The PLC control and DCS control system is not a logical level concept. It can be seen from the name: PLC is named after function, and DCS is named after architecture. In principle, PLC can form DCS. Of course, the difference in performance between the two still exists, depending on the product and needs. From the application point of view, simply distinguishing by PLC and DCS will often lead to misunderstanding.
The difference between DCS control system and PLC control:
DCS is a “decentralized control system”, while PLC (can be edited
The controller is just a kind of control “device”, which is the difference between “system” and “device”. The system can realize the function and coordination of any device, and the PLC device only realizes the functions of this unit.
The DCS network is the central nervous system of the entire system, and the DCS system usually adopts the international standard protocol TCP/IP. It is a safe and reliable dual redundant high-speed communication network, and the system is more expandable and open. PLC is basically used for a single small system, when it communicates with other PLCs or host computers, the network used. The forms are basically single-network structures, and network protocols often do not conform to international standards. In network security _ 匕 PLC does not have good protection measures. The DCS overall consideration scheme, the operator station has the function of the engineering station, and the station and the station are closely combined after being installed in the running program. Any station, any function, and any controlled device are interlocked with each other. Coordinated control; while the system consisting of PLCs connected to each other, the connection between the station and the station (PLC and PLC) is a loose connection method, and the function of coordinated control cannot be performed.
DCS has a large number of extensibility interfaces in the whole design. The external system or the expansion system is very convenient. After the whole system of the PLC is connected, it is difficult to increase or decrease the operator station at will. In order to ensure the safety and reliability of the DCS controlled equipment, the DCS adopts a dual redundant control unit. When the important control unit fails, the relevant redundant unit will be switched to the working unit without any disturbance in real time, ensuring the safety and reliability of the whole system. . The system to which the PLC is connected needs to be configured with dual PLCs for redundancy. The update of various process control schemes is one of the most basic functions of DCS. When a certain program changes, the engineer only needs to execute the download command after the engineer webmaster compiles the changed plan. The process is completed by the system in white, and does not affect the operation of the original control scheme. Various control software and algorithms of the system can improve the control precision of the process object. For a system composed of PLC, the workload is extremely large. First, you need to determine which PLC you want to edit and update, and then use the compiler corresponding to the program to compile, and finally use a dedicated machine (reader). The transfer of the program to the PLC for one, greatly increases the commissioning time and commissioning cost during system debugging, and is extremely unfavorable for future maintenance.
There is a big difference in control accuracy. This determines why in the large and medium-sized control projects (500 points or more), the system that is connected by the PLC is basically not used. All the I/O modules of the DCS system have a CPU, which can realize the acquisition and output signals. Quality judgment and scalar transformation, faulty electrification, random replacement.
The PLC module is just a simple electrical conversion element, there is no smart chip, and the corresponding units are all defective after the fault.