PLC control and DCS control systems are not concepts at the same logical level.
PLC is named after its functionality, while DCS is named after its architecture.
In principle, PLC can form DCS.
Of course, there is still a performance difference between the two, which depends on the specific product and requirements.
The difference between DCS control system and PLC control system:
DCS is a decentralized control system, while PLC (Programmable Logic Controller) is just a control “device”, the difference between “system” and “device”. The system can achieve the functions and coordination of any device, while the PLC device only realizes the functions that this unit has.
The DCS network is the central nervous system of the entire system, and the international standard protocol TCP/IP is commonly used in DCS systems. It is a safe, reliable, and dual redundant high-speed communication network, with better scalability and openness of the system. However, PLC, because it basically works as a single small system, uses a single network structure when communicating with other PLCs or upper computers, and the network protocol often does not comply with international standards.
DCS considers the overall plan, and the operator station has the function of an engineer station. After the program is installed in the operation plan, there is a close joint relationship between stations. Any station, any function, or any controlled device is interlocked and controlled in a coordinated manner; The system composed solely of PLCs connected to each other has a loose connection between stations (PLCs and PLCs), which cannot achieve the function of coordinated control.
DCS has a large number of expandable interfaces in the entire design, making it very convenient for external or extended systems. After the completion of the entire system connected by PLC, it is difficult to arbitrarily add or reduce operator stations.
To ensure the safety and reliability of DCS controlled equipment, DCS adopts dual redundant control units. When important control units fail, relevant redundant units will switch to working units in real-time without disturbance, ensuring the safety and reliability of the entire system. The system connected to the PLC needs to be configured with dual PLCs to achieve redundancy.
Updating various process control schemes is the most basic function of DCS. When a certain scheme changes, the engineer only needs to compile the modified scheme in the engineer station master and execute the download command. The download process is completed by the system and does not affect the operation of the original control scheme. Various control software and algorithms in the system can improve the control accuracy of process objects. For systems composed of PLCs, the workload is extremely large. First, it is necessary to determine which PLC to edit and update, then use the corresponding compiler to compile the program, and finally use a dedicated machine (reader) to one-on-one transmit the program to this PLC. During system debugging, it greatly increases debugging time and costs, and is extremely detrimental to future maintenance.
There is a significant difference in control accuracy. This determines why in large and medium-sized control projects (above 500 points), systems connected entirely by PLC are not used. The reason is that all I/O modules in the DCS system are equipped with CPUs, which can achieve quality judgment and scalar transformation of acquisition and output signals, fault electrification, and random replacement. The PLC module is just a simple electrical conversion element without intelligent chips, and after a fault, all corresponding units will collapse. 1. Distributed Control System (DCS) is a control system that integrates computer technology, control technology, communication technology, and CRT technology to monitor, control, operate, and manage the production process.
The common understanding between DCS and PLC is the basic configuration of software configuration, but DCS is more powerful, such as database generation, historical data generation, graphic generation, report generation, and control configuration.
The objects controlled by PLC are generally relatively simple, while DCS is different. It can control all the equipment of the enterprise.
There is no one who controls who. PLC can provide signals to DCS, and in turn, DCS can also provide signals to PLC. The two cooperate with each other, mainly depending on the requirements of the equipment.