What is Distributed Control Systems (DCS)

We discuss about the basics of Distributed Control Systems (DCS), Distributed Control System architecture and Distributed Control Systems Parts.

Distributed Control Systems (DCS)

A radically new concept appeared in the world of industrial control in the mid-1970’s: the notion of distributed digital control.

Direct digital control during that era suffered a substantial problem: the potential for catastrophic failure if the single digital computer executing multiple PID control functions were to ever halt. Digital control brings many advantages, but it isn’t worth the risk if the entire operation will shut down (or catastrophically fail!) following a hardware or software failure within that one computer.

Distributed control directly addressed this concern by having multiple control computers – each one responsible for only a handful of PID loops – distributed throughout the facility and networked together to share information with each other and with operator display consoles.

With individual process control “nodes” scattered throughout the campus, each one dedicated to controlling just a few loops, there would be less concentration of liability as there would be with a single-computer DDC system.

Such distribution of computing hardware also shortened the analog signal wiring, because now the hundreds or thousands of analog field instrument cables only had to reach as far as the distributed nodes, not all the way to a centralized control room.

Only the networking cable had to reach that far, representing a drastic reduction in wiring needs. Furthermore, distributed control introduced the concept of redundancy to industrial control systems: where digital signal acquisition and processing hardware units were equipped with “spare” units designed to automatically take over all critical functions in the event of a primary failure.

The following illustration shows a typical distributed control system (DCS) architecture:

Each “rack” contains a microprocessor to implement all necessary control functions, with individual I/O (input/output) “cards” for converting analog field instrument signals into digital format, and vice-versa.

Redundant processors, redundant network cables, and even redundant I/O cards address the possibility of component failure. DCS processors are usually programmed to perform routine self-checks on redundant system components to ensure availability of the spare components in the event of a failure.

If there ever was a total failure in one of the “control racks” where the redundancy proved insufficient for the fault(s), the only PID loops faulted will be those resident in that rack, not any of the other loops throughout the system.

Likewise, if ever the network cables become severed or otherwise faulted, only the information flow between those two points will suffer; the rest of the system will continue to communicate data normally.

Thus, one of the “hallmark” features of a DCS is its tolerance to serious faults: even in the event of severe hardware or software faults, the impact to process control is minimized by design.


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