Fujian Sanan Steel Company's engineering automation system uses a SIEMENS SS7-400 PLC system. The PLC acts as the control station, and industrial computers act as operator stations, forming a distributed structure to meet the control needs of the raw material yard, sintering plant, blast furnace, converter, and continuous casting. However, harsh environments, strong interference, improper installation and use, inadequate maintenance, and insufficient protection programs can all compromise the normal, reliable, and safe operation of the PLC. Therefore, discussing how to improve the reliability of the PLC control system is crucial.
1. Main technical performance and technical requirements
(1) The Siemens S7-400 control system requires the equipment to operate in an ambient temperature of 20-25℃, be shockproof, dustproof, and electromagnetic interferenceproof, and have a relative humidity of 20%-70% without condensation; the principle for arranging templates on a base frame is: the number of templates that can be inserted is limited by the current value they obtain from the super backplane bus (see the technical requirements of each template for details).
(2) Arrangement principles for S7-400 modules in multiple rack configurations. Interface modules are always located in slot 3, to the left of the first signal module. No more than 8 slots should be used per rack, and modules are always located to the left of the interface modules. The number of modules that can be inserted is limited by the current allowed by the S7-400 backplane bus, per row or per rack.
The current consumption should never exceed 1.2A .
(3) Selectable power supply for CPU operation: 200-240VAC, 24VDC.
(4) The total power consumption of the selected power supply module shall not be lower than or equal to the total current consumption of the input and output units.
(5) The CPU module has a backup battery to prevent program loss.
(6) The system should be grounded separately, and the grounding resistance R≤4n.
2. Main factors affecting the reliability of PLC control systems
The reliability of a PLC control system directly affects overall production efficiency and safety. A problem in any part of the system—whether hardware, software, or peripheral equipment—will lead to production stoppages. Therefore, it is a primary factor in discussing technical requirements. Figure 1 shows a fault diagram for analyzing a PLC control system.
As shown in Figure 1, only 5% of the failures occur within the programmable logic controller (PLC), indicating that the reliability of the PLC itself is far higher than that of external devices. 90% of the failures occur in the I/O modules, while only 10% occur in the controller itself. This means that failures in the PLC's CPU, memory, system bus, and power supply account for only 0.5 % of total system failures, while failures in the I/O modules account for only 4.5 %. Analysis suggests that in practical applications, the following aspects are typically considered.
(1) Pay attention to the selection of external equipment to shorten a lot of maintenance time.
(2) Data and program protection. Most PLC control systems use lithium-ion battery-powered RAM to store user applications. A typical battery lasts about 5 years, after which all applications are lost. Therefore, memory cards are often used to store user applications in critical locations. Programs and technical data are archived for backup purposes.
(3) Certain anti-interference measures should be taken during installation and wiring. Generally, PLC power supplies use shielded 1:1 isolation transformers. When used in environments with strong interference sources, the shielding layer and the PLC floating ground terminal should be grounded, and the grounding wire area should not be less than 2mm². The grounding wire should be grounded independently and should not be connected in series with other equipment. PLC power lines, I/O power lines, input and output signal lines, and AC and DC lines should be wired separately as much as possible. Switch signal lines and analog signal lines should also be wired separately, and the latter should use shielded wires, and the shielding layer should be reliably grounded, etc.
3. Methods to Improve the Reliability of PLC Control Systems
3.1 Emphasis on installation
Improving the reliability of PLC control systems is a long-term and ongoing task. First, construction and installation are crucial and must be strictly controlled to reduce the failure rate after commissioning. Second, ensuring the quality of maintenance, especially for technical upgrades, wiring modifications, and system upgrades, is currently a top priority. Otherwise, after several years of system upgrades, the replacement of numerous wires, loss of wire numbers, and program changes, along with the lack of proper record keeping, will increase maintenance workload and compromise reliability. This is an area easily overlooked and must be given high priority.
3.2 Aging Screening Method
We typically use the "aging screening" method, which involves ending the "early" phase, extending the "accidental" phase, and replacing components in a timely manner during the "wear-out" phase to improve the reliability of the PLC system. This method is mainly used for non-repairable components. The failure rate of a PLC control system is time-dependent. We divide the change in the failure rate y(t) of equipment components over time into three periods for analysis, as shown in Figure 2. This change curve is commonly referred to as the failure rate curve or the bathtub curve.
As can be seen from Figure 2:
(1) Higher failure rate in the early stage (from 0 to t0). This is mainly caused by unreasonable reasons such as inherent design errors in the system, component quality, installation and process defects, etc., but the failure rate decreases rapidly with the passage of time. The main task during this period is to identify unreliable factors as early as possible and stabilize the system as soon as possible.
(2) The random failure period (t0 to t1) is relatively stable and can also be called the random failure period. During this period, failures occur randomly, the system failure rate is the lowest and stable, and can be regarded as a constant. This period is the optimal state period of the system. During operation, maintenance should be strengthened to extend the time of this period, and regular inspection and maintenance should be carried out.
(3) The failure rate increases during the wear-out period (after t1) because some components of the system have aged and worn out over time, resulting in mechanical and electrical wear and insulation aging. During this period, most components will begin to fail. If the start time of wear-out can be known in advance, components can be replaced in advance to extend the effective life of the system and postpone the arrival of the wear-out failure period.
Safety Design Methods for 4PLC Control Systems
Although PLCs are safe and reliable in operation, stability and reliability remain crucial issues for any system . System safety design should fully utilize the characteristics of PLCs to ensure their safe and reliable operation.
(1) Hardware protection. This mainly includes: interlock protection, limit protection and emergency stop protection, etc.
(2) Software protection. This mainly includes interlock protection, limit protection, time-limit protection, and self-test protection. PLCs use stored programs for control and protection, and can also ensure that they only execute correct operations and reject incorrect commands. Software protection mainly utilizes self-test information to promptly detect hidden dangers and eliminate faults; it can also be tailored to the characteristics of the project to eliminate faults, thereby ensuring the reliable, stable, and safe operation of the PLC.
5 PLC control system reliability management
5. One-person management
With the development of technology, the scientific content of automation is constantly increasing. This will place higher demands on the quality of staff. Staff must not only be familiar with equipment operation, maintenance, planning, and design, but also possess computer skills and a certain level of technical expertise. Therefore, it is essential to focus on the intensity, methods, and content of staff training. Only by continuously improving the professional and ideological qualities of staff can they be competent in their positions.
5.2 Maintenance and Management
The reliability of a PLC control system depends primarily on proper maintenance and management. Key maintenance components include: power supply, external environment, installation status, power supply (PS), central processing unit (CPU), signal modules (SM), and peripheral devices such as input/output intermediate relays and lifespan components.
6. Conclusion
As long as one is good at observing and summarizing experience in practical work, one can fully consider all the adverse factors of PLC and take appropriate measures in practical applications to ensure the reliable and safe operation of the control system.
