As the name implies, the DCS is a system of sensors, controllers, and associated computers that are distributed throughout a plant. Each of these elements serves a unique purpose such as data acquisition, process control, as well as data storage and graphical display. These individual elements communicate with a centralized computer through the plant’s local area network – often referred to as a control network.
As the ‘central brain’ of the plant the DCS makes automated decisions based on production trends it sees them in real-time throughout a plant. As an example, the DCS at a power plant might automatically increase steam generation capacity of multiple turbines in order to keep up with changing demand for electricity during hot Summer days and then decrease it as outdoor temperatures cool overnight and demand subsides. Whereas a PLC could adjust a single unit operation, the DCS can make adjustments to each of a plant’s many interacting unit operations.
While DCSs are used across the process control industries to supervise complex production processes, they are more widely deployed at large, continuous manufacturing plants such as those in the petrochemical industry. With the help of a DCS these and other manufactures can efficiently coordinate adjustments in a top-down fashion using a centralized network of computers. Instructions from the DCS are deployed throughout a plant and fed to individual controllers. When configured appropriately the DCS can improve safety while also enhancing production efficiency.
They're use primarily for continuous, complex controls, and have an integrated control center much like a SCADA, which is the core of the system versus the processors in a PLC system. The DCS has a number of predefined functions that come ready to customize and deploy for various applications. Processing times are somewhat slower. Operators interact with the control system via an integrated graphical display. DCSs are also often the more reliable system to go with. The reason for this is because the manufacturer supplies both the control and supervisory equipment as an integrated package, the risks of integration errors are greatly reduced.
Over the years, the term DCS has evolved from the original description for the acronym as a “Distributed Control System” to the use of the term “Decentralized Control System” and they seem to be somewhat interchangeable nowadays. Regardless of which description is used, this is a structure that, at the high-level view, coordinates and supervises an entire plant of many varying processes.
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