PLC Control System Design Principles
1) Practicality
Practicality is a fundamental principle of control system design. Engineers must not only study the controlled object but also understand the operating environment of the control system to ensure that the designed system meets all user requirements. The hardware should be as compact and flexible as possible, while the software should be simple and convenient.
2) Reliability
Reliability is an extremely important principle for control systems. For systems that may pose a danger, it is essential to ensure the long-term stable, safe, and reliable operation of the control system. Even if the control system itself malfunctions, it should at least be guaranteed that there will be no significant loss of personnel or property. In the initial stages of system planning, potential problems should be fully considered, different design schemes should be proposed, and a highly reliable and easily implemented scheme should be selected. During hardware design, appropriate backups or redundancies should be considered based on the importance of the equipment. In software design, corresponding protective measures should be implemented, and online commissioning and operation should only commence after repeated testing to ensure there are no major oversights.
3) Economic efficiency
This requires engineers to prioritize practicality and reliability while ensuring the system's hardware and software configurations are economical and cost-effective, avoiding the blind pursuit of new technologies and high performance. Hardware selection should prioritize economy and suitability; software development should strike a balance between development cycle and product functionality. Furthermore, the availability of complete technical documentation and after-sales service for the products used should be considered to reduce development costs.
4) Scalability
This requires engineers to fully consider the user's future production development and process improvement needs when planning the overall system. They should leave appropriate margins in the controller's computing power and the number of I/O ports, and also provide external expansion interfaces to meet the needs of system expansion and monitoring.
5) Advanced nature
This requires engineers to prioritize the use of technologically advanced and widely used mature products in the hardware design phase to compose the control system, ensuring that the system remains advanced for a certain period and is not eliminated by the market. This principle, along with economic considerations, ensures that the control system has a high cost-performance ratio.
PLC Control System Design Flow
When designing a control system, a specific design process should be followed. Mastering this process can increase the efficiency and accuracy of the control system design. The general design process for a PLC control system is shown in Figure 1-1.
Analysis and description of the controlled object
Analyzing the controlled object involves a detailed analysis of its technological process and an understanding of its operational characteristics. This stage requires in-depth communication with the user to ensure a comprehensive and accurate analysis. Control system design often necessitates meeting specific indicators and requirements, i.e., satisfying practical applications or customer needs. These indicators and requirements must be considered when analyzing the controlled object. Following a comprehensive analysis, the controlled object must be accurately described using engineering methods according to certain principles, laying a solid foundation for the control system design.
1) System Scale
The system size is determined based on the process flow and complexity of the controlled object, as well as the customer's technical requirements. It can be categorized into large, medium, and small sizes. It's crucial to ensure that hardware resources have sufficient margin to avoid waste.
Small-scale control systems are suitable for single-machine or small-scale production processes, primarily using sequential control. The signals are mostly switching quantities, and the number of I/O points is relatively small (less than 128). Accuracy and response time requirements are not high. Generally, an S7-200 microcontroller is sufficient to meet the control requirements.
Medium-scale control systems are suitable for production processes with complex logic and closed-loop control. They have a relatively large number of I/O points (between 128 and 512) and need to perform certain special functions, such as PID control. S7-300 is generally chosen as an example.
Large-scale control systems are suitable for large-scale process control, DCS systems, and factory automation network control. They have a large number of I/O points (more than 512 points), the controlled object's process is complex, and high requirements are placed on accuracy and response time. High-end PLCs with intelligent control, high-speed communication, database, and function operation capabilities, such as the S7-400, should be selected.
2) Hardware configuration
Estimate the number of I/O points for the control system based on system scale and customer technology. Analyze the controlled object's process flow and compile statistics on the number and types of I/O points. Divide the system according to different equipment and production areas, clarifying the location and function of each I/O point. Add 10% to 20% as a reserve and create a detailed I/O point list.
3) Software Configuration
Select appropriate software based on the design requirements of the control system, including system platform software and programming software.
Choosing the right host computer monitoring software. First, consider the limit on the number of monitoring points; whether it has alarm display, trend analysis, report printing, and historical record functions.
4) Control Function
To correctly select the scale of a control system, it's essential to first understand the characteristics of various controllers, such as performance parameters, application scenarios, industry solutions, reliability, and versatility. Generally, the following points should be considered when choosing a control system:
Does the control system need redundancy? Does the I/O signal module need redundancy? Does the communication need redundancy?
The number of control points includes the number of digital input and output points and the number of analog input and output points.
Is the process of the controlled object complex? Does it require special functions, such as anti-surge control?
When the system is running normally, does the controller have sufficient working margin in terms of load rate? Do the I/O signal points require a certain margin?
For digital signals, is relay isolation required? Consider the voltage and current levels of the input signal; and whether the output signal requires a solid-state relay output.
For analog signals, is a safety isolation barrier required? What is the signal type—voltage or current? What are the measurement ranges for voltage and current? Different signal types require different I/O signal modules.
For the signal module used for temperature measurement, consider whether it is a resistance temperature detector (RTD) or a thermocouple.
Does the signal module require hot-swappable replacement in-line? If so, additional special backplane slots should also be considered.
When system or external faults occur, such as signal short circuits or forging furnace failures, should the signal module automatically switch the input and output signals to pre-set safe values? If so, fail-safe controllers and signal modules should be considered.
When communication with third-party devices is required, the length of the communication distance and the corresponding communication interface protocol should be considered, and different communication modules should be selected accordingly.
For important interlocking signals in the system, is a special SOE module needed to record the temporal sequence of signal changes?
Familiarity with the controlled object is fundamental to designing a control system. Only through in-depth understanding of the controlled object and the controlled process can a reasonable and scientific control scheme be proposed.
1) Analyze the controlled object. Conduct a detailed analysis of the controlled object's process flow to understand its operating characteristics. In-depth communication with the user is crucial at this stage to ensure a comprehensive and accurate analysis.
2) Draw the process flow diagram. After the first step, you should have a thorough understanding of the entire process flow of the controlled object. To represent it more intuitively and concisely, draw the process flow diagram to prepare for the subsequent system design.
3) Analyze and define the control tasks. Based on the existing process flow diagram, engineers can translate the user's control requirements into technical terms, break them down one by one, and transform the requirements into multiple control loops from a control perspective. For process control systems, P&ID diagrams can be used to represent the control relationships.
PLC Control System Overall Design
Before designing a control system, a feasibility study of the system's design is necessary. This primarily involves making a predictive estimate of the overall system's feasibility. At this stage, it is crucial to comprehensively consider all potential problems that may arise during the design and implementation of the system. If there is no prior experience with related projects, a thorough on-site investigation and detailed feasibility study of each step in the system design are essential. Especially during the hardware implementation phase, even slight oversights can lead to significant problems, ranging from system failure to severe loss of personnel and property. Obstacles during project implementation often stem from insufficient attention to this crucial step.
The overall design of the system relates to the overall architecture of the entire system, and every detail must be carefully considered. First, it must meet the basic requirements put forward by the users; second, it must ensure the reliability of the system, so that failures are not frequent and, even if they do occur, will not cause significant losses; and then, economic efficiency and other aspects must be considered.
Generally, the following issues need to be considered during the overall system design:
(1) Determine whether the system is controlled by a standalone PLC or a networked PLC; determine whether the system uses remote I/O or local I/O. The choice mainly depends on the size of the system and the functions required by the user. For general small and medium-sized process control systems, standalone PLC control is generally sufficient to meet functional requirements. However, the concept of distributed control systems can be adopted, which disperses hazards and controls while centralizing management and monitoring. This can greatly improve the reliability of the system.
(2) Whether communication with other parts is required. A complete control system will include at least three parts: controller, controlled object, and monitoring system. Therefore, the controller must communicate with the monitoring system at least. Whether to communicate with other control units or departments depends on the user's requirements. Generally speaking, such communication interfaces will be provided even if the user does not require them.
(3) Which communication method is used? Generally speaking, PROFIBUS/USDP is used at the field control level, while PROFINET is used for communication from the field control level to the monitoring system. However, sometimes they can be used interchangeably, and the appropriate communication method should be selected according to the specific situation.
(4) Is a redundant backup system required? Different methods should be selected based on the required security level of the system. For data archiving, OS server redundancy can be used to prevent data loss; in automation stations (AS), controller redundancy can be used to prevent system downtime or unpredictable consequences due to malfunctions. Choosing appropriate redundancy backups can significantly improve system reliability.
Before selecting a control system, we should first consider how the system's network structure is built. The network structure diagram is shown in Figure 1-2.
Determine the number and location of the system's operator stations and process control stations, and how they are interconnected. Determine if an industrial Ethernet switch is needed.
Typically, the field control room and main control room are located in separate, relatively far locations from the electrical control cabinets. To ensure stable and reliable signals, fiber optic cables are used to connect their respective switches. Furthermore, for communication line redundancy, industrial Ethernet switches with honor management capabilities are selected to form a fiber optic ring network between the field operator stations and process control stations. This way, even if communication is lost in one direction, it can continue through the other.
Traditionally, the connection between the process control station and field signals involves directly connecting the field signals to the process control station via hardware. However, this method suffers from signal loss over long distances, especially for analog signals. Furthermore, with numerous signal points, wiring becomes complex and wasteful of materials. Therefore, it is generally necessary to install distributed I/O slave stations in the field (if the field is a hazardous area, intrinsically safe distributed I/O slave stations should be selected). Field signals are directly connected to the I/O slave stations, and then transmitted to the process control station via a fieldbus.
As a crucial foundation for automation, understanding and learning PLCs is a major stepping stone into the industrial control field. I believe that in your industrial control career, you have certainly encountered, learned about, and used PLCs; they have undoubtedly brought you both joy and sorrow.
