In modern industrial production, programmable logic controllers ( PLCs ) are widely used. While PLCs can operate normally in harsh environments, control systems based on them sometimes experience malfunctions due to design, installation, and interference issues. Some problems arise from inadequate consideration during system design. Based on practical experience and lessons learned, this article discusses the issues that should be considered when designing programmable control systems.
1. Mature technologies should account for at least 75% of the total technology used in a system.
"Mature technology" refers to two aspects: first, programmable logic controller (PLC) products or similar designs that have undergone practical production testing, or designs that are certain to withstand future production challenges; second, the design personnel must have experience with or master the technology required. The technology and design scheme chosen for designing and configuring a programmable control system must be practical and feasible. This is because once a production process control system is built, it is intended for long-term use, and opportunities for repeated modifications are rare. Some deficiencies in the designed hardware system and programming software may remain hidden within the completed system. If conditions for damage occur, the consequences are unpredictable.
2. The system's hardware structure and network should be simple and clear.
The hardware structure should not be overly complex, and the network configuration should not have too many overlapping factors; efforts should be made to utilize the networking capabilities inherent in the programmable logic controller (PLC). When assembling the modules配套 with the I/O chassis, it is recommended to use simple and consistent model numbers, and the module density should not be too high. The number of wiring points should not be excessive; given current chassis manufacturing and wiring processes, the input and output wiring density should not be too high.
3. The functions of the control system and the management system should be strictly separated.
Because process control systems composed of PLCs have high real-time requirements, and network communication allows for temporary loss of communication and subsequent recovery, the PLC will be out of control during this interval before recovery. Furthermore, when multiple PLC systems are used to form a large system, for critical commands of the main control, in addition to using the PLC's own network communication to transmit information, it is best to have hardware interlocks made using its I/O points, especially for handling "emergency stops" between the two systems. Although both systems exchange "stop" or "emergency stop" commands during their own communication scans, if one system stops operating during an emergency stop failure, the other system may not receive the stop information and continue operating normally, with unpredictable consequences. For critical "emergency stops" in PLC control systems, the power supply to the actuator should be cut off first, and then its signal should be sent to the PLC. This allows time for equipment safety protection.
4. The programmable logic controller (PLC) program should be concise and readable.
User software is written in a straightforward manner. It can be viewed as an ordered series of "black boxes," each of which, according to structured languages, can be divided into several typical statements. While the style and techniques of each statement may be quite monotonous, they must be clear. If too many complex logical relationships, too many statement factors, or too many special conditions are used in the design and writing of these statements, they will become very difficult to understand. Therefore, the readability of a programmable controller's user software—that is, the ability for most people to understand what happens when the programmable controller executes a statement—is extremely important. Each program segment should strive to be functionally singular and fluent; this is a crucial condition for the use and maintenance of the software.
5. The programmable control system has pre-configured hardware and software features, along with key monitoring conditions for operational detection.
The programmable control system is equipped with a color graphic workstation/screen monitor, but screen monitoring is still inconvenient in terms of price and time to reflect the on-site status. For critical faults, or near critical mechanical equipment, some indicator lights can be configured. These can be made using digital outputs to monitor the normal operation of the program or to debug the program. Function labels next to the indicator lights can help operators confirm the normal operation of the programmable control system and promptly report faults.
6. When designing large and medium-sized programmable control systems, avoid exhausting their hardware and software resources.
For newly designed systems, at least 15% redundancy should be reserved in the hardware. Similarly, during software development, the user software's requirements and usage of computer resources must be estimated. In particular, sufficient margin should be left for the use of intermediate relays and counters/timers. This is because software will always be modified, supplemented, or even rewritten after debugging and operation. It is impractical in engineering for existing software to be unmodifiable and unimprovable.
