Remote data acquisition and monitoring as the main function of the SCADA system and DCS, PLC, is the same as the industrial process automation and informationization of the indispensable basic system.
SCADA System
SCADA is SupervisoryControI And Data AcquiSition System (Data Acquisition and Supervisory Control System) acronym, SCADA system is the distribution of long-distance, decentralized production units of the production system of a data acquisition, monitoring and control system.
It has a wide range of applications, can be used in electric power, metallurgy, petroleum, chemical and other fields of data acquisition and monitoring control and process control and many other fields, which is more widely used in the power system, the development of technology is also the most mature. It occupies an important position in the telecontrol system, and can monitor and control the on-site operating equipment to realize the functions of data acquisition, equipment control, measurement, parameter adjustment and various types of signal alarms, i.e., what we know as the "four-remote" function.
RTU (Remote Terminal Unit) and FTU (Feeder Terminal Unit) are its important components. In today's substation automation construction plays a very important role. It is a production automation control system structured on a PC. Of course, the required functions are different for different fields of application, but they all have the following basic features: graphical operation interface; dynamic simulation of system status; instant and historical data trend curve display; alarm processing system; data acquisition and recording; report output.
SCADA system development history
SCADA system since its inception on the date of the development of computer technology is closely related to the development of SCADA systems to date has experienced four generations.
The first generation is based on a dedicated computer and a dedicated operating system SCADA system, such as the Power Automation Research Institute for the North China Power Grid SD176 system developed by Hitachi in Japan for China's railway electrification system designed by the remote H-80M system. This stage is from the computer used to SCADA system when started to the 70s.
The second generation is the SCADA system based on general-purpose computers in the 80's. In the second generation, other computers such as VAX and other general-purpose workstations are widely used, and the operating system is generally the general-purpose UNIX operating system. At this stage, SCADA systems were combined with economic operation analysis, automatic generation control (AGC) and network analysis in grid dispatch automation to form EMS systems (energy management systems).
The common feature of the first and second generation SCADA systems was that they were based on centralized computer systems and were not open, which made maintenance, upgrading and networking with other systems very difficult.
In the 90's in accordance with the principle of openness, based on distributed computer networks and relational database technology to achieve a wide range of networking EMS/SCADA system called the third generation. This stage is China's SCADA/EMS system is the fastest growing stage, a variety of the latest computer technology are brought together into the SCADA/EMS system. This stage is also China's power system automation and grid construction of the largest investment period, the country invested 270 billion yuan to transform the urban and rural power grids can be seen in the country's power system automation and grid construction of the importance of the degree.
The main feature of the fourth generation of SCADA/EMS system is the use of Internet technology, object-oriented technology, neural network technology and JAVA technology and other technologies, continue to expand the integration of SCADA/EMS system and other systems, comprehensive security and economic operation and the need for commercial operation.
The SCADA system will include the following subsystems:
1. Human Machine Interface (or HMI for short)
A device that can display the status of the program, the operator can monitor and control the program according to this device, HMI will be linked to the SCADA system database and software, read the relevant information to display trends, diagnostic data and related management information, such as regular maintenance procedures, logistics information, specific sensors or machine detail wiring diagrams, or can assist in the troubleshooting of the expert system.
HMI systems often display system information graphically and simulate the actual system with images. The operator can see a diagram of the system to be controlled. For example, a connection to the pipeline pump icon, can show the pump is running, and the flow of liquid in the pipeline, the operator can make the pump stop, the HMI software will show the liquid flow in the pipeline over time down. Simulation will include wiring diagrams and schematics to represent the elements of the process, but also may use pictures of the process equipment, coupled with animation to illustrate the process situation.
The HMI software of a SCADA system generally includes drawing software that allows the system maintainer to modify the presentation of the system in the HMI. Presentation can be as simple as only the lights on the screen, with the lights indicate the actual state of the situation, can also be as complex as using multiple projectors to display all the elevators in the skyscraper location or all the trains in the railroad location.
When implementing a SCADA system, warning handling is an important part of the system. The system monitors whether specified warning conditions hold to determine if a warning event has occurred. When there is a warning event, the system will take corresponding action, such as activating one or more warning instructions, or sending an e-mail or SMS to the system administrator or SCADA operator to inform that there is a warning event. SCADA operators need to confirm the warning event, some warning events in the confirmation of the warning instructions will be turned off, and some warning indicators will be turned off only after the warning conditions are cleared.
2. (Computerized) Monitoring System
Data can be collected and commands can be submitted to monitor the progress of the program.
3. Remote Terminal Unit (RTU)
The RTU can be connected to many sensors used in the program, and the digital data is transmitted to the monitoring system after data acquisition.
Remote terminal control systems (RTUs) can be connected to other equipment, and RTUs can convert electrical signals from equipment into digital values, such as the open/close status of a switch or valve, or the pressure, flow, voltage, or current measured by an instrument. Can also be converted and transmitted by the signal to control the device, such as a specific switch or valve open/close, or set the speed of a pump.
4. programmable logic controller (programmable logic controller, PLC for short)
Because of its inexpensive, versatile, but also commonly used as field devices, instead of special functions of the remote terminal control system.
SCADA is the scheduling management layer, PLC is the field equipment layer. PLC system, that is, programmable controller, suitable for industrial field measurement and control, field measurement and control functions, stable performance, high reliability, mature technology, widely used and reasonably priced. SCADA focuses on the monitoring and control, and can be realized in part of the logic function, basically used for the upper; PLC purely to achieve the logic function and PLC simply realize the logic function and control, do not provide human-machine interface, realize the operation with the help of button indicator, HMI and SCADA system;
5. Communication network
Is to provide monitoring system and RTU (or PLC) between the pipeline to transmit data.
Traditional SCADA system will use broadcast, serial or modem to achieve the communication function, some large-scale SCADA system (such as power plants or railroads) will also often use the architecture in the synchronous optical network (SONET) or synchronous digital system (SDH) on the Ethernet or network protocols. SCADA system in the remote management or monitoring function is often called telemetry.
Typical SCADA System Architecture
The development of SCADA system has gone through three stages: centralized SCADA system stage, distributed SCADA system stage and networked SCADA system.
Centralized SCADA system is that all the monitoring functions depend on a host (mainframe), using a wide area network to connect the field RTU and the host. The network protocol is relatively simple, with poor openness and weak functionality.
Networked SCADA system is based on various network technologies, with more decentralized control structure and more centralized information management. The system is generally based on client / server (C / S) and browser / server structure (B / S), most of the system structure contains both structures, but the C / S structure is mainly B / S structure is mainly to support Internet applications to meet the needs of remote monitoring.
Compared with the second-generation SCADA system, the third-generation SCADA system is more open in structure, better compatibility, and can be seamlessly integrated into the whole plant comprehensive automation system. Since the scale of SCADA system can be from several hundred points to tens of thousands of points, the user's demand for SCADA system is diverse, so it puts forward high requirements for its system architecture.
SCADA system belongs to the typical distributed computer application system, in such a system, the architecture is the most essential thing in the software system, a good architecture means universal, efficient and stable. It can efficiently handle a wide variety of individual needs. At the same time, the architecture remains stable for a certain period of time. When the requirements change, the programmer can do without modifying the architecture of the system.
1. Client/server architecture
C / S structure of the client and server communication between the "request - response" way. The client sends a request to the server and the server responds to the request.
The most important feature of the C/S architecture is that it is not a master-slave environment, but an equal environment, i.e., the computers in a C/S system may be both clients and servers on different occasions. In C / S applications, the user is only concerned with the complete solution of their own applications, not concerned about these applications by the system which computer or computers to complete.
For example, in a SCADA system, when the SCADA server requests data from the PLC, it is the client, and when other operating stations request services from the SCADA server, it is the server. Obviously, this structure can make full use of the advantages of the hardware environment at both ends, and reasonably allocate tasks to the client and server side to realize, reducing the communication overhead of the system.
2. Browser/server structure
With the popularity and development of the Internet, the previous host / terminal and C / S structure can not meet the current global network open, interconnected, information everywhere and information sharing of the new requirements, so the emergence of a B / S structure.
B / S structure is characterized by: the user can go through the browser to access the Internet text, data, images, animation, video-on-demand and sound information, this information is generated by many Web servers, and each Web server can be connected to a variety of ways and database servers, a large amount of data actually stored in the database server. The biggest advantage of this structure is: the client is unified using the browser, which not only makes the user easy to use, but also makes the client does not have the problem of maintenance.
3. Comparison of the two structures
(1) the advantages and disadvantages of the B / S model
The advantages of the B / S structure is shown in:
Distributed characteristics, you can query, browse and other business processes at any time, anywhere.
Business expansion is simple and convenient, through the addition of web pages can increase the function of the server.
Maintenance is simple and convenient, only need to change the web page, you can realize the synchronization of all users to update.
Simple development and strong sharing.
The disadvantages of B／S structure are as follows:
The personalized features are obviously reduced, and it is impossible to realize the requirements of personalized functions.
The operation is based on the mouse as the most basic mode of operation, unable to meet the requirements of rapid operation.
Dynamic page refresh, response speed is significantly reduced.
Functions are weakened, and it is difficult to realize the special function requirements in the traditional mode.
(2) Advantages and disadvantages of the C / S model
The advantages of the C / S structure:
As the client realizes the direct connection with the server, there is no intermediate link, so the response speed is fast.
The operation interface is beautiful, diverse, and can fully meet the customer's own personalized requirements.
C / S structure of the management information system has a strong transaction processing capabilities, can realize complex business processes.
C / S structure of the shortcomings of the performance:
The need for a specialized client installation program, the distribution function is weak, for a wide range of points and does not have the network conditions of the user group, can not realize the rapid deployment of the installation and configuration.
Poor compatibility, for different development tools, has greater limitations. If you use different tools, you need to rewrite the program.
Higher development costs, requiring a certain level of professional technical staff to complete.
The difference between SCADA and DCS, PLC
Industrial control systems cover a variety of types of control systems, before we talked about distributed control systems (DCS), programmable logic controllers (PLC), they and data acquisition and monitoring system (SCADA) more common and easy to confuse. So, what is the difference between SCADA, DCS and PLC?
DCS
A DCS system, or Distributed Control System, is a system that is primarily used to control production processes in the same geographic environment.
DCS systems use centralized monitoring and control to coordinate local controllers to execute the entire production process. By modularizing the production system, DCS reduces the impact of individual failures on the entire system. In many modern systems, DCS systems are interfaced with enterprise systems to enable the production process to be reflected in the overall operation of the business.
DCS systems are commonly used in industrial control areas such as oil refineries, wastewater treatment plants, power plants, chemical plants and pharmaceutical plants. These systems are typically used for process control or discrete control systems.
SCADA
SCADA systems, or data acquisition and monitoring systems, are core systems for industrial control, primarily used to control decentralized assets in order to perform centralized data collection that is just as important as control.
SCADA systems integrate data acquisition systems, data transmission systems, and HMI software to provide centralized monitoring and control for process inputs and outputs.SCADA systems are designed to collect information from the field, transmit that information to a computer system, and display that information in image or text form. As a result, an operator can monitor and control the entire system in real time from a centralized location, control any of the individual systems according to the complexity and relevant settings of each system, and automate relevant operations or tasks, which can also be performed automatically by operator commands.
SCADA systems are primarily used in distributed systems such as water treatment, oil and gas pipelines, power transmission and distribution systems, railroads and other public transportation systems.
PLC
PLC system, i.e. Programmable Logic Controller PLC is a new generation of industrial control devices based on the introduction of microelectronics technology, computer technology, automatic control technology and communication technology into the traditional sequential controllers with the aim of replacing the sequential control functions such as relays, execution logic, timekeeping, counting, etc. and establishing flexible programmed control systems. The International Electrotechnical Commission (IEC) promulgated the provisions of the PLC: programmable controller is a digital computing operation of the electronic system, designed for application in industrial environments. It uses a programmable memory, which is used to store instructions for performing logical operations, sequential control, timing, counting and arithmetic operations within it, and to control various types of machinery or production processes through digital and analog inputs and outputs. Programmable controller and its related equipment, should be easy to form a whole with the industrial control system, easy to expand its functionality of the principle of design.
In the network architecture of industrial automation and control system, PLC, as an important control component, is usually used in SCADA and DCS systems to realize specific operation and process control of industrial equipment and provide local process management through loop control.
In SCADA systems, the PLC functions as an RTU (i.e. Remote Terminal Unit). When used in DCS systems, PLCs are used as local controllers with a supervisory control program. At the same time, PLCs are often used as important components to configure smaller control systems.
PLCs have user-programmable memories for storing instructions to achieve specific functions such as I/O control, logic, timing, counting, PID control, communications, arithmetic, data and file processing. With the development of communication technology, PLC also from the closed private communication protocols to the use of open public protocols, greatly improving the compatibility of the system, and facilitate the maintenance and updating of the system.
Conclusion
From the above it is easy to see: SCADA, DCS is a concept, and PLC is a product, the three are not comparable:
1, PLC is a product, by which it can constitute SCADA, DCS;
2, DCS is a process control developed, PLC is a relay-logic control system developed;
3, PLC is a device, DCS, SCADA is the system.
Narrowly speaking, 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, stretches for thousands of miles of pipelines. If from the computer and network point of view, they are unified, the reason why there is a difference, mainly in the application requirements, DCS often require advanced control algorithms. Such as in the oil refining industry, PLC requires high processing speed, because it is often used in interlocking, and even fail-safe systems, SCADA also has some special requirements, such as vibration monitoring, flow calculation, peak and trough regulation and so on.
Therefore, it can also be simply considered:
SCADA is the scheduling management
DCS is the plant management layer
PLC is the field equipment layer
SCADA system is a long history, but still in the rapid development of the "newborn" things, with a complex system, open interface standards and Internet network depth of integration and other characteristics, the need to integrate production, academia, research, and use of all aspects of the research SCADA system of information security theory and technology, to encourage domestic SCADA system manufacturers are encouraged to accumulate and innovate technology, cultivate SCADA system technology research and development team to reach the world's advanced level, and gradually promote the use of localized SCADA system with independent intellectual property rights to replace foreign systems in key industries.