The DCS control system is also called the distributed control system in the domestic self-control industry. The so-called distributed control system is a new type of computer control system relative to the centralized control system. It is developed and evolved on the basis of the centralized control system.
As a computer integrated system integrating process control and process monitoring, DCS has become a complete system of integrated computer, communication, display and control 4C technology under the continuous driving of communication network. Its main features are decentralized control, centralized operation, hierarchical management, flexible configuration and convenient configuration.
Nowadays, the DCS system can be widely used in the production control and operation management of industrial devices, and has been widely used in the field of process automation such as chemical, electric power and metallurgy.
PLC, or logic programmable controller, is an electronic system for digital computing operations designed for industrial environments. It uses a class of programmable memory for its internal memory program, performing user-oriented instructions such as logic operations, sequence control, timing, counting, and arithmetic operations, and controlling various types of machinery through digital or analog input/output. Or the production process is a core part of industrial control.
PLC is mainly used in aerospace, power generation, petrochemical, steel, tobacco, pharmaceutical, food, petrochemical, metallurgy, mining, water treatment, transportation and other fields in process control. The power plant is used in auxiliary workshops and hydropower of large thermal power plants. Master control, etc.
DCS is mainly used in process control, mainly in the fields of power generation, petrochemical, steel, tobacco, pharmaceutical, food, petrochemical, metallurgy, mining, etc., where the main control units of aerospace, thermal power, nuclear power, large petrochemical, and steel must be Use DCS for control.
The application of DCS and PLC in process control is mutually infiltrated. In general, DCS can be used in all areas where PLC can be used; except direct combustion engine, large thermal power main control, nuclear power main control, steel mill PLC+SCADA has all the functions of DCS in other fields, such as main control and large petrochemical main control.
Hardware composition and architecture of DCS and PLC
We compare the PLC and DCS hardware components. The PLC+SCADA architecture is basically the same as the DCS architecture, or the PLC architecture is more flexible.
The subtle difference is that the hardware hot standby redundancy mode of the PLC is dual rack, dual power supply, dual processor, and dual network card redundancy. The early DCS is a dual power supply and dual DPU (distributed processing unit) on the same rack. Redundant architecture, the current DCS is a two-in-one DPU that forms a hot standby redundancy architecture.
Here is not to compare the advantages and disadvantages of the two methods, from the hardware cost, PLC is unfavorable, integrated CPU (DPU / CPU means that the processor does not distinguish) is more conducive to reducing costs, PLC redundant architecture has not changed In the early years of design architecture, some domestic PLC and DCS redundancy architectures are the same. There is a difference between PLC and DCS hardware. PLC does not have redundant IO switching module. It may be that there is no requirement for this module in PLC application field.
In recent years, there has been a new development in PLC design architecture. The CPU and non-local bracket IO modules and intelligent devices no longer push the bus communication mode, but instead use industrial Ethernet instead. The bus is only in the local communication module and equipment. Inter-use, even the Ethernet 1 station in the end structure, DTM function can directly configure the standard protocol of the TCP protocol device, communication is more convenient than DCS.
Difference between DCS and PLC controllerThe main difference between the DCS and the PLC controller is that in the calculation of the switching quantity and the analog quantity, even if the two are somewhat infiltrated with each other, there is still a difference. After the 1980s, in addition to logic operations, PLC also added some control loop algorithms, but it is still difficult to complete some complex operations. PLC uses ladder diagram programming, and analog calculations are not intuitive in programming, and programming is troublesome. But in terms of solving logic, it shows a quick advantage. DCS uses functional blocks to encapsulate analog and logical operations. The expressions of both logical and complex analog operations are very clear, but the computational efficiency of logic operations is relatively low compared to PLC.
Application of DCS and PLC in thermal power plantsIn the field of thermal automation in thermal power plants, DCS and PLC are two completely different concepts that are inextricably linked. Both DCS and PLC are products of computer technology and industrial control technology. The power plant main control system uses DCS, and PLC is mainly used in power plant auxiliary workshop. Both DCS and PLC have the means of man-machine interaction provided by the operator station, relying on the controller based on computer technology to complete the control operation, and all the data exchange with the primary component and the execution device through the I/O card, all have the so-called Network communication system.
With the continuous expansion of the installed capacity of domestic power plants and the advancement of power system reform, the requirements for the control of auxiliary workshops have also been continuously improved. In this large environment, the control of DCS systems into auxiliary workshops has become a trend. Due to its comprehensive technical and economic advantages, NT6000DCS has played and will continue to play an increasingly important role in auxiliary workshop control.
The application of a wide range of PLCs in the auxiliary workshop will not exit the historical stage of thermal automation. Unprecedented competitive pressure will prompt PLC manufacturers to move closer to the DCS standard in technology and make greater efforts in price. The result of competition between DCS and PLC market will enable users to obtain greater benefits.
DCS and PLC control processing capabilitiesA PLC controller can often handle thousands of I/O points (up to 8,000 I/Os). DCS controllers can only handle hundreds of I/O points (no more than 500 I/Os).
From the requirements of the distribution system, it is not allowed to have centralized control. Too many points of controllers are useless in practical applications. DCS developers simply do not need to develop many I/O point controllers. Drive, their main focus is to provide system reliability and flexibility. The PLC is different. As an independent flexible control device, the stronger the point capability, of course, the higher the technical level. As for the application level of the entire control system, this is mainly the matter of the engineering company and the user, not the The core goal of the PLC manufacturer. Another indicator for controlling processing power, the speed of operation, in people's impressions, PLC is also much faster than DCS.
The new DCS controller has learned the design of large PLCs and has greatly improved its performance in terms of control cycles. Take the NT6000DCS T2550 controller as an example. The controller can set up four tasks with different priorities. The minimum calculation period can be set to 10ms. With high-speed I/O cards, the control cycle can reach 15~20ms. The analog operation is set in other tasks with longer periods.
In the field of thermal automation, the main plant control system basically uses DCS without exception. The PLC is used in the auxiliary workshop. The main reason is that the early DCS system is very expensive. It is considered that the operation of the auxiliary workshop can be interrupted, the reliability requirements are not very high, and the analog control requirements are less. From the perspective of cost reduction, the PLC is often selected to construct the control system. The control systems of boilers, steam turbines and generators require long-term stable and reliable operation. The signal contains a considerable proportion of analog quantities. From the performance of the system, people have to choose expensive DCS.
In addition, we will find an interesting phenomenon by analyzing the market competition of the main plant DCS and the auxiliary workshop control system. The competition of the main plant DCS is often carried out between suppliers or agents of different brands. The competition is fierce and the price of DCS is continuously lowered. The competition of the auxiliary workshop control system is often carried out among the various engineering companies of the same brand PLC. The threshold is lower and the competition is more intense. However, the price reduction of PLC is not as obvious as DCS. The main reason is that manufacturers of DCS are directly involved in the competition, and under the pressure of huge market, the equipment manufacturing costs and project implementation costs are continuously lowered. The PLC manufacturers do not directly participate in the competition, and each engineering company can only reduce its limited engineering costs, and the space is limited. From the current situation, the price gap between DCS and high-end PLC is not obvious, and the auxiliary workshop still uses PLC more, which is the inertia of the market.
The market for PLC and DCS products is booming and fiercely competitive. Throughout the PLC market, there are more than 200 companies in the world that produce more than 400 varieties of PLCs for use in electric power, petroleum and petrochemical, metallurgy, materials, packaging, paper, automotive and municipal industries. From an industry perspective, foreign manufacturers occupy the mountain as the king and have their own sphere of influence.
The DCS market is almost the same as PLC, which is basically controlled by foreign industry leaders. What is gratifying is that a number of local manufacturers such as Hollysys, Supcon and Xinhua have gradually grown stronger. Due to the high content of DCS technology, many of the demand for products are based on the project, so the demand for DCS will also be a cyclical process. Therefore, the DCS market structure is unlikely to change significantly in a short period of time. Of course, due to the irregularity of the industry's development speed, companies that focus on different industries may have some changes.
The development direction of DCS and PLC1) Development direction of PLC
Miniaturized PLCs will be developed to a more professional use perspective, such as more specific functions and more targeted applications.
Large-scale PLCs already have some of the features of DCS, but their expansibility and open-ended are limited.
2) The development direction of DCS
DCS will develop in the direction of FCS. In addition to the more decentralized control system, the core of the FCS is instrumentation. The application of FCS in foreign countries has developed to the instrument level. The control system needs to deal with only the signal acquisition and human-machine interface and logic control. The whole analog control is distributed to the field instrument. There is no need for traditional cable connection between the instrument and the control system. The fieldbus is used to connect the entire instrument level.
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