DCS and PLC have been pillar for control of process plant for quite some time now with advancement in technology. I understand that the gap in technical capabilities between these 2 breeds of process control systems have narrowed, but still I understand there are some fundamental differences like.

1. Scalability

2. closed loop control is better in DCS than PLC

3. digital control of motion based equipments is better in PLC than DCS on account of better scan time for PLC than DCS

Can someone help me with some more points?

 

Hmmmm.... I'm surprised the Friendly Moderator didn't point you to at least one other very similar thread about this topic on control.com. (There have been several, I think. Archived threads can be searched using the cleverly hidden 'Search' feature at the far right of the Menu bar at the top of every control.com webpage. It's suggested to use the Search Help before searching, as it's not intuitive--but it is very powerful.)

It's helpful to just think of most control systems as PACs--Programmable Automation Controllers. The lines are really blurred these days, and continue to be so as technology--and marketing propaganda--advances. DCSs and PLCs and PLCs used as DCSs are virtually the same things (though manufacturers will argue vociferously they aren't--especially when they only offer one or the other!).

To my way of thinking, there are PACs and there are purpose-built PACs. What's a purpose-built PAC? One that's built for a particular industry, piece of equipment or process. What sets purpose-built PACs apart from other generic PACs? Usually there are one or more printed circuit cards that have some special features that make the system work better with certain devices or pieces of equipment. Examples? High speed sensing (running at scan rates faster than the PACs "base" scan rate), for sensing overspeed, or underspeed, as the case may be; high-speed position control, taking a reference generated at "base" PAC scan rate and then adjusting the output a a very fast rate between reference updates to make feedback equal to reference (think servo-operated devices; and so on.

Siemens, for example, have special software and a couple of special printed circuit cards that enable their PCS7 to be a turbine control system. Emerson has some special printed circuit cards that allow their DCS to be a turbine control. Some control system integrators have designed and built special cards for various PLCs (PACs) over the years to allow high-speed data acquisition and monitoring or servo positioning. I know of one control system integrator that is using inexpensive Automation Direct PLC (PAC) hardware, along with some proprietary cards, to control large- and medium- and even small aircraft-derivative turbine-geneators. Solar Turbines uses Rockwell (Allen-Bradley) hardware along with some printed circuit cards they've worked with Rockwell to design to control their gas turbines.

The lines are truly blurred. As are the definitions of DCS and PLC--they are all just PACs, some with more specific industries or processes or equipments in mind.

The challenge is to understand the application and to choose a PAC that meets the hardware requirements of the application. And the one theme that keeps coming out again and again and again on these and similar threads on control.com is: Choose a supplier that has experience in the industry and with the application <b>AND</b> that has a proven track record of <b>supporting</b> the installation <b>after</b> commissioning. That's the differentiator--support and after-sales support. Just about any PAC can be made to control just about any application--some better than others. But, the real value is in the design and application and the after-sales, after-commissioning support. Hardware costs aren't that much different between PAC manufacturers. It's the knowledge, experience and integrity of the supplier that's important--and can be costly.

There are many cliche's about the cost of quality, but it's the long-term value that a supplier brings to a PAC application--along with the experience in the industry or equipment the PAC is being applied to--that makes the difference. Don't get too baffled by all the sales talk about features--it can be mind-boggling. Try to concentrate on the application knowledge and experience of the PAC supplier, and then talk to various customers of theirs (references they should be able to give you) to ask how they feel about the installation support and after-commissioning support. Because, that's priceless--and what differentiates one supplier from another. You may find that a lesser advanced or not-so-feature-rich PAC (hardware-wise) may actually be a better choice from a knowledgeable and capable supplier, someone who has the knowledge of the application and has demonstrated the ability to apply the hardware to be a very successful control system even though it wasn't the most advanced or feature-rich system to begin with. The supplier had the knowledge and the experience to use the available configuration and programming tools to make that system "dance and sing" perform better than a more expensive, advanced and feature-rich system.

Quite often, advanced features are not well thought-out and are released for sale well in advance of when they should be--they haven't been thoroughly vetted and tested. This can cause a lot of problems for the control system integrator/supplier when trying to make them work. The purchaser thinks they were getting the latest-and-greatest and the most technologically advanced system, but in fact, they didn't. And that can be a real problem.

The choice isn't in the words or acronyms or abbreviations used to describe a control system--it's in the knowledge and experience of the people applying and supporting the control system. DCS or PLC or PAC, or whatever you want to call it. Look for a supplier with knowledge and experience and a record of supporting the installations after they've been installed and commissioned--and then choose the hardware. Let them steer you to the proper hardware for the application. I've seen some very good control system integrators forced to use one system or another that they weren't familiar with that resulted in lots of installation and commissioning and operational problems.

Hope this helps!

 

In additon to CSAs comparison, I would add one more item: PLC's and DCS are both very similar in terms of hardware, where they differ is in the maturity and robustness of the software and management tools. In general terms the DCS platform is designed for continuous process control with excellent software features, hardware management, programming, monitoring and tuning the process. The programming software is typically graphical in nature (e.g. SAMA drawings with boolean logic gates) which I find much more intuitive than ladder logic used in PLC's. Many features like quality flag processing are built into the software and tools for using the quality are already built-in. With PLC's many of these features are not built-in and have to programmed by the developer for each application which can be time consuming if they didn't previously develop the functionality themselves.

My 2 cents.

 

Forget about all the buzzwords. The terms PLC or DCS have mainly historical roots.

But today borders between them become blurred.

The computer and digital devices are very common today. But the real world is still analog. According to Fourier and his sampling theory the analog world can be mapped to the digital space.

 

Ah geez - PAC terminology is a load of crock! It is only an advanced PLC and nearly all of them are that these days. Ladder, function block, ST, SFC (graphical) - most have it to be compliant with IEC - stupid - oh well. The latest gig is with fast control of motion - EtherCat is the go. DCS is no better at controlling anything than a good PLC these days and it is quite slow by comparison. Everyone is on the Ethernet IP bandwagon - gets too congested and too slow - too many crashes too. I use Omron and I also use there remote I/O solutions - they are great. I do not use Ethernet I/O - too many problems - there are much better solutions out there. CompoNet remote I/O system has an update speed over 30 metres of 1000 I/O in 1 millisecond - way faster than Ethernet and no issues - it just happens.

 

Hi ,

Function of DCS is for data acquisition/trending, process control loops group, history of process parameters, and event and reports.

PLC is meant for STARTUP & SHUT DOWN of process equipment.