Overview
Various types of PLCs used in automation systems are either centrally installed in the control room or distributed across individual machines on the production floor. Although they are mostly located in harsh electromagnetic environments created by high-voltage circuits and equipment, PLCs are control devices specifically designed for industrial production environments. Their design and manufacturing incorporate multi-layered anti-interference measures and carefully selected components, giving them strong adaptability to harsh industrial environments, operational stability, and high reliability. Therefore, they can generally be used directly in industrial environments without special measures. However, because they are directly connected to field I/O devices, external interference can easily intrude through power lines or I/O transmission lines, causing malfunctions in the control system. Interference experienced by PLCs can be divided into external and internal interference. In actual production environments, external interference is random, unrelated to the system structure, and the interference source cannot be eliminated; it can only be limited according to specific circumstances. Internal interference is related to the system structure and is mainly caused by the AC main circuit and analog input signals within the system. Reasonable system circuit design can weaken and suppress internal interference and prevent external interference. To improve the reliability of a PLC control system, it is necessary to improve the system's anti-interference capability from multiple aspects.
Analyze the hardware circuit and propose hardware anti-interference measures.
1. Installation and operating environment of PLC control system
PLCs are designed specifically for industrial control and can generally be used directly in industrial environments without special measures. However, in PLC control systems, harsh environments or improper installation and use can reduce system reliability. The operating temperature range for PLCs is typically 0℃ to 55℃. Direct sunlight should be avoided, and the installation location should be far away from components that generate significant heat, ensuring sufficient space for heat dissipation and ventilation. Ambient humidity should generally be less than 85% to ensure good insulation of the PLC. In environments containing corrosive gases, dense fog, or dust, the PLC must be installed in a sealed enclosure. Furthermore, if the PLC installation location is near a strong vibration source, system reliability will also decrease; therefore, appropriate vibration damping measures should be taken.
2. PLC power supply and grounding
PLCs generally have strong anti-interference capabilities. Typically, the PLC power supply should be wired separately from the system's power supply, and it usually has sufficient suppression capabilities against interference from power lines. However, in special cases where power interference is particularly severe, a shielded isolation transformer can be added to reduce interference between the equipment and ground, improving system reliability. If a system contains expansion units, their power supplies must share a single switch control with the basic units; that is, their power-on and power-off must occur simultaneously. Proper grounding is a crucial condition for ensuring the safe and reliable operation of the PLC.
Grounding typically serves two purposes: safety and interference suppression. A robust grounding system is a crucial anti-interference measure for PLC control systems. Grounding plays a significant role in eliminating interference. Here, grounding refers to the ground that determines the system potential, not the grounding of the signal system's return path. PLC control systems contain many suspended metal frames, which are susceptible to aerial interference and require ground wires that determine their potential. AC ground is essential for the PLC control system's power supply, forming one of the two power supply loops through the transformer's center point. The current and various harmonic currents in this loop are a significant source of interference. Therefore, AC ground wires, DC ground wires, analog ground, and digital ground must be separated. The common ground point of digital and analog grounds is best placed in a floating configuration. The potential difference between ground wire points should be minimized, and ground wires should be as thick as possible; a ring ground wire can be used if feasible. The system ground terminal (LG) is the neutral terminal for interference suppression and usually does not require grounding. However, when electromagnetic interference is severe, this terminal needs to be connected to the earth-grounded terminal (GR).
3. PLC input and output devices
The input circuit is the port through which the PLC receives input signals such as digital and analog signals. The quality of its components, wiring methods, and reliability are all important factors affecting the reliability of the control system. Taking digital input as an example, the contacts of pushbuttons and limit switches must be kept in good condition, and the wiring must be secure and reliable. Mechanical limit switches are prone to failure; therefore, high-reliability proximity switches should be used instead of mechanical limit switches whenever possible. Furthermore, the choice of pushbutton contacts also affects system reliability. When designing the circuit, high-reliability components should be selected whenever possible. For analog input signals, commonly used signals include 4–20mA and 0–20mA DC current signals; and 0–5V and 0–10V DC voltage signals, with a 24V DC power supply.
For digital outputs, PLCs offer three types of outputs: relay outputs, thyristor outputs, and transistor outputs. The specific type of output to choose depends on the load requirements. Inappropriate selection can reduce system reliability and, in severe cases, cause the system to malfunction. For example, thyristor outputs can only be used with AC loads, while transistor outputs can only be used with DC loads. Furthermore, the load capacity of PLC output terminals is limited. If the maximum load is exceeded, an external relay or contactor must be connected for normal operation. The quality of external relays, contactors, solenoid valves, and other actuators is a crucial factor affecting system reliability. Common faults include coil short circuits and mechanical failures causing contact malfunctions or poor contact. Reliability can be improved by using high-quality components. In applications requiring high system reliability and intelligence, abnormal current in the circuit can be used to diagnose key components of the output unit. When an abnormal signal is detected, the system automatically initiates fault handling according to the program, thereby improving system reliability. If inductive components are connected to the PLC output terminals, appropriate protective measures should be taken to protect the PLC output contacts.
To prevent or reduce interference from external wiring, AC input and output signals should use separate cables from DC input and output signals. For input and output signal lines of integrated circuits or transistor devices, shielded cables must be used. The shielded cables are left floating on the input and output sides and grounded on the control side, as shown in Figure 2.
Software anti-interference measures
The purpose of hardware anti-interference measures is to cut off interference from entering the control system as much as possible. However, due to the randomness of interference, especially in industrial production environments, hardware anti-interference measures cannot completely keep out various interferences. In this case, the flexibility of software can be combined with hardware measures to improve the anti-interference capability of the system.
1. Use the "watchdog" method to monitor the system's operating status.
PLCs have a wealth of built-in software components, such as timers, counters, and auxiliary relays. These can be used to design programs that can mask erroneous signals from input components and prevent malfunctions in output components. When designing application programs, a "watchdog" method can be used to monitor the operating status of various system components. For example, when using a PLC to control a moving part, a timer can be defined as a "watchdog" to monitor the moving part's operating status. The timer's set value is the maximum possible time required by the moving part. When the action command for the part is issued, the "watchdog" timer is simultaneously started. If the moving part reaches the specified position within the specified time, an action completion signal is issued, resetting the timer to zero, indicating that the monitored object is working normally; otherwise, it indicates that the monitored object is not working normally, and an alarm or stop signal is issued.
2. Anti-shake
In vibrating environments, limit switches or buttons often emit false signals due to jitter. The jitter time is usually short. To address this short jitter time, a reliable and effective signal after eliminating the jitter can be obtained by using an internal timer of the PLC after a certain delay, thereby achieving the purpose of anti-interference.
3. Improve the signal-to-noise ratio of the input signal using software digital filtering.
To improve the signal-to-noise ratio of the input signal, software digital filtering is often used to enhance the authenticity of the useful signal. For systems with significant random interference, a program-based amplitude limiting method is employed, which involves sampling five times consecutively; if a sample value is significantly larger than the amplitudes of the other samples, it is discarded. For parameters such as flow rate, pressure, liquid level, and displacement, which often fluctuate frequently within a certain range, an arithmetic mean method is used. This involves using the average of n samples to replace the current value. Generally, n=12 for flow rate and n=4 for pressure are considered most suitable. For slowly changing signals such as temperature parameters, three consecutive samples can be taken, and the middle sample value is selected as the valid signal. For A/D conversions with integrators, the sampling time should be an integer multiple of the power frequency period (20ms). Practice has shown that its ability to suppress power frequency interference exceeds that of a simple integrator.
