An Analysis of SCADA, DCS, and PLC in One Article
With the improvement of intelligent application level in industrial enterprises, more and more people are beginning to come into contact with various industrial automation systems and products, including SCADA, DCS, and PLC.
Among them:
DCS: Distributed control systems.
PLC: Programmable Logic Controller.
From the perspective of origin and development, DCS has evolved from traditional dashboards. Therefore, DCS places greater emphasis on instrument control from an innate perspective.
PLC has evolved from traditional relay circuits. The original PLC did not even have simulation processing capabilities. Therefore, PLC emphasizes logical operation capability from the beginning.
In terms of system scalability and compatibility, although DCs have their own system during the development process, most systems have different internal communication protocols, but the operation level network platform uses Ethernet. As long as the network reaches that location, you can freely increase or decrease the number of nodes and arrange node positions. In addition, the new universal operating station is based on open protocols such as OPC and DDE for Windows systems, which can facilitate communication between various systems and achieve resource sharing.
SCADA, also known as Data Acquisition and Monitoring System, is the core system of industrial control, mainly used to control dispersed assets to achieve centralized data acquisition DCS – distributed control system, which is equally important as control. It is mainly used to control production processes in the same geographical environment.
PLC is an important control component commonly used in SCADA and DCS systems to achieve specific operations and process control of industrial equipment, and to provide local process management through loop control.
It can be seen that SCADA and DCS are a system, while PLC is a product. PLC can form SCADA and DCS.
PLC was first applied in the automotive manufacturing industry in the 1970s, using only switch logic control. That is to say, PLC will be more inclined towards logical quantities (digital quantities). It is also possible to have one PLC master station and multiple PLCs of the same type as slaves to form a PLC network. This is more convenient than using a PC as the main station: users do not need to know the communication protocol when programming, just manually write the format. A system with the same I/O points has the advantage of PLC over DCS, and its cost is lower (can save about 40%).
PLC does not have a dedicated operation station, and the software and hardware used are universal, so the maintenance cost is much lower than DCS. PLC can serve as both an independent DCS and a subsystem of DCS.
The CPU of the PLC has a program counter that indicates the storage address of the program steps. During program execution, the counter automatically increments by 1 for each step. The program executes sequentially from the beginning step (with 0 steps) to the last step (usually the END instruction), and then returns to the beginning step loop operation.
The time required for PLC to complete each cycle operation is called a scanning cycle. PLC has become easier to design enterprise management information systems due to the use of universal monitoring software.
DCS: DCS was developed based on operational amplifiers.
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Form all the relationships between functions and process variables into a functional block (referred to as an expansion block in some DCS systems). That is to say, DCS is more inclined towards simulation (process control direction). DCS is much better than PLC in terms of controllers, I/O boards, communication network redundancy, advanced operations, and industry specific requirements. In terms of grounding resistance, PLC may not require high requirements, but DCS must meet several requirements ω Below (usually in 4) ω Below). Simulated isolation is also very important. Safety barriers should be set up in areas with explosion hazards.
DCS is a three-level structure consisting of control (engineer station), operation (operator station), and on-site instruments (on-site monitoring station). Used for large-scale continuous process control, such as petrochemical industry. DCS was first applied in the chemical industry. The main difference in performance between DCS and PLC lies in the logical solution for switching and the operation of analog signals, although there may be some mutual penetration between the two in the later stage. For example, Siemens’ PCS7 process control system is a product of DCS+PLC.
Generally speaking, DCS is mainly used for process automation, PLC is mainly used for factory automation (production lines), and SCADA is mainly aimed at a wide range of needs, such as oil fields and thousands of miles of pipelines. From the perspective of application functionality, DCS typically requires advanced control algorithms. For example, in the refining industry, PLC has high requirements for processing speed because it is often used for interlocking and even for fault safety systems. SCADA also has some special requirements, such as vibration monitoring, flow calculation, peak valley regulation, etc.
As manufacturing enterprises accelerate the planning, implementation, and transformation of intelligent factories, there is an urgent need to select appropriate industrial automation control systems based on cost and demand.
For analog quantities greater than 100 points, DCS is generally used; For analog quantities within 100 points, PLC is generally used.
For the same industrial control application scenario, different solutions may be proposed.
For control processes with widely distributed monitoring points, SCADA systems are preferred.
For more centralized continuous process control requirements, such as in process industry enterprises, DCS systems are preferred.
For on-site process control, choose PLC.
However, in the actual implementation and application process, enterprises still need to fully evaluate requirements and costs, and make flexible choices.