary to discuss the interference problem in PLC application systems. This article mainly discusses three anti-interference techniques for PLCs.
2. Analysis of Anti-interference Technical Countermeasures
To prevent interference, both hardware and software anti-interference measures can be adopted. Among them, hardware anti-interference is the most basic and important anti-interference measure. It generally starts from both resistance and prevention to suppress and eliminate interference sources, cut off the coupling channel of interference to the system, and reduce the system's sensitivity to interference signals.
2.1 Interference introduced by the power supply system
Interference from the power grid and frequency fluctuations directly affect the reliability and stability of a PLC system. Suppressing power supply system interference is a key aspect of improving the PLC's anti-interference performance.
(1) Add a filtering, isolation, shielding, and switching power supply system.
The purpose of setting up a filter is to suppress interference signals transmitted from the power line into the system. When using an isolation transformer, the following must be noted: the shielding layer must be properly grounded; the secondary connection line must use double winding (to reduce interference between wires); the primary and secondary windings of the isolation transformer should be shielded separately; the primary shielding layer should be connected to the neutral wire of the AC power grid; and the shielding layer between the secondary and primary windings should be connected to the DC terminal.
To suppress voltage fluctuations caused by the start-up and shutdown of large-capacity equipment (such as water pumps) and maintain stable power supply voltage, a switching voltage regulator can be used.
(2) Separate power supply system
The PLC controller and I/O system are powered by their own isolation transformers and are separate from the main power supply. This way, when the input and output power supply is interrupted, the power supply to the controller will not be affected.
2.2 Suppressing interference introduced by the grounding system
PLC systems are divided into logic circuit grounding and power circuit grounding, with three methods: common ground, floating ground, and chassis common ground and circuit floating ground. Generally, it is best to use separate grounding for the controller and other equipment. When grounding, pay attention to the following: the grounding wire should be as thick as possible, generally greater than 2mm²; the grounding point should be as close to the controller as possible, and the distance between the grounding point and the controller should not exceed 50m; the grounding wire should avoid high-voltage circuits and main circuit wires as much as possible. If it is not possible to avoid them, they should intersect perpendicularly, and the length of parallel lines should be minimized.
Practice has shown that grounding is often an important means of suppressing noise and preventing interference. A good grounding method can greatly suppress the coupling of internal noise, prevent the intrusion of external interference, and improve the anti-interference ability of the system.
2.3 Suppressing interference introduced by input/output circuits
To achieve complete isolation between input and output circuits, optocouplers have been widely used in control systems in recent years and have become one of the most effective measures to prevent interference.
Optocouplers have the following characteristics: First, because they are sealed within a casing, they are not affected by external light interference; second, because they transmit signals via light, the ground connection between various component circuits is severed; third, the dynamic resistance of the LED is very small, while the internal resistance of interference sources is generally large, thus the interference signal transmitted to the input and output of the optocoupler becomes very small; fourth, the transfer ratio of an optocoupler is generally much smaller than the amplification factor of a transistor, making it far less sensitive to interference signals than a transistor, and the LED in an optocoupler only emits light when a certain current flows through it. Therefore, even under conditions of high interference voltage amplitude, the LED cannot emit light due to insufficient energy, thus effectively suppressing the interference signal. Since the linear region of an optocoupler is generally limited to a specific range, it is essential to ensure that the range of variation of the transmitted signal remains within the linear region. To ensure linear coupling, both strict selection of the optocoupler and appropriate nonlinear correction measures must be implemented; otherwise, significant errors will occur.
The above describes the processing methods for digital input and output signals. For analog input and output signals, in order to eliminate the influence of transient interference in the industrial field, in addition to adding A/D and D/A conversion circuits and optocouplers, software digital filtering techniques such as median method and first-order recursive digital filtering method can be adopted as needed.
3. Conclusion
The anti-interference design of a PLC control system is a complex systems engineering project, involving specific input and output devices and the industrial environment. When designing an anti-interference system, it is necessary to comprehensively consider various factors.
