1. dcs is a "decentralized control system", and plc is just a (programmable controller) control "device", the two are 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.
2. On the network side, the DCS network is the central nervous system of the whole system. The system network in the MACS system of Lishi Company adopts dual redundant 100Mbps industrial Ethernet, adopting 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 an individual work, and its network form is used when communicating with other PLCs or host computers. Basically, they are single-network structures, and network protocols often do not conform to international standards. In terms of network security, PLC does not have good protection measures. We use power, CPU, network dual redundancy.
3. 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, any controlled device is mutual Chain control, coordinated control; and the system consisting of PLCs connected to each other, the connection between the station and the station (PLC and PLC) is a loose connection, which can not do the coordination control function.
4. DCS has a large number of extensible interfaces in the whole design. The external system or the extended 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. of.
5. DCS security: In order to ensure the safety and reliability of the DCS controlled equipment, the DCS adopts a dual redundant control unit. When an important control unit fails, the relevant redundant unit will be switched to the working unit in real time without interference. Ensure the safety and reliability of the entire system. There is basically no concept of redundancy in the system that the PLC is connected to, and there is no more redundant control strategy. Especially when one of its PLC units fails, the entire system has to be stopped before it can be replaced and reprogrammed. Therefore, the DCS system is one level higher than its safety and reliability.
6. System software, 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 compile the changed plan on the engineering station and execute the download command. Yes, the download process is automatically completed by the system 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 requirements control object. For the system composed of PLC, the workload is extremely large. First, it is necessary to determine which PLC to edit and update, and then use the corresponding compiler to compile the program, and finally use a dedicated machine (reader). One-to-one transfer of the program to this PLC greatly increases commissioning time and commissioning costs during system commissioning, and is extremely detrimental to future maintenance. There is a big difference in control accuracy. This determines why in large and medium-sized control projects (500 points or more), basically all systems connected by PLCs are not used.
7. Module: All I/O modules of the DCS system are equipped with a CPU, which can realize the quality judgment and scalar conversion of the acquisition and output signals, faulty hot plugging and replacement, and random replacement. The PLC module is just a simple electrical conversion unit, there is no smart chip, and the corresponding units are all defective after the fault.
We have to start from the origin of PLC and DCS: the development of PLC is based on the on-site control needs of the manufacturing industry, and the development of DCS is based on continuous process control and monitoring in the chemical industry. From the classification of control requirements, the control system can be divided into: sequential control (basic function of PLC), process control (DCS), position control (CNC), transmission control (speed regulation or synchronization), but from a development perspective, this Several parts of the control method are gradually merging. If you have to say something different, I think there are the following points: 1. Object-oriented differences: PLC is oriented to general industrial control and has strong versatility. DCS is heavily weighted for process control and is used in the chemical industry. 2. DCS emphasizes the precision of continuous process control, and can realize complex control such as PID, feedforward, cascade, multi-level, fuzzy, adaptive, etc. The general PLC only has PID function, and the control precision is not as high as DCS. 3, DCS system has a powerful SCAND software package, with recipe function, and developed expert software (chemical process parameter configuration and control algorithm) for different lines, which is more convenient than PLC application.
Finally, modern PLC has been able to perform most of the functions of DCS, and can do redundancy or hot standby, and modules can also be plugged in and out. The choice of PLC or DCS key depends on what you are facing. Economic, reliable and convenient are the most important.