Comparing PLC and DCS

Very good explanation for beginners or other engineers who are not specialized in this field, but have to deal with it on one side.

I appreciate your time and knowledge.
 
> Can you provide some Input on cost comparison of PLC & DCS?

The basic difference is DCS and PLC lies in application. THE PLC IS used for start up and shut down activities for equipments like pumps or compressors because of faster response of PLC scantime shall be less.

DCS is used for for process parameters trends, history of parameters
data acquisitions reports, snapshots, collection of process history data.

regards
saradhi
 
Ok, being devils advocate, please explain why you don't see PLC's providing primary control in a refinery or petrochemical facility. Conversely, can you explain why you don't see DCS systems in discrete manufacturing facilities, grain terminals or bottling facilities for example.

There are very practical reasons why this is the case.

As some history, PLC's have there roots in discrete manufacturing while DCS systems were born in the process industries. In the 1970's before the days of digital electronics, companies like IBM built the hardware and wrote the software for each plant individually. Some of the large process companies got in the business and developed their own systems. Did you know that the Honeywell TDC system was originally developed by Exxon and later sold to Honeywell. Similarly, the Fisher Provox system was originally developed by Monsanto corporation and later sold to the Fisher corporation, Provox was the predecessor to DeltaV that was developed by Emerson.

DCS systems were born out of necessity as the existing control systems of the day in discrete manufacturing were not able to handle the large numbers of analog I/O that is typically seen in a process plant. For those of us old enough to remember analog electronics, or even pneumatics, the PID controllers required to operate these facilities were panel mounted in the control rooms in long rows, and the board operator spent much of his day walking back and forth in front of these panels monitoring strip charts and adjusting these controllers as necessary.

It is true that the advent of digital electronics has blurred the lines between these systems, but there are applications where each would be considered a better choice than the other. As control engineers, as a rule of thumb, we tend to look at the ratio of analog to digital IO in a plant to help make the selection. If the ratio of analog to digital IO is high, most manufactures will select a DCS to control their plant and conversely, if this ratio is low, a PLC is most often a better choice. No doubt there are exceptions, but as I said its a rule of thumb.

The primary reason for this, is that over the many years the vendors have developed these systems, their architectures have been optimized to handle either large numbers of analog I/O (DCS) or large numbers of discrete I/O (PLC). Will either one work in place of the other ? Of course, just not as efficiently as the other.

In my 30+ years in the business, I have yet to see a PLC running a world scale cat cracker, crude unit or ethylene furnace for example. Similarly, I have yet to see a DCS system installed in a world scale discrete manufacturing plant (packaging, bottling, assembly plants, etc)

Where it often gets grey is when analog to discrete ratio is near 1:1 like water and waste water treatment plants, facilities with a lot of materials handling like minerals processing or a cement production for example. In these instances I've seen both technologies applied successfully.

Other considerations:
- wiring costs can be up to 10% of the capital cost of a greenfield facility. To address this, some vendors have begun to distribute their I/O in the field and then network the IO cabinets back to the main controllers. There can be huge savings in wiring and marshaling costs if these systems are engineered properly. The allow for late stage I/O changes in a greenfield project without impacting dozens of drawings and changes to marshaling cabinet sizes.

- advanced control technologies like model predictive control were pioneered by the some of the DCS vendors and 3rd party software vendors like AspenTech. If you want to take advantage of the benefits of APC technologies like MPC, you pretty much need a DCS system. Again these technologies were originally developed to work with single database platforms like a DCS.

Just some thoughts from an old control hand. Cheers to all.

Warren
 
B

BOLARINWA GBENGA JOSHUA

DCS (Distributed Control System) is superior to PLCs( Programmable Logic Controllers), though some high-end PLCs can now deliver reliably as DCS. When the task has to do with lots of Analog and Digital IOs, and the feedback response time is below 10ms, go for a DCS; if up to 10ms and above, use a PLC. However, most high-end PLCs are very expensive e.g Siemens S7-400.
 
>I am new in the field of automation. Could anybody tell me
>the difference between PLC and DCS other than I/O handling
>capacity?

Went through all 44 replies here so far, was surprised no one mentioned PACs (Process Automation Controllers). Many not knowing any better often refer to a PAC as a PLC (as it has one built in). PACs are the evolution of PLCs about 10 years ago, thus my surprise for the lack of them being mentioned. (An example of PAC is Rockwell's Controllogix/RSLogix 5000.) But PACs can, and often do have 128 thousand available distributed I/O (4 thousand analog) for control. See chart at http://bin95.com/PLC-PAC-Difference.htm for reference.

PAC easily replace proprietary DCS with a more flexible open system, that has a much lower life cycle cost than DCS. (Although PAC's have a much higher life cycle cost than PLCs. Greater complexity and functionality typically do result in higher life cycle cost, but proprietary systems like DCS increase that cost even higher. Thus the answer to one person here who asked, 'why doesn't oil industry use them [PLC/PAC] more?' The answer is simple, and age old. Industry is slow to change. On new systems you see PACs being used more at an increasing rates, but changing out old DCS systems, well if it ain't broke, don't fix it mentality takes priority often. :)
 
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