Analyze the similarities and differences between D

  • Detail

Analyze the similarities and differences between DCs and PLC

there are many kinds of control products with different names. Commonly used control products include DCS and PLC. We also extended the concept of DCS to FCS. DCS (distributedcontrillsystem), distributed control system, also known as distributed control system. PLC (programlogiccontrol), programmable logic controller. FCS (fieldbusconvertorsyestem), fieldbus control system has developed to the present, there is no strict boundary between DCs and PLC. In most people's view, the large system is DCS, and the small system is called PLC. Of course, it's not impossible to say so, but it's not right. Now let's re-establish this concept

first, what is the difference between DCs and PLC

1. In terms of development, DCS is developed from the traditional instrument panel monitoring system. Therefore, DCS pays more attention to the control of instruments from the congenital point of view. For example, the yokogawacs3000dcs system we use does not even have the limit of the number of PID (PID, proportional differential integral algorithm, is the standard algorithm for the closed-loop control of regulating valves and frequency converters. Generally, the number of PID determines the number of regulating valves that can be used). PLC developed from the traditional relay circuit. The original PLC even had no analog processing capacity. Therefore, PLC emphasized the logic operation ability from the beginning

2. From the perspective of system scalability and compatibility: there are many control products in the market, and many manufacturers, whether DCS or PLC, are producing and selling them. For PLC system, there is generally no or little need for expansion, because PLC system is generally used for equipment. Generally speaking, PLC has few compatibility requirements. For example, two or more systems require resource sharing, which is also very difficult for PLC. Moreover, PLC generally adopts a special network structure, such as Siemens' MPI bus network, and even adding an operator station is not easy or costly. In the process of DCS development, each manufacturer has its own system, but most DCS systems, such as Yokogawa, Honeywell, abb, etc., although the internal (process level) communication protocols of the system are different, the operation level network platform is different, and the country has implemented the major project of "research and development and utilization of key new materials", and has also selected Ethernet, using standard or modified tcp/ip protocol. This provides convenient scalability. In this network, the controller is used to serve the index module in the data index, and the computer exists as a node. As long as the network reaches the place, the number of nodes and the location of nodes can be increased or decreased at will. In addition, based on OPC, DDE and other open protocols of windows system, each system can also communicate conveniently to realize resource sharing

3. From the perspective of database: DCS generally provides a unified database. In other words, once a data exists in the database in the DCS system, it can be referenced in any case, such as in the configuration software, in the monitoring software, in the trend chart, in the report... While the database of the PLC system is usually not unified, the configuration software, the monitoring software and even the archiving software have their own numbers, and the aluminum foil produced by Zongyang Jinyu metal materials Co., Ltd The aluminum scrap can be recycled. Why is it often said that Siemens S7400 is called DCS only when it is above 414? Because Siemens' PCS7 System uses a unified database, and PCS7 requires the controller to be at least s or above

4. In terms of time scheduling, PLC programs generally cannot run according to the preset cycle. The PLC program is executed from beginning to end and then from the beginning. (now some new PLCs have been improved, but there are still restrictions on the number of task cycles) and DCs can set task cycles. For example, fast tasks. It is also the sampling of the sensor. The change time of the pressure sensor is very short. We can use a 200ms task cycle for sampling, while the lag time of the temperature sensor is very large. We can use a 2S task cycle for sampling. In this way, DCs can reasonably dispatch the resources of the controller

5. From the perspective of network structure: Generally speaking, DCS usually uses a two-layer network structure. The first layer is process level network. Most DCS use their own bus protocols, such as Yokogawa modbus, Siemens and ABB PROFIBUS, abb CANbus, etc. these protocols are based on the standard serial port transmission protocol RS232 or RS485 protocol. The on-site IO module, especially the sampling data of analog quantity (machine code, 213/scanning cycle) is very large, and there are many on-site interference factors, so the network standard with large data throughput and strong anti-interference ability should be adopted. The bus structure based on RS485 serial port asynchronous communication mode meets the requirements of field communication. The sampling data of IO is converted into shaped data or real data by CPU and transmitted on the operation level network (layer 2 network). Therefore, the operation level network can adopt the network standard of moderate throughput, fast transmission speed and convenient connection. At the same time, because the operation level network is generally arranged in the control room, the requirements for anti-interference are relatively low. Therefore, using standard Ethernet is the best choice. Tcp/ip protocol is a standard Ethernet protocol. Generally, we use 100mbit/s communication speed. The work task of PLC system is relatively simple, so the amount of data that needs to be transmitted is generally not too large, so the common PLC system is a one-layer network structure. The process level network and the operation level network are either combined, or the process level network is simplified into the internal connection between modules. PLC does not or rarely use Ethernet

6. In terms of the scale of the application object: PLC is generally used in small-scale automatic control places, such as equipment control or a small amount of analog control and interlocking, while large-scale applications are generally DCS. Of course, this concept is not very accurate, but it is very intuitive. Traditionally, we call systems larger than 600 points DCS and systems smaller than this scale PLC. The control system of our heat pump, QCS and horizontal products is generally called PLC

what are the similarities between DCs and PLC

having said so much about the differences between PLC and DCS, we should realize that PLC and DCS are actually moving closer to each other today. Strictly speaking, PLC and DCs can't be cut off at the same time, and the concept between them has been blurred many times. Now, let's discuss the similarities (similarities) between us

1. In terms of function, PLC has the control function of analog quantity, and some PLC systems have even quite strong analog quantity processing capacity, such as Yokogawa fa-ma3, Siemens S7400, abb ControlLogix and Schneider quantum system. DCs also has quite strong logic processing ability. For example, we have realized all possible process interlocking and equipment linkage start and stop on CS3000

2. From the perspective of system structure, the basic structure of PLC and DCS is the same. Today, PLC has been fully transplanted to computer system control, and the traditional programmer has long been eliminated. Small application PLC generally uses touch screen, and large-scale application PLC fully uses computer system. Like DCS, the controller and IO station use fieldbus (generally based on RS485 or RS232 asynchronous serial port communication protocol). If there is no expansion requirement between the controller and the computer, that is, only one computer is used, this bus communication will also be used. However, if more than one computer is used, the system structure will be the same as DCS, and the upper computer platform will use Ethernet structure. This is one of the reasons for the fuzzy concept of PLC and DCS after the large-scale

3. Development direction of PLC and DCS: miniaturized PLC will develop to a more professional perspective, such as more targeted functions, more targeted application environment, etc. The boundary between large-scale PLC and DCS is gradually diluted until it is fully integrated. DCS will continue to develop in the direction of FCS. In addition to the more decentralized control system, the core of FCS is instrumentation. The application of FCS in foreign countries has developed to the instrument level. The control system only needs to deal with signal acquisition and provide man-machine interface and logic control. The control of the whole analog quantity is distributed to the field instrument. There is no need for traditional cable connection between the instrument and the control system, and the field bus is used to connect the whole instrument system. (at present, Yokogawa has used FCS in the CNOOC Shell Petrochemical Project in China, and the instrument level adopts intelligent instruments, such as ejx, which has the world's most advanced control level)

how to treat PLC and DCS correctly

personally, I never emphasize the advantages and disadvantages between PLC and DCS. I use a new term "control products" for them. We provide users with the most suitable control system. Most users will not use DCS because they want to use a set of DCS. Control products must be positioned on the basis of meeting users' 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. Overemphasizing this thing will only lead to a dispute of words. From the differences and similarities between PLC and DCS, we understand the general situation of control products. Note that in this context, as professionals, we should not define PLC or DCS for our products, and we should not treat products differently in this way psychologically

Copyright © 2011 JIN SHI