1, in terms of development:
Developed from the traditional dashboard monitoring system. Therefore, DCS is essentially more focused on instrument control. For example, the Yokogawa CS3000 DCS system we use has no limit on the number of PID (PID, proportional differential integral algorithm, is the standard algorithm for closed-loop control of regulating valves and frequency converters, and usually the number of PID determines the number of regulating valves that can be used).
PLC is developed from the traditional relay circuit, and the original PLC didn't even have the ability to process analog signals. Therefore, PLC emphasized the ability of logical operation from the beginning.
2. In terms of system scalability and compatibility:
There are many control products in the market, both DCS and PLC are produced and sold by many manufacturers. For PLC systems, there is generally no need or little need for expansion, because PLC systems are generally used in equipment. Generally speaking, PLC rarely requires compatibility. For example, two or more systems need to share resources, which is also very difficult for PLC. Moreover, PLC generally adopts a special network structure, such as the MPI bus network of Siemens. Even if an operator station is added, it is not easy and the cost is not high.
In the process of development, DCS is also a self-contained system of various manufacturers, but most DCS systems, such as Yokogawa, Honeywell, ABB, etc. Although the communication protocols in the system (process level) are different, the network platform at the operation level adopts Ethernet without exception and adopts standard or variant TCP/IP protocol. This provides very convenient scalability. In this network, both the controller and the computer exist as a node. As long as the network reaches the place, it can increase or decrease the number of nodes and arrange the location of nodes at will. In addition, based on the open protocols such as OPC and DDE of windows system, all systems can communicate conveniently and realize resource sharing.
3. From the database:
DCS generally provides a unified database. In other words, once the data exists in the database of DCS system, it can be referenced under any circumstances, such as in configuration software, monitoring software, trend chart and report ... The database of a PLC system is usually not unified, and configuration software, monitoring software and even archiving software all have their own databases. Why is it often said that Siemens S7 400 is called DCS when it reaches 4 14? Because the PCS7 system of Siemens uses a unified database, PCS7 requires the controller to be at least S7 4 14-3.
4. In terms of time arrangement.
Generally, the program of PLC can't run according to the preset cycle. The PLC program is executed once from beginning to end, and then executed from the beginning. (Now some new PLCs have been improved, but the number of task cycles is still limited. ) and DCS can set the task cycle. Such as fast tasks. Similarly, sensor sampling, the change time of pressure sensor is very short, we can sample with a duty cycle of 200ms, while the lag time of temperature sensor is very large, we can sample with a duty cycle of 2 s, so that DCS can reasonably schedule the resources of the controller.
5. From the network structure:
Generally speaking, DCS usually adopts two-layer network structure, one is process-level network, and most DCS adopt their own bus protocols, such as Modbus of Yokogawa, Profibus of Siemens and ABB, CAN bus of ABB, etc. These protocols are based on standard serial port transmission protocols RS232 or RS485. The sampling data (machine code, 2 13/ scanning period) of the field IO module, especially the analog quantity, is huge, and there are many interference factors in the field, so the network standard with large data throughput and strong anti-interference ability should be adopted. Based on the bus structure of RS485 serial asynchronous communication mode, the sampled data meeting the requirements of field communication are converted into plastic data or real data by CPU and transmitted on the operation level network (layer 2 network). Therefore, the network standard of moderate data throughput, fast transmission speed and convenient connection can be adopted in the operation level network. Because the operation level network is generally arranged in the control room, the requirements for anti-interference are relatively low. Therefore, adopting standard Ethernet is the best choice. TCP/IP protocol is a standard Ethernet protocol. Generally, we use the communication speed of100 mbit/s. ..
The task of PLC system is relatively simple, so the amount of data to be transmitted is generally not too large, so the common PLC system is a layer of network structure. Process-level network and operation-level network are either merged or simplified as internal connections between modules. PLC does not use or rarely uses Ethernet.
6. Judging from the scale of the applicants:
PLC is generally used in small automatic control places, such as equipment control or a small amount of analog control and interlocking, and DCS is generally used in large-scale applications. Of course, this concept is not accurate, but it is intuitive. Traditionally, we call the system above 600 points as DCS, and the system below 600 points as PLC. Our control systems of heat pump, QCS and horizontal products are collectively called PLC.
Having said so much about the differences between PLC and DCS, we should realize that today's PLC and DCS are actually getting closer and closer. Strictly speaking, PLC and DCS can no longer be uniform, and the concept between them has been blurred in many cases.
Now, let's discuss our similarities.
1, function:
PLC already has the control function of analog quantity, and some PLC systems are even quite powerful in analog quantity processing, such as Yokogawa FA-MA3, Xizi S7 400, ABB Control Logix and Schneider's Quantum system. Moreover, DCS also has quite strong logical processing ability. For example, we have realized all possible process interlocking and equipment linkage start-stop on CS3000.
2, from the system structure:
The basic structure of PLC and DCS is the same. Today, PLC has been completely transplanted into computer system control, and the traditional programmer has long been eliminated. Small application PLC generally adopts touch screen, and large application PLC adopts computer system in an all-round way. Like DCS, the controller and IO station use field bus (generally based on RS485 or RS232 asynchronous serial communication protocol). If there is no expansion requirement between the controller and the computer, that is, only one computer is used, then this bus will also be used for communication. However, if multiple computers are used, the system structure will be the same as DCS, and the upper computer platform will adopt Ethernet structure. This is also one of the reasons why the concepts of PLC and DCS are vague after large-scale PLC.
3, the development direction of PLC and DCS:
Miniaturized PLC will develop to a more professional use angle, such as more targeted functions, more targeted application environment and so on. The boundary between large PLC and DCS is gradually weakened until it is completely integrated.
District councils will continue to develop into functional constituencies. The core of FCS is not only a more decentralized control system, but also instruments. The application of FCS abroad has developed to instrument level. The control system only needs to deal with signal acquisition, man-machine interface and logic control. The control of the whole analog quantity is distributed to the field instrument, and the instrument and the control system do not need to be connected through the traditional cable, and the whole instrument system is connected through the field bus. (At present, Yokogawa has used FCS in CNOOC Shell Petrochemical Project in China, and the instrument level is intelligent instruments such as EJX, which has the most advanced control level in the world).
How to treat PLC and DCS correctly?
Personally, I never emphasize which is better than PLC or DCS. I used a new term "control products" for them. We provide users with the most suitable control system. The vast majority of users will not use DCS because they want to use a set of DCS, and the control products must be positioned on the basis of meeting the user's process requirements. In fact, users who propose to use DCS or PLC have never been exposed to automatic control products or have some special needs. Too much emphasis on this thing will only lead to arguments.
Understand the general situation of control products from the similarities and differences between PLC and DCS. Attention, as professionals, don't define products as PLC or DCS, and don't treat products differently in this way psychologically.