Please clarify following before u decide PLC or DCS 1. Is plant going to expand further in future?? 2. Is plant is operated by AUTO or by process operator?? If the answer is yes ,then go for DCS else PLC. regards Bhargava [email protected] > I have to decide on the control system of a new medium sized plant. I am really in confusion which one to choose. Shoud it be a PLC or a DCS control. The plant has about 50 AI, 16 AO, 100 DO, 300 DI and 16 PID loops. Thank you very much for your comments. M. BICER
> > Lou,
> In your response, you say "However, it is a reasonable rule of thumb
> that PLCs are going to be more cost effective in factory
> (discrete control)
> applications and DCS will be moreso in process (regulatory control)
> applications. Why? Because the focus of those products is
> on where their
> primary market opportunity lies. It's where their inherent design
> advantages are found.". And your reason is due to the
> inherent design of each system respectively. I hear this same argument
> around here ( meaning my place of work ). There point is that DCS
> systems are easier to program PID loops with and with all the pretty
> accessories ( like limit blocks square root function for analog inputs,
> etc.) that are usually not in PLCs, regulatory control in a DCS is
> superior to a PLC. To me, that seems like a weak argument for using
> a DCS over a PLC.
It seems like a strong arguement to me. At least until PLCs / PC MMIs catch up (and they are). The time it take to program/maintain control system is a significant cost.
> Anyway, what makes regulatory control so unique that DCS systems are
> best suited for this type of control? Is it the speed of data
> processing? Are all DCS systems use a form of parallel processing of
> data? Do they have a superior means of signal
> processing ( digital filtering ) over PLC systems?
No, DCS's are not fast, they are usually pretty slow. but then scan times of .1 to .5 seconds are much faster than what is typically needed in most processes.
> For me, the part of the answer is that there are those that love
> DCS systems over PLC systems and there are those that love PLC systems
> over DCS systems.
> I guess where I am coming from is that if I look at the
> Boiler control system example in the SAMA functional diagramming of
> instrument and control systems standard dated September 1981, it seems
> to that both the combustion controls and the three element feedwater
> control can easily be implemented in a PLC. I would even say that a
> multi-Boiler system could be implemented on multi-processor PLC system
> ( like using a few ControlLogix processors in a single rack and
> having a bunch of remote IO for all the instruments ). I would even
> gamble that a multi-fuel multi-Boiler systems could be implemented
> using a multi-processor PLC system. Now unless DCS systems have this
> incredible processor and IO speeds, I do not see the reason outside of
> what the designing engineer likes, what the consumer likes, what the
> maintenance people can handle, and most importantly the dynamics of
> the process ( heat and mass balancing, chemical kinetics, etc.).
The designing engineer likes the DCS over the PLC for the boiler control because it is easier to configure/maintain/ensure 100% control system uptime, not because it can control better.
> I don't know, maybe I should just sit down and reread chapter 7 on
> DCS systems in the book, "Instrument Engineers' Handbook 3rd Edition"
> by Bela G. Liptak, in order to seem what I am missing here.
> Would you use a DCS system or a PLC system for a distillation
> process like the separation of petroleum products from crude oil )?
It depends on how you want I needed to run the system. If it is a big system that needs to run 24/7/365, I'd probably lean toward the DCS. If is was a small system, I'd consider a PLC, or a
Fieldbus system.
Mark Blunier
Any opinions expressed in this message are not necessarily those of the
company.
> Today you can get the best from both a PLC and a DCS system. These units are called hybrid controllers. PLC are very popular for their ease of programming, DCS provide better methods for data handling, storage and external communications. Visit www.controlwave.com for more information about hybrid controllers. Also look for standards such as IEC61131 and OPC, TCP/IP to set a benchmark for future products.
