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PLC process control common fault analysis and maintenance - Database & Sql Blog Articles
In the context of extending the lifespan of a PLC control system, it is essential to have a clear understanding of both the equipment consumption and the likelihood of component failures within the system. This knowledge allows for proactive measures to be taken in advance. By analyzing the failure distribution of a PLC process control system used in our special cement line 1, we aim to provide insights that can aid in the design and maintenance of similar systems.
System failure generally refers to any malfunction within the entire production control system, which can be categorized into two main areas: PLC-related failures and on-site equipment failures. The PLC system includes components such as the central processing unit (CPU), main chassis, expansion chassis, I/O modules, and related network devices. On-site equipment encompasses elements like I/O ports, relays, contactors, valves, motors, and other field control devices.
In our factory's special cement line 1, the system was upgraded in 2000 with a PLC-based process control system using Mitsubishi's A2 series PLCs. The system features two centralized control rooms—the kiln tail and kiln head control rooms—with the latter serving as the main station. There are also two on-site workstations and two TV monitoring systems for preheater and kiln head operations. These on-site workstations operate independently but communicate with the main station through analog methods.
According to statistical data, the system experienced a total of 126 faults, with approximately 4.7% attributed to PLC failures and 95.3% to field equipment failures. Given the relatively short operational period, this distribution closely resembles general trends observed in PLC systems. Typically, about 5% of failures occur in the PLC itself, while 95% stem from on-site equipment.
The most common points of failure in the PLC system include the power supply and communication networks. Continuous operation and heat generation make voltage and current fluctuations inevitable. Communication systems are also vulnerable to external interference, making environmental factors a major cause of failure. Additionally, long-term use of plug-in modules can lead to damage to the system bus, Connectors, or printed circuit boards due to aging and oxidation.
To mitigate these issues, it's crucial to maintain a clean and controlled environment in the control room, ensure proper cooling, and follow strict operating procedures during maintenance. Modern PLCs often use rewritable ROMs, whose lifespans depend on both manufacturing quality and power supply stability.
PLC I/O ports are another critical area, as they represent the technical strength of the PLC. External interferences should be minimized by following manufacturer guidelines and implementing isolation techniques. Field control equipment, such as relays, contactors, valves, and sensors, are the most prone to failure. Harsh environments contribute significantly to wear and tear, requiring regular inspection and maintenance.
For example, relays and contactors often experience contact degradation due to heat, oxidation, or mechanical stress. Valves and actuators may fail due to misalignment or mechanical wear. Limit switches and proximity sensors can become unreliable due to dust accumulation or corrosion. Even small components like junction boxes, wiring terminals, and bolts can cause failures if not properly installed or maintained.
Power and signal line noise is another common issue, often resolved through proper shielding, grounding, and filtering techniques. Ensuring reliable signal transmission is key to maintaining system integrity.
Ultimately, reducing failure rates requires adherence to safe operating procedures, regular maintenance, and attention to environmental conditions. By focusing on these areas, system reliability can be significantly improved, leading to more efficient and stable operations.