DCS or PLC? 1

[bmsg]

We regularly have heated debates about whether to have a DCS or a PLC for the instrumentation portion of our projects.
Typically, I find that what a DCS can do, a PLC can also do. Only, a DCS is generally costlier than a PLC, for the same I/Os.
In the end, the choice boils down to the whims of someone ("We'll have a PLC, because I say so" or "For instrumentation, we'll have a DCS, because we say so"].

Will someone please let me know the following:
1. The technical difference between an DCS and a PLC TODAY (i.e., not historically).
2. Niche applications for a DCS and a PLC TODAY.
3. In the end, is the DCS on its way out, having been vanquished by the PLC?

[I am an instrumentation engineer and work for steel plant projects. In any 'shop' in a steel plant, there is an Electrical Department and an Instrumentation Department in the plant - with more or less water-tight responsibilities. I have a enough of a grounding in PLC & DCS and their evolution path - so don't write about the history of these things - only about where either of them stand today.

 

[JLSeagull]

The DCS includes higher levels of application software for regulatory and batch control; and higher degree of systems integration. The technologies overlap. Each use open display software and communications.

 

[ADMIRALKEN]

IMHO DCS systems are much more robust, redundancy is much easier to implement. I currently work with both, and only use the DCS for critical applications. PLC's are better for high speed operations. Our current system has a DCS system as a master, so to speak, and we have many PLC's running little stand alone operations, the DCS monitors the PLC's, records data, and passes permissives etc.

 

[JLSeagull]

PLC's are available with far higher reliability than the DCS. I regard safety shutdown systems (safety instrumented systems) as a PLC. Triple and quad are common. Shelf GE PLC's are sometimes packaged as a triple modular system. Many plants use a PLC for small plant operations integrated with Wonderware etc. The Emerson Delta V hardware and the Honeywell Experion P200 series use PLC hardware.

 

[bmsg]

To clarify my query further:
1. Today, any PLC can provide as much redundancy (processor, communication modules, power supply, communication bus, etc.) as a DCS.
2. PLCs provide all the function blocks for regulatory process control that a DCS does (say, 'P-I-D' blocks, setpoint tracking, auto-tuning, etc.).
3. Typically, PLCs are assigned operations for small units (in a plant, you may have a PLC for water supply, another for some other function, etc.) - because they are cheaper and one can afford to have many of them around at the same time. A DCS is more expensive, and it is common to have only one DCS in a plant.
4. PLCs are normally faster than DCS - at least for sequential operations (that will always be there in a plant).
5. PLCs are cheaper than a DCS.
6. Even if you have a DCS, you will always have a separate PLC for such applications as Emergency Shutdown (ESD), Furnace Safeguard Supervisory System (FSSS), etc.
7. Many of the DCS in the market now use the same modules as used in PLCs from the same manufacturer (eg., Honeywell Experion PKS, ABB make DCS, etc. use cards also used in PLCs from these manufacturers).
8. Both PLCs and DCS support open architecture, these days (whether at the system bus level or at a higher level).
9. 3rd party software such as Wonderware, Intellution FIX, etc. support graphics, trending and historian functions,as good as, if not more versatile than, a DCS.
10. PLCs and DCS use similar programming software (such as Functional Block Diagrams (FBD)).
11. And, the list goes on...
Mind you, as I said earlier, I am an Instrumentation engineer, and my sympathies are for the DCS.
But, technically, is there any reason to defend use of the DCS in place of a PLC in today's environment?
That is what I want to know from you people.

 

[JLSeagull]

Most of your points are valid.

The control software for a PLC is more difficult to use than the DCS control software for batch. The DCS supplier provides more system integration. A typical panel shop is not as ideal to package the system as the DCS assembly, staging and testing facility. Still the DCS suppliers will quote a hardware only job and ship the parts to a plant or panel shop. The higher level software and integration are a key difference. If the supplier does not spend their time to design, assemble and test the system then those hours still apply to the purchaser.

You failed to point out that systems vendors charge lots for their engineering services. This is another point in the balancing act.

Not everyone needs the advanced software available with a DCS. Not everyone needs the systems integration.

If the same hardware applies to the DCS and PLC then the execution speed can be the same. However, analog PID loop execution takes longer than discrete logic.

In the hydrocarbon process industry the PLC that controls a pump as regulatory control in a manner similar to a DCS is not the same PLC used for safety shutdown. For the safety system ISA 84 and 29 CFR 1910.119 move the plant toward a dedicated safety shutdown system with higher availability. Also, the reliability requirements such as the need to detect measurement instrumentation failure moves the plant to analog transmitters and eliminates most process switches.

Whether selecting the DCS or PLC platform it is necessary for the plant to develop the skills to support the system. It may be practical for many facilities to use the technical support within instead of the outside support for the systems integration.

 

[Ashereng]

bmsg,

The terms DCS and PLC are more legacy terms than descriptive right now. In times past, maybe 15 year ago, they served more distinct purposes. Today, they are pretty much interchangeable.

On a recent job, we sent bid packages to all the usual suspects - Honeywell (Experion/Plantscape), Foxboro/Invensys (IA/MicroI/A), Emerson (Delta V), Modicon (Quantum) and AB (PLC5). Their solutions are all pretty much the same - redundancy, HMI, fully integrated, data historian with software, remote I/O, Modbus interfaces, LAN connectivity and data exchange, etc.

The final decision came down to pricing. Doesn't it always?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[fribby]

From my experience (large chemical and petrochem installations) I tend to view the DCS as a overlord system dealing with all 'normal' IO, the PLC's we have serve as small system solutions, i.e. on a complex voting trip system we would use a SIL2 PLC which deals with the trip logic/functions, the PLC would then communicate its status to the DCS.

The DCS acts as the main controller and operator interface, PLC's perform very much behind the scenes.

 

[JLSeagull]

Didn't Honeywell use an Allen Bradley PLC for control for Experion?

Didn't Emerson Process Fisher Rosemount Delta V use the MTL PLC hardware for control?

Don't the GE and Solar turbines use a PLC for autonomous control?

Are the ICS Triplex, Triconix, HIMA and other SIL 3 TMR Quad etc. safety systems PLC's?

...and the displays for all of this use a desktop PC running Gates Window?

 

[Ashereng]

The Honeywell TDC3000 series did not use desktop PC at all. It used a Honeywell "operator station" running their own software.

The ICS Triplex, Triconex, HIMA, and other SIL 3 SIS are "programmable logic controllers", but they are not your run of the mill PLC's, like AB and Modicon. These are special "boxes" with a bit more to them. Sort of comparing drill bit diamonds with what you get in a jewellery store. No?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[danw2]

Does anyone know of a DCS installation that doesn't use the DCS vendor's own software? I don't.

Yet there are a multitude of PLC installations that run on 3rd party HMI/supervisory/SCADA software.

The connection goes back to JLSeagull's observation about the difference in the level of integration of the software to the product.

It is my impression that one marries the DCS software along with the DCS hardware, but that is not necessarily the case with PLCs.

Dan

 

[controlnovice]

BMSG,

We're struggling with the same questions.

We consider the latest systems as Hybrid or PAC (Process Automation Controller), although the latter is really all encompasing. So the DeltaV, Experion, PCS7 are all hybrids to us. You can make any of the systems do what you want it seems now-a-days.

Our main struggle right now is trying to determine a global position on control systems for the company. Having one manufacturer would be beneficial in many ways, with a few drawbacks. One of the main drawbacks is new plant acquisitions. Do we spend the money to change the existing to our "global position", or do we maintain the existing system until it's obsolete with no support?

This doesn't answer your questions, but it does let you know there are people in the same situation.....

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[Ashereng]

Dan,

The old Moore APAC system used Wonderware, a third party software. Does that count?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[danw2]

>The old Moore APAC system used Wonderware, a third party software. Does that count?

Yes, it does, some people called APAC a DCS.

At this point, is APAC a milk-the-installed-base, or is it a viable product since the Siemens buy-out?

Dan

 

[Ashereng]

I think it's more service the installed base, and don't buy new?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[controlnovice]

Yes, APAC is only support now by Siemens. Siemens is steering customers to the PCS7 for new service, no new sales of APAC.

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This is normally the space where people post something insightful.

 

[glowing]

I feal it is a decision that must be strongly decided on process size. It is undeniable that a good DCS set-up blows any PLC system out of the water, but at the same time, modern PLCs can achieve the same things most DCS systems could in years past.

There are not too many instances inwhich PLC will not suffice. I personaly like Delta-V...not that the programming end is amazing or anything, but because of the integrated card controllers.

 

[dcasto]

My two cents. for robustness, try several small PLC's, that is true distributed control. Each area of systems has its own local control system with a PLC located nearby. Then use amaster PLC that links them together. For the HMI, nothing beats a simple WonderWare package.

PLC's, AB, GE you can't go wrong. Unless you have 10 or 20 towers and 50 + pumps and 1000 loops, stay away from Delta-V overpriced, over complicated, overkill....