Application of PLC in Industrial Distributed Control Systems
PLC, or Programmable Logic Controller, is an electronic system used to perform various complex industrial control functions. By handling inputs and outputs through relays, converters, and other systems, it executes numerous logic operations, such as sequential control, timing, counting, and digital arithmetic instructions, achieving the goals of the control system. Currently, the integration of PLC with DCS (Distributed Control Systems) is a developing trend in the control field. In the future, PLC will be extensively integrated into industrial distributed control systems. Furthermore, as the internal compatibility, flexibility, interchangeability, and stability of the systems continue to improve, it is foreseeable that classic control systems will be replaced by DCS integrated configuration systems, leading their application and development into a new phase. From practical use, the average failure interval time of PLC control systems reaches around 40,000 to 60,000 hours, with the ultimate lifespan even reaching 100,000 hours.
1. Analysis of Distributed Control Systems
DCS, also known as Distributed Control System, is a complex industrial system used for centralized control or classified parallel control of production processes. It belongs to a comprehensive equipment control device. The emergence of DCS came with the continuous improvement of large-scale industrial production automation, leading to increasingly complex system requirements in the production process. This necessitated an industrial system capable of parallel centralized control, thus giving rise to DCS, which combines communication networks with automatic control systems to significantly enhance efficiency in industrial production. A classic DCS system typically consists of three parts: a centralized management system, decentralized control devices, and communication network connection modules.
Workflow Analysis of DCS Systems
DCS can set and control hardware and program modules using configuration software according to actual process control requirements, such as selecting control circuits and control schemes and generating trend screens after configuration. Before starting, DCS generates a series of configuration files that can be loaded into process control units and operator stations based on various file attributes before the system runs. Subsequently, the process control unit finds the corresponding control algorithm from the function module library solidified in EPROM and initiates the process according to the content of the configuration information file.
Differences between DCS and PLC
DCS control systems are artificial systems developed by instrument manufacturers on overall disc-shaped control devices, capable of analyzing related analog quantities through monitoring data from the field and providing correct information to control personnel. DCS system manufacturers can adopt different communication protocols, but the network transmission platform generally uses Ethernet, offering convenient external expansion interfaces for future functional extensions. In contrast, PLC systems are generally fixed devices with little demand for functional extensions after installation and startup, making compatibility checks and related functional expansions difficult.
DCS control systems can use redundant control units to ensure overall system stability, allowing for interference-free switching to backup modules in case of major failures, ensuring that system operation status is not lost. However, despite PLC devices using a high-precision cyclic sequential scanning method, there is no redundant control program for hot-swappable modules, requiring system maintenance and replacement after shutdown. DCS control systems usually have a complete system database where each link in the database system provides real-time references to related data, supporting other links flexibly and conveniently. Similarly, in the DCS’s PLC configuration devices, each link has corresponding data links for real-time application during dynamic invocation, ensuring normal system operation.
2. Application of PLC in DCS and Industrial Distributed Systems
Functions of PLC in DCS
PLC’s processing speed, operation, and control functions can be integrated with DCS systems, evolving PLC from a specialized controller to an open-type controller. In terms of application scope and level, PLC opens new avenues for electrical control, instrument control, and computer control integration. In DCS control systems, increasingly more intelligent digital regulators and PLC configurations are being used. New digital voltage regulators and PLCs not only have large capacity and high speed but also strong network communication capabilities. They can use relatively inexpensive twisted-pair cables as transmission media for fieldbus networks, allowing controllers, PLCs, or digital intelligent transmitters and field control stations to serve as main nodes, connecting dozens of digital regulators as slave nodes to form high-speed data streams and perform staged sequential control. With the continuous improvement of PLC’s various characteristics and functions, the control functions of DCS and PLC will be further integrated in the future.
Advantages of Combining PLC and DCS
1. Ease of Operation and High Efficiency: PLC systems are easy to operate, have fast running efficiency, and simple hardware replacement and maintenance, ensuring high system reliability. In industrial systems, using PLC not only avoids the complexity of maintaining large system circuits but also ensures the system is not tampered with, preventing significant safety accidents and improving overall system stability, work efficiency, and reducing personnel work time. When designing PLC systems, it is usually necessary to use sequential controllers to create ladder diagrams according to control system requirements and verify data through simulation systems to ensure standardized design.
2. Integration into Powerful Standardized Control Systems: During the control process of industrial automation systems, PLC can be integrated into powerful standardized control systems according to the characteristics of specific control objects, providing simple control and flexible hardware replacement features. PLC mainly includes modules for input and output, data-related analog quantity control, and network system communication. During the control process of industrial automation systems, PLC can control analog quantities and customize control equipment through programming statements. PLC’s application in industrial automation systems significantly improves the precision of work systems, avoiding work type limitations caused by low precision. Additionally, PLC can completely control the entire process of industrial heat treatment, avoiding manufacturing errors due to excessively high or low temperatures, greatly facilitating industrial automation in heat treatment.
3. Position Control in Automated Manufacturing: Position control is crucial in the industrial automation process. During industrial equipment manufacturing, the system needs to perform compensation control for various tools’ positions and positioning of transport tools.
4. Automated Intelligent Control: In the process of automated intelligent control of industrial production, PLC can achieve fully automated unmanned control and self-health monitoring of the system. If the PLC configuration module detects a fatal error or equipment failure in the production line, it will send a stop signal for repair. For minor faults, PLC can repair by restarting related modules, saving work. The fault detection principle of PLC is: during production line operation, the working components of the electronic control system are connected to the database. If a fatal fault occurs and cannot be recovered, the logical operations in the relevant database will be interrupted, and the PLC system will receive a notification signal and begin the troubleshooting process. In summary, after integrating with DCS systems, PLC has powerful functions and rich instructions, so efforts should be increased to promote it.
