The difference between SCADA and DCS, PLC
SCADA systems with remote data collection and monitoring as their main functions, like DCS and PLC, are essential basic systems for industrial process automation and informatization.
SCADA system
SCADA is the abbreviation for Supervisory Control I And Data Acquisition System. SCADA system is a data acquisition, monitoring, and control system for production systems that are distributed far away and have dispersed production units.
It has a wide range of applications and can be applied in various fields such as data acquisition, monitoring and control, and process control in the fields of power, metallurgy, petroleum, and chemical engineering. Among them, its application in the power system is more extensive, and its development technology is also the most mature. It plays an important role in the remote control system, which can monitor and control the operating equipment on site to achieve various functions such as data acquisition, equipment control, measurement, parameter adjustment, and various signal alarms, known as the “four remote” functions.
RTU (Remote Terminal Unit) and FTU (Feed Line Terminal Unit) are important components of it. It has played a significant role in the comprehensive automation construction of substations today. It is a production automation control system built on top of a PC. Of course, applications in different fields require different functions, but they all have the following basic characteristics: graphical operation interface; Dynamic simulation of system state; Real time and historical trend curve display; Alarm processing system; Data acquisition and recording; Report output.
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Monitoring system of a certain sewage treatment plant
The Development History of SCADA Systems
The SCADA system has been closely related to the development of computer technology since its inception. The development of SCADA systems has gone through four generations today.
The first generation was SCADA systems based on dedicated computers and operating systems, such as the SD176 system developed by the Electric Power Automation Research Institute for North China Power Grid and the H-80M system designed by Hitachi in Japan for China’s railway electrification remote control system. This stage began from the application of computers to SCADA systems until the 1970s.
The second generation was a SCADA system based on general-purpose computers in the 1980s. In the second generation, other computers such as VAX and other general-purpose workstations were widely used, and the operating system was generally the general-purpose UNIX operating system. At this stage, the SCADA system is combined with economic operation analysis, automatic generation control (AGC), and network analysis in power grid dispatch automation to form the EMS system (energy management system).
The common feature of the first and second generation SCADA systems is that they are based on centralized computer systems and do not have openness, making system maintenance, upgrading, and networking with other systems very difficult.
In the 1990s, according to the principle of openness, EMS/SCADA systems based on distributed computer networks and relational database technology that could achieve large-scale networking were called the third generation. This stage is the fastest developing stage of SCADA/EMS system in China, where various latest computer technologies are integrated into the SCADA/EMS system. This stage is also the period when China has invested the most in power system automation and power grid construction. The national investment of 270 billion yuan in renovating urban and rural power grids shows the country’s emphasis on power system automation and power grid construction.
The main features of the fourth generation SCADA/EMS system are the use of Internet technology, object-oriented technology, neural network technology, and JAVA technology, continuing to expand the integration of SCADA/EMS system with other systems, meet the needs of comprehensive security and economic operation, and commercial operation.
Module composition of SCADA system
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The SCADA system will include the following subsystems:
1. Human Machine Interface (HMI)
A device that can display program status, allowing operators to monitor and control programs based on this device. HMI will link to the SCADA system’s database and software, read relevant information to display trends, diagnostic data, and related management information, such as regular maintenance programs, logistics information, detailed circuit diagrams of specific sensors or machines, or expert systems that can assist in troubleshooting.
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HMI systems often display system information in the form of images and simulate actual systems using images. The operator can see a schematic diagram of the system to be controlled. For example, a pump icon connected to a pipeline can display that the pump is running and the flow rate of liquid in the pipeline. The operator can shut down the pump, and the HMI software will display that the flow rate of liquid in the pipeline decreases over time. Simulation diagrams may include circuit diagrams and schematic diagrams to represent the elements in the process, or may use images of the process equipment, accompanied by animations to illustrate the process situation.
The HMI software of SCADA systems generally includes drawing software, which allows system maintainers to modify the presentation of the system in HMI. The presentation method can be as simple as using only the lights on the screen to represent the actual status of the scene, or as complex as using multiple projectors to display the positions of all elevators in skyscrapers or trains on railways.
Warning handling is an important part of implementing SCADA systems. The system will monitor whether the specified warning conditions are valid to determine if any warning events have occurred. When there is a warning event, the system will take corresponding actions, such as activating one or more warning indications, or sending an email or text message to the system manager or SCADA operator to inform them of the existing warning event. SCADA operators need to confirm warning events. Some warning events will turn off their warning indications after confirmation, while others will only turn off after the warning conditions are cleared.
2. (Computer) Monitoring System
Data can be collected and commands can be submitted to monitor the progress of the program.
3. Remote Terminal Unit (RTU)
It can connect many sensors used in programs, and after data acquisition, the digital data is transmitted to the monitoring system.
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The remote terminal control system (RTU) can be connected to other devices. RTU can convert electrical signals on equipment into digital values, such as the open/closed state of a switch or valve, or the pressure, flow rate, voltage, or current measured by instruments. Equipment can also be controlled through signal conversion and transmission, such as opening/closing specific switches or valves, or setting the speed of a pump.
4. Programmable Logic Controller (PLC)
Due to its low price and wide range of uses, it is also commonly used as a field device to replace remote terminal control systems with special functions.
SCADA is the scheduling management layer, while PLC is the on-site equipment layer. PLC system, which can be programmed as a controller, is suitable for measurement and control in industrial sites. It has strong on-site measurement and control functions, stable performance, high reliability, mature technology, wide use, and reasonable price. The focus of SCADA is on monitoring and control, which can achieve some logical functions and is basically used for upper level; PLC only implements logical functions and control, without providing a human-machine interface, and operation requires the use of button indicator lights, HMI, and SCADA system;
5. Communication network
It is a pipeline that provides data transmission between monitoring systems and RTUs (or PLCs).
Traditional SCADA systems use broadcasting, serial, or modems to achieve communication capabilities, and some large SCADA systems (such as power plants or railways) often use Ethernet or network protocols built on synchronous optical networks (SONET) or synchronous digital hierarchy (SDH). The remote management or monitoring function in SCADA systems is often referred to as telemetry.
Typical Architecture of SCADA System
The development of SCADA systems has gone through three stages: centralized SCADA system stage, distributed SCADA system stage, and networked SCADA system stage.
A centralized SCADA system relies on a mainframe for all monitoring functions, using a wide area network to connect the on-site RTU and the host. The network protocol is relatively simple, with poor openness and weak functionality.
The networked SCADA system is based on various network technologies, with a more decentralized control structure and centralized information management. The system is generally based on client/server (C/S) and browser/server architecture (B/S), and most system architectures include these two structures. However, the C/S structure is the main one, and the B/S structure is mainly used to support Internet applications and meet the needs of remote monitoring.
Compared with the second generation SCADA system, the third generation SCADA system is more open in structure, has better compatibility, and can be seamlessly integrated into the comprehensive automation system of the entire factory. Due to the fact that the scale of SCADA systems can range from a few hundred to tens of thousands of points, users have diverse requirements for SCADA systems, thus placing high demands on their system architecture.
The SCADA system belongs to a typical distributed computer application system, in which the architecture is the most essential thing in the software system. A good architecture means universality, efficiency, and stability. It can efficiently handle a variety of individual needs. Meanwhile, the architecture remains stable for a certain period of time. When requirements change, programmers do not need to modify the system architecture.
1. Client/Server Architecture
The communication between the client and server in the C/S structure is carried out in a request response manner. The client first sends a request to the server, and the server responds to this request, as shown in Figure 1.3.
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The most important feature of C/S architecture is that it is not a master-slave environment, but an equal environment, where each computer in the C/S system can be either a client or a server in different situations. In C/S applications, users only care about solving their own application problems completely, without caring about which computer or machines in the system will complete these application problems.
In the SCADA system, when the SCADA server requests data from the PLC, it is the client, while when other operating stations request services from the SCADA server, it is the server. Obviously, this structure can fully utilize the advantages of the hardware environment at both ends, allocate tasks reasonably to the client and server sides, and reduce the communication overhead of the system.
2. Browser/Server Architecture
With the popularization and development of the Internet, traditional host/terminal and C/S structures cannot meet the new requirements of global network openness, interconnection, ubiquitous information, and information sharing. Therefore, B/S structures have emerged.
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The biggest feature of the B/S structure is that users can access text, data, images, animations, video on demand, and sound information on the Internet through a browser. This information is generated by many web servers, and each web server can connect to the database server through various methods. A large amount of data is actually stored in the database server. The biggest advantage of this structure is that the client adopts a unified browser, which not only makes it convenient for users to use, but also eliminates maintenance issues for the client.
3. Comparison of Two Structures
(1) The advantages and disadvantages of B/S mode
The advantages of B/S structure are reflected in:
It has the characteristic of distribution and can be used for business processing such as querying and browsing anytime and anywhere.
Business expansion is simple and convenient, and server functionality can be added by adding web pages.
Easy and convenient to maintain, just change the webpage to achieve synchronized updates for all users.
Simple development and strong sharing.
The drawbacks of B/S structure are:
The personalized characteristics are significantly reduced, making it impossible to achieve personalized functional requirements.
Operation is based on the mouse as the most basic mode of operation, which cannot meet the requirements of fast operation.
The page is dynamically refreshed, and the response speed is significantly reduced.
Weakened functionality makes it difficult to meet the special functional requirements of traditional modes.
(2) The advantages and disadvantages of C/S mode
The advantages of C/S structure are reflected in:
Due to the direct connection between the client and the server without any intermediate links, the response speed is fast.
The operation interface is beautiful and diverse, which can fully meet the personalized requirements of customers.
The C/S structured management information system has strong transaction processing capabilities and can achieve complex business processes.
The drawbacks of the C/S structure are:
A specialized client installation program is required, with weak distribution functions. It is not suitable for users who have a wide range of points and do not have network conditions, and cannot achieve rapid deployment, installation, and configuration.
Poor compatibility and significant limitations for different development tools. If different tools are used, the program needs to be rewritten.
The development cost is high and requires technical personnel with a certain level of expertise to complete.
The difference between SCADA and DCS, PLC
Industrial control systems cover various types of control systems. Previously, we talked about distributed control systems (DCS) and programmable logic controllers (PLC), which are more common and easily confused with data acquisition and monitoring systems (SCADA). So, what are the differences between SCADA, DCS, and PLC?
DCS
DCS system, also known as distributed control system, is mainly used to control the production process in the same geographical environment.
The DCS system uses centralized monitoring to coordinate local controllers to execute the entire production process. Through modular production systems, DCS reduces the impact of individual faults on the entire system. In many modern systems, interfaces are set up between DCS systems and enterprise systems to reflect the production process in the overall operation of the business.
DCS systems are commonly used in industrial control fields such as oil refining, sewage treatment plants, power plants, chemical plants, and pharmaceutical plants. These systems are typically used for process control or discrete control systems.
SCADA
SCADA system, also known as data acquisition and monitoring system, is the core system of industrial control, mainly used to control dispersed assets for centralized data collection that is equally important as control.
The SCADA system integrates data acquisition system, data transmission system, and HMI software to provide centralized monitoring and control for process input and output. The SCADA system is designed to collect on-site information, transmit this information to computer systems, and display this information in the form of images or text. Therefore, operators can monitor and control the entire system in real-time from a centralized location, control any individual system based on its complexity and related settings, and automatically execute related operations or tasks, which can also be automatically executed by operator commands.
SCADA systems are mainly used for distributed systems, such as water treatment, oil and gas pipelines, power transmission and distribution systems, railways, and other public transportation systems.
PLC
PLC system can be used to program logic controllers. PLC is a new generation of industrial control equipment that introduces microelectronics, computer technology, automatic control technology, and communication technology on the basis of traditional sequential controllers. Its purpose is to replace relay, execution logic, timing, counting and other sequential control functions, and establish a flexible programmable control system. The International Electrotechnical Commission (IEC) has issued regulations for PLCs: Programmable controllers are electronic systems designed for digital operations, specifically designed for use in industrial environments. It uses programmable memory to store instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations within it, and controls various types of machinery or production processes through digital and analog inputs and outputs. Programmable controllers and related equipment should be designed according to the principle of easy integration with industrial control systems and easy expansion of their functions.
In the network architecture of industrial automation and control systems, PLC, as an important control component, is usually applied in SCADA and DCS systems to achieve specific operation and process control of industrial equipment, providing local process management through loop control.
In SCADA systems, the function of PLC is the same as that of RTU (remote terminal unit). When used in DCS systems, PLCs are used as local controllers with monitoring and control plans. At the same time, PLC is often used as an important component for smaller scale control systems.
PLC has user programmable memory to store instructions for implementing specific functions, such as I/O control, logic, timing, counting, PID control, communication, arithmetic, data and file processing, etc. With the development of communication technology, PLCs have shifted from closed private communication protocols to open public protocols, greatly improving system compatibility and facilitating system maintenance and updates.
Summary
It is easy to see from the above content that SCADA and DCS are concepts, while PLC is a product, and the three are not comparable:
1. PLC is a product that can form SCADA and DCS;
2. DCS evolved from process control, while PLC evolved from relay logic control systems;
3. PLC is the equipment, DCS, SCADA is the system.
Narrowly speaking, DCS is mainly used for process automation, PLC is mainly used for factory automation (production lines), and SCADA is mainly aimed at wide-ranging needs, such as oil fields and thousands of miles of pipelines. From the perspective of computers and networks, they are unified, and the main difference lies in the application requirements. DCS often requires advanced control algorithms. In the refining industry, PLC has high processing speed requirements because it is often used in interlocking, and even in fault safety systems. SCADA also has some special requirements, such as vibration monitoring, flow calculation, peak shaving, and valley shaving.
Therefore, it can also be simply assumed that:
SCADA is the scheduling management layer
DCS is the management team of the factory station
PLC is the on-site equipment layer
The SCADA system is a new and rapidly developing thing with a long history, characterized by complex systems, open interface standards, and deep integration of Internet networks. It needs to integrate research on information security theory and technology of SCADA systems from various aspects such as industry, academia, research, and application. Domestic SCADA system manufacturers are encouraged to accumulate technology and innovate, cultivate SCADA system technology research and development teams that reach the world’s advanced level, and gradually promote the use of domestically produced SCADA systems with independent intellectual property rights to replace foreign systems in key industries.