In a narrow sense, DCS is mainly used for process automation, PLC is mainly used for factory automation (production line), SCADA is mainly for wide-area needs, such as oil fields, pipelines that stretch for thousands of miles. If they are unified from a computer and network perspective, the difference is mainly in the application requirements, DCS often requires advanced control algorithms. For example, in the refining industry, PLC requires high processing speed. Because it is often used in interlocking or even fail-safe systems, SCADA also has some special requirements, such as vibration monitoring, flow calculation, peaking and valley adjustment.
SCADA is the scheduling management
DCS is the management of the plant
PLC is the field device layer
PLC system:
The programmable controller is suitable for measurement control in industrial field. It has strong on-site measurement and control function, stable performance, high reliability, mature technology, wide use and reasonable price.
DCS system:
That is, the distributed control system is a large-scale international advanced control system in the 1990s. It is suitable for industrial sites with many measurement and control points, high precision of measurement and control, and fast measurement and control speed. It features distributed control and centralized monitoring. It has networking communication capability, strong measurement and control function, reliable operation, easy expansion, convenient configuration and easy operation and maintenance. But the system is expensive.
SCADA system:
That is, the distributed data acquisition and monitoring system is a small and medium-scale measurement and control system. It concentrates on the two advantages of the on-site measurement and control function of the PLC system and the networking communication capability of the DCS system, and the performance price ratio is high.
Distinction and misunderstanding:
SCADA, DCS is a concept, PLC is a product, and the three are not comparable.
A product of PLC that can constitute SCADA and DCS.
DCS is developed by process control, and PLC is a relay-logic control system developed.
PLC is the device, DCS, SCADA is the system.
The difference between DCS and PLC:
1. DCS is a "decentralized control system", and PLC (programmable controller) is just a kind of 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. In terms of network, DCS network is the central nervous system of the whole system. It is a safe and reliable dual redundant high-speed communication network, and the system is more expansive and open. Because PLC basically works for individuals, when it communicates with other PLCs or host computers, the network form used is basically a single network structure, 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, and network dual redundancy.
3. DCS overall considerations, the operator station has the function of the engineering station, and the station and the station are closely related 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, and 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, 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 any disturbance. Ensure the safety and reliability of the entire system. The system that the PLC is connected to has basically no concept of redundancy, let alone the 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 modified scheme 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), the system that is connected by PLCs is basically not used.
7. Module: DCS system All I/O modules are equipped with CPU, which can realize the quality judgment and scalar conversion of acquisition and output signals, faulty hot plugging, 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.
8. Now the functions of high-end PLC and DCS are almost the same. DCS has a strong function for timed scanning of network and distributed database, and at the same time, it is better at comparing the calculation and analog quantity.
9. PLC is also divided into large, medium, small, micro PLC, which only sells a few hundred to 2,000 pieces, the number of points is also small, the large one can bring thousands of points, the computing power is similar to DCS, but for multi-machine networking The function is weak. Now both technology platforms are similar, but the focus is different.
Since DCS and SCADA are concepts? In fact, it should be understood as an architecture...
PLC and DCS are gradually evolved from different early product forms and applications. It can be said that PLC has developed relays in the manufacturing industry.
DCS evolved from the development of process-controlled instruments.
Each product has its own use occasions and characteristics, with the advancement of technology, the mutual penetration of the industry and fierce competition.
Both PLC and DCS products are infiltrating each other while developing their own advantages.
It should be known that the logic control function of the PLC is very powerful, and the loop control is not worth mentioning (you can look at the PLC with a slightly older model) and the DCS system is just the opposite. This is due to the difference in the internal processing mechanisms of the two systems.
Look at it now?
The old PLC manufacturers all involve DCS applications such as Siemens and Rockwell. The system is just a PLC plus network plus software plus enhanced controller.
DCS vendors such as Honeywell's systems also incorporate PLCs to enhance their logic control capabilities. It should be noted that PLC is a kind of controller;
DCS is a system consisting of controller + IO acquisition + network + software.
The SCADA system is another concept that has a system that is presented by another application requirement.
As its name suggests, it is:
Distributed data acquisition and monitoring system, its origin and application are different from the former two. It is mainly used for data acquisition, such as power monitoring system, oil pipeline monitoring system, which is characterized by control point dispersion. A system may cover thousands of functions [such as oil and gas pipeline monitoring systems in North America];
The communication structure is complex, not comparable to the general control system [from fiber to wireless or even satellite communication]. Its basic unit RTU.
I found two problems:
1. Define a framework for a system;
2. Explain and educate current students with original system definitions
To know that products are constantly evolving, there is competition between different products, that is, to constantly learn the strengths of others while exerting their own advantages, this is integration.
The focus of SCADA is on monitoring and control, and some logic functions can be realized, which are basically used for upper level;
PLC simply implements logic functions and controls, does not provide a human-machine interface, and requires operation with button indicators, HMI, and SCADA systems;
DCS has both functions, but it is basically used in relatively large systems and in some systems with high control requirements, and the price is also more expensive;
The three interpenetrate each other. In some production enterprises, there will be 3 systems, SCADA as the production management level, and DCS to achieve complex control, while PLC realizes single machine and simple control.
However, with the development of technology, some manufacturers' PLCs can also realize very complicated and precise control, and gradually occupy the market of DCS......
Simply put, PLC+SCADA=DCS, as far as I understand it, they can achieve crossover between the control functions. It is also based on the control of this system to develop the advantages of the other party.
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