1. Introduction Programmable Logic Controllers (PLCs) are products combining microcomputer technology and conventional relay control concepts, developed from programmable controllers and microcomputer controllers. Broadly speaking, a PLC is a computer system with more robust input/output interfaces for connecting to industrial processes than a typical computer, and has become a fundamental device in automated control systems. PLCs are widely used in industries such as machinery, metallurgy, chemical engineering, automotive, and light industry, and have largely replaced traditional relay and contactor logic control. Using PLCs to control system equipment significantly improves reliability compared to simple relay and contactor control. The mean time between failures (MTBF) of a PLC is generally 30,000 to 50,000 hours; Mitsubishi's F series reportedly boasts an MTBF of 300,000 hours. Therefore, the reliability of the entire PLC control system primarily depends on its peripheral devices, such as limit switches, pushbuttons, and proximity switches in the input devices, and contactors, relays, and solenoid valves in the output devices. Furthermore, from a software programming perspective, developing a program with monitoring capabilities greatly enhances system reliability. The following discussion explores how to improve the reliability of PLC control systems. 2. Improving PLC Reliability by Considering Peripheral Equipment PLCs are control devices specifically designed for industrial production environments. When the working environment is harsh, such as strong electromagnetic interference, high humidity, or when power supplies, input and output circuits are susceptible to interference, the reliability of the control system will be affected. 2.1 Working Environment Requirements Except for PLCs designed for special working environments, the ambient temperature for general PLC operation should be within the range of 0–55℃, and direct sunlight should be avoided. During installation, it should be kept away from large heat sources, ensuring sufficient heat dissipation space and ventilation. The relative humidity of the air should be less than 85%, without condensation, to ensure good insulation of the PLC. PLCs should avoid installation in vibrating locations; for vibration sources where permissible conditions exist, vibration-damping rubber pads should be installed or other anti-vibration measures should be taken according to the product manual. When there is dust or harmful gases in the air, the PLC should be enclosed in an enclosed installation. 2.2 Power Supply Requirements Different PLC products have different power supply requirements, including the voltage level, frequency, AC ripple coefficient, and input/output power supply methods. In applications with strong electromagnetic interference and high PLC reliability requirements, the PLC power supply should be separated from the power supply and control circuit power supply. If necessary, a shielded isolation transformer or a series LC filter circuit can be used. During design, the external DC power supply should be a regulated power supply with a 20%–30% margin. For the DC power supply provided by the controller itself, the maximum current it can provide should be known to prevent overcurrent damage to the equipment. 2.3 Grounding and Wiring 1) Proper grounding of the PLC is a prerequisite for normal operation. During design, the PLC grounding should be separated from the power equipment grounding, using a dedicated grounding; if separate grounding is not possible, a shared grounding should be used; the use of a common grounding method is absolutely prohibited. As shown in Figure 1, the grounding point should be as close to the PLC as possible, the grounding wire diameter should be greater than 4mm², and the grounding resistance should generally be less than 10Ω. Figure 1 Grounding Method 2) PLC wiring includes input wiring and output wiring. The length of the input wiring should not be too long, generally not exceeding 30m; when the line distance is long, intermediate relays can be used for signal conversion. The COM terminal of the input wiring and the COM terminal of the output wiring must not be connected together. Input and output cables should be installed separately. If necessary, separate junction boxes can be installed on-site. Shielded cables must be used for the input and output signals of integrated circuits or transistor devices; the grounding terminal of the shielding layer should be a single-point ground, preferably on the controller side. 2.4 Redundancy Design and Degradation Operation Design 1) For applications with high reliability requirements, redundancy design and degradation operation are necessary. Redundancy design can use hot standby or cold standby. In hot standby mode, the redundant backup system also runs simultaneously. When the outputs of both are consistent, it indicates that the system is operating normally; if the results are inconsistent, an alarm signal is issued, and the system switches to the normal system based on the self-diagnosis results. In cold standby mode, the cold standby system does not run; it only switches to the backup system after the self-diagnosis detects a fault in the operating system. For PLCs, the scope of the redundancy system mainly includes the CPU, storage unit, power system, and communication system. Redundancy of input and output units is only included when high reliability requirements are required. 2) Degraded operation refers to the design that includes manual operation. Examples include emergency stop design and the design of start/stop and restart functions for critical equipment. This allows for degraded operation in case of a fault, i.e., manual start/stop of some or all equipment to avoid equipment damage or personnel injury. Furthermore, the design can consider transitions from fully automatic to semi-automatic, and finally to manual operation. 2.5 PLC I/O Circuits 1) Since the PLC receives input signals such as digital and analog signals through input circuits, the quality of the components and the connection method of the input circuits directly affect the reliability of the control system. For example, the contact of input switches such as buttons and limit switches must be good, and the wiring must be secure. Mechanical limit switches on equipment are relatively prone to failure. In the design, high-reliability proximity switches should be used instead of mechanical limit switches whenever possible. In addition, the selection of normally open and normally closed contacts for buttons also affects the reliability of the system. A simple start/stop control circuit is shown in Figures 2 and 3, which illustrate two control circuits and their corresponding ladder diagrams. The two control circuits have the same control function: pressing the start button activates the output; pressing the stop button disconnects the output. However, their reliability differs. Assuming the output disconnection is a safe state, the reliability of Figure 3 is higher than that of Figure 2. This is because both SB1 and SB2 can fail, with the most common phenomenon being an open circuit in the input circuit. When using the circuit in Figure 3, regardless of whether the SB1 and SB2 switches themselves are open or the wiring is open, the output remains safe, ensuring the system's safety and reliability. 2) When there is an inductive load at the input, to prevent the backflow induced electromotive force from damaging the module, a capacitor C and a resistor R (AC input signal) are connected in parallel across the load, or a freewheeling diode D (DC input signal) is connected in parallel. As shown in Figure 4: In AC input mode, the selection of CR must be appropriate to achieve better results. Through testing with experimental devices, when the load capacity is below 10VA, 0.1μF + 120Ω is generally selected; when the load capacity is above 10VA, 0.47μF + 47Ω is generally more suitable. When using DC input, testing shows that the diode's rated current should be 1A, and its rated voltage should be more than three times the power supply voltage. 3) When there is an inductive load at the output, testing shows that: for AC loads, a CR surge absorber should be connected in parallel across the load; for example, if the AC voltage is 100V or 200V and the power is around 400VA, the CR surge absorber should be 0.47μF + 47Ω, as shown in Figure 5. The closer the CR is to the load, the better its anti-interference effect; for DC loads, a freewheeling diode D should be connected in parallel across the load, as shown in Figure 6. The diode should also be close to the load. The reverse withstand voltage of the diode should be four times the load voltage. 3. Improving the reliability of the control system from the perspective of PLC software programs To improve the reliability of the PLC control system, a dedicated timer can be set up as part of the monitoring program to detect the system's operating status. If the program finishes running normally, the timer is immediately reset. If the program malfunctions, such as entering an infinite loop, the timer will not reset before the set time expires, and the PLC will issue an alarm signal. This method is used in application design to monitor the operating status of various parts of the system. When using a PLC to control an object, a timer can be defined in the program to monitor the object's operating status: the timer's set time is the maximum time required for the object to work; when the object starts running, the timer is also started. If the object's program finishes working within the specified time, a work completion signal is issued, causing the timer to reset, indicating that the object's program is running normally; otherwise, it is running abnormally, and an alarm signal or a stop signal is issued. The ladder diagram of the monitoring program is shown in Figure 7. In Figure 7, timer T1 is the detection element, X001 is the control object's action signal, X002 is the action completion signal, and M2 is the alarm or stop signal. Assuming that the controlled object's program takes 50 seconds to complete one cycle, the timer K value can be 510 (T1 is a 100ms timer). When X001=1, the controlled object starts running, and T1 starts timing. If the controlled object's program completes normally within the specified time, X002 activates, M1 resets, and timer T1 is cleared, waiting for the start of the next cycle. If no signal indicating completion of the controlled object's operation is issued within the specified time, a fault is identified, the contact of T1 closes, and M2 is activated to issue an alarm or stop signal. 4. Conclusion The reliability of a PLC control system is related to many factors, and some objective factors also interfere with the stability of the control system. By designing correct hardware circuitry, selecting high-quality components, improving the working environment, and developing monitoring programs, the reliability and stability of the PLC control system can be greatly improved. References [1] Chang Dounan et al. Programmable Logic Controller Principles, Applications and Experiments [M]. Beijing: Machinery Industry Press, 2003. [2] Qi Congqian et al. PLC Technology and Applications [M]. Beijing: Machinery Industry Press, 2000. [3] Mitsubishi FX2 Series Programmable Logic Controller User Manual [Z]. 2001. [4] Liu Wenyong. Discussion on Anti-interference Problems of Large-scale Measurement and Control Systems [J]. Ordnance Automation, 1997 (4): 10-13. [5] Chen Qiming. Anti-interference Measures in the Use of Measurement and Control Instruments [J]. Instrument Technology and Sensors, 2001 (5): 45-46.