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In this video, we are going to do a deep dive into a DCS
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and discuss some of the why's and how's associated with a DCS.
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Let's first clarify, for the purpose of this lesson, what we mean by DCS.
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Over the years, the term DCS has evolved from the original description
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for the acronym as a Distributed Control System to the use of the term Decentralized Control System
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and they seem to be somewhat interchangeable nowadays.
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Regardless of which description is used,
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we are discussing a structure that,at the high-level view,
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is a system that coordinates and supervises an entire plant of many varying processes.
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Briefly, as a point of the historical review, when PLC's were invented,
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they were really good at handling single processes
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and were primarily used for repetitive, discrete control.
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The advent of the DCS was for controlling many autonomous controllers
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that handled many continuous operations, mainly using analog control.
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Through time and innovation,the lines have blurred a bit between the two systems
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but each, in the current day, has some principal differences.
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PLCs, traditionally, were used for single batch or high-speed control,
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have a relatively simple, low-cost design, and are the core of the system.
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Their design is flexible and generic but completely customizable.
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Processing time for tasks are typically very fast,
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operators usually interact and control the system using some sort of graphical display such as SCADA.
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A DCS is used for continuous, complex controls,
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they have an integrated control center much like a SCADA,
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which is the core of the system versus the processors in a PLC system.
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The DCS has a number of predefined functions that come ready to customize
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and deploy for various applications.
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Processing times are somewhat slower.
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Operators interact with the control system via an integrated graphical display.
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DCS's also have a claim that when safety is a top priority, the DCS is the most reliable system.
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The reason for this is because the manufacturer supplies both the control and supervisory equipment
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as an integrated package, the risks of integration errors are greatly reduced.
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There are indeed scenarios in which a PLC system would be the best option such as smaller sized processes
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where you could employ redundant components to negate the possibility of process shutdowns.
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Without redundancy, you risk production halts due to the nature of a single processor controlling an entire plant.
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Redundancy may be deployed in either the PLC or DCS applications.
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We will talk more about redundancy in a future video.
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Just as there are circumstances for a PLC system, the use of DCS would be for larger,more complex processes
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that require a lot of interaction between many processors.
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Now that we've touched on some of the differences in the systems,
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let's focus on the DCS and some of its components.
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The DCS is a process-oriented system that uses closed-loop control.
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A typical plant starts with a centralized operator control center typically called Operator Stations.
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Operator Stations, in a DCS, are the heart of the system.
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This is where the operator can observe the operations of the plant,
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view process warnings and alarms, monitor production, and more.
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The next level of components may contain servers, archiving computers, and engineering stations.
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Communications with the Operator Station level is typically Industrial Ethernet.
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Servers are used for the collection of data at the processor level.
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They are responsible for the data that moves between the Operator Station and the processors on the plant floor.
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Archiving computers are used for storing historical data that may be used for trends or compliance.
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Engineering stations are used for creating the projects on which the processes run.
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This includes hardware configurations, logic for tasks, graphical displays for operator interaction,
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and the administration of all of those tasks through installed software packages.
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This is the station that is used to download the projects to the processors and the graphical displays.
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At the next level, you have the master controllers that supervise the individual processors as well as I/O modules.
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These controllers are also responsible for providing the data to the servers,
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which in turn, supply the data for the graphical interface.
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Industrial Ethernet is typically used for communication with the previous level.
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Fiber Optic may be used here when Ethernet cabling runs would be too long.
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At this level, the processor executes the logic and does what it needs to do in order to control the process.
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The next level is the field device level.
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Communications between this level and the processor level
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can be nearly any type that may be compatible with the components.
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Those include Industrial Ethernet, Profibus DP, EtherCAT,
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Fiber Optic, or other proprietary communication protocols.
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Components at this level would be devices
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such as transmitters, switches, valves, motors, remote or distributed I/O, etc.
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In short, both PLCs and a DCS have their place in the market today.
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PLCs would work best in a small production environment
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where component failure if no redundant system is deployed, is a small risk for the application,
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the budget is restricted, or the tasks and I/O count are minimal.
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A DCS would be better used in an environment where there are large I/O counts
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with many continuous processes,
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a processor failure in one section of the plant is not a problem for production,
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or risk assessment has determined that an integrated package would be the best option.
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The line between the two systems is nearly invisible nowadays
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and it may not be long before there is no differentiation at all.
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In future videos, we are going to discuss SCADA and contrast DCS and SCADA systems so stay tuned!
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Want to learn PLC programming in an easy to understand format
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and take your career to the next level?
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Head on over to realpars.com
