Discussion on the Reliability of PLC Control Systems

I. Main Technical Performance and Requirements

1. Siemens S7-400 Control System (Digital and Analog)
   Operating Environment: Temperature range of 20-25°C, vibration-proof, dust-proof, and electromagnetic interference-proof.
   Humidity: 20%-70% relative humidity without condensation.
   Module Arrangement: The number of modules that can be inserted on a rack is limited by the current they can draw from the super backplane bus (specific technical requirements for each module should be referred to).
2. Multiple Rack Configuration for S7-400 Modules
   Interface Modules: Always positioned in slot 3, to the left of the first signal module.
   Slots per Rack: No more than 8 slots per rack, with modules always to the left of the interface module.
   Current Limitation: The number of modules that can be inserted is limited by the S7-400 backplane bus’s allowable current, with a maximum current consumption of 1.2 A per row or rack.
3. Optional CPU Power Supplies: 200-240 VAC, 24 VDC.
4. Power Module Selection: The total power of selected power modules must not be less than or equal to the total current consumption of input and output units.
5. CPU Module Backup Battery: Prevents program loss in case of power failure.
6. System Grounding: Should be grounded independently with a ground resistance.

II. Main Factors Affecting the Reliability of PLC Control Systems

The reliability of a PLC control system is directly related to overall production efficiency and safety. Issues in any part, whether hardware, software, or peripheral devices, can lead to production stoppages. Hence, it is a primary consideration in technical requirements.

PLC Fault Distribution: Only 5% of faults occur within the programmable controller, indicating that the PLC’s reliability is much higher than that of external devices. About 90% of faults occur in the I/O modules, and only 10% in the controller. Faults in the CPU, memory, system bus, and power supply make up only 0.5% of total system faults, while faults in I/O modules account for 4.5%.

1. External Equipment Selection: This can significantly reduce maintenance time.
2. Data and Program Protection: Most PLC systems use lithium batteries to support RAM for storing user applications. Typically, a battery lasts about 5 years. Once depleted, the application program will be lost. Therefore, important applications are often stored on memory cards.
3. Installation and Wiring: Interference mitigation measures must be taken. PLC power should be supplied via a shielded 1:1 isolation transformer. In environments with strong interference, the shield and PLC floating ground terminal should be grounded independently. Various signal and power lines should be wired separately to avoid interference.

III. Methods to Improve PLC Control System Reliability

Improving reliability should be considered throughout the system’s design process, including software, hardware, and peripheral devices.

1. Attention to Installation: Proper installation and commissioning are crucial to reduce initial fault rates. Ensuring quality during repairs, especially during technical upgrades and system modifications, is essential to maintain reliability.
2. Aging Screening Method: This method helps to eliminate early faults, extend the period of random faults, and promptly replace worn components to maintain reliability. It is mainly used for non-repairable components.
   Early Failure Period: High failure rates due to design, component quality, installation, and manufacturing defects, which decrease rapidly over time.
   Random Failure Period: Stable and low failure rate; this is the system’s operational period.
   Wear-out Failure Period: Failure rates rise due to aging and wear of system components. Timely replacement of components can extend the system’s effective life.

IV. Safety Design Methods for PLC Control Systems

Although PLC operation is generally safe and reliable, ensuring stability and reliability in system design is crucial.

1. Hardware Protection: Includes interlock protection, limit protection, and emergency stop protection.
2. Software Protection: Includes interlock protection, limit protection, time limit protection, and self-diagnosis. Using stored programs for control and protection ensures only correct operations are executed and errors are rejected. Self-diagnosis information can identify and address potential issues promptly.

V. Management of PLC Control System Reliability

1. Personnel Management: With the advancement of technology, higher requirements are placed on staff quality. Staff must be familiar with equipment, maintenance, planning, design, and have some computer knowledge. Continuous training and improvement of staff’s professional and ideological quality are necessary.
2. Maintenance Management: Effective post-installation maintenance is key to system reliability. Key areas include power supply, external environment, installation status, power supply (PS), central processing unit (CPU), signal modules (SM), and other peripheral devices.