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DCS (Distributed Control System), also known as a distributed control system, is a type of computer control system that is developed and evolved from centralized control systems.PLC (Programmable Logic Controller).
PLC and DCS
PLC:
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Developed from switch control to sequential control and transport processing, it is an upward evolution.
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Continuous PID control and other multifunctions, with PID in interrupt stations.
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A single PC can serve as the master station, with multiple identical PLCs as slave stations.
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Also, one PLC can serve as the master station with multiple identical PLCs as slave stations, forming a PLC network. This is more convenient than using a PC as the master station because when programming, users do not need to know the communication protocol, just follow the manual format.
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The PLC grid can serve as an independent DCS or as a subsystem of DCS.
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PLC is mainly used for sequential control in industrial processes, and new PLCs also have closed-loop control functions.
DCS:
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DCS integrates 4C (Communication, Computer, Control, CRT) technologies into a monitoring technology.
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A large system with a top-down tree topology, where communication is key.
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PID in interrupt stations, connecting computers with field instruments and control devices in a tree topology and parallel continuous link structure, with many cables running from relay stations to field instruments.
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Analog signals, A/D – D/A, with microprocessor hybrids.
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Each instrument connects to I/O with a pair of wires, linked to the local area network (LAN) by the control station.
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DCS has a three-level structure of control (engineer station), operation (operator station), and field instruments (field measurement and control station). It is used for large-scale continuous process control, such as in petrochemicals.
Industry Experts Discuss the Differences Between PLC and DCS
PLC and DCS cannot be compared; PLC is a controller, an isolated product, while DCS is a system. However, PLC can be compared with the control station of DCS; the cycle time of PLC is around 10 milliseconds, while that of DCS control stations is around 500 milliseconds. PLC has better openness and stronger independent operational capability as a product.
—— OMRON Company
DCS is a system that includes upper-level software, networks, and controllers, while PLC is just a controller; to form a system, it also needs an upper-level SCADA system and the connected network.
Regarding PID loop control, Mitsubishi’s process controllers can now achieve FBD programming similar to SAMA configuration; DCS systems are larger, with more control loops and more control and algorithms, allowing for more complex inter-loop control. The hardware reliability is about the same. DCS can achieve I/O redundancy, while PLC cannot. Relatively speaking, systems built with PLCs are more cost-effective.
—— Manager of Precision Control Department, Mitsubishi Electric Automation
DCS is a type of “distributed control system,” hardware-wise including field controllers, operator station computers, engineer station computers, and the network system connecting them; DCS software is a complete solution addressing all technical issues of a system, with tight integration between system components.
PLC is a device, hardware-wise equivalent to the field controller in DCS; software-wise, it is a local solution with loose organization between stations.
—— Technical Manager for APROL Product Application Development, B&R Automation
Analyzing the differences between DCS and PLC, the two key points are: first, DCS is distributed control with a global database; second, PLC uses a sequential scanning mechanism, while DCS is time-based control. Our system meets the first point; for example, a modification of an I/O tag can also be reflected synchronously in the HMI.—— Product Manager for Process Market, Rockwell Automation
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