Distributed Control Systems

Hi,

Please forgive me if I'm talking out of turn, but are you by any chance the same person who posted http://cr4.globalspec.com/thread/13878/Option for Process Control is DCS or PLC ?

The thing that makes me think this may be the case (apart from the subject matter of DCS) is the end of your post, viz.: "and why?"

The thing is, you're asking a hell of a lot here, and you haven't even given a clue as to what is being controlled. And we're asked to justify our answers?

Please clarify (and, just for me, please say "please")

In regards to http://cr4.globalspec.com/thread/13878/Option for Process Control is DCS or PLC, no I am not the person who posted it.

I know my question was a little broad but I wanted to know what DCS Systems people are most familiar with, their advantanges, disadvantages and the types of projects they would best be suited for.

Sorry to have suspected the worst - CR4 is often plagued with people who want everything for nothing.

However, I think your question is more than a little broad. We can perhaps spend hours chronicling our experiences with this or that brand of system; information which could be useless to you, because our system was turning 3 titanium widgets/hour, and your system needs to injection mould 55 replica glass peas/second. Unless you can find someone who has nothing better to do than write their engineering memoirs (unpaid), I suggest you ask rather more specific questions.

Regards, John.

Thank you for your comment, however it depends on how vast your knowledge is and how much you want to share. To a number of people it is stright forward. What DCS have you used and how do you like it? if you have no experience in this area you have no need to answer the question.

Please buy Emersons Delta V. It is by far the best for your application.

(Why? I am a Emerson shareholder, so it helps my retirement out....)

Gotta respect the honesty...

I will go for SIMATIC PCS 7,

Why:

SIMATIC PCS 7 is more than only a DCS (Distributed Control System). SIMATIC PCS 7 combines a unique scalable architecture with high performance engineering tools and comprehensive additional functions such as Management, Process Safety or Asset Management, so that you can operate your plant more efficiently. With SIMATIC PCS 7 you can increase availability, reduce the life cycle costs of your plant and increase the efficiency of your processes. In addition, SIMATIC PCS 7 offers you a flexible option for modernizing your existing systems, even during runtime. Your advantages at a glance:

• Minimizing TCO by Integration of all new and existing automation systems (process, discrete and safety) and devices (process, discrete and electrical) within your facility
• Delivering higher Performance by maximizing system reliability and availability
• Continuous technology Innovation - from one of the world's leading automation suppliers - Siemens
• System Scalability from 100's of I/O to greater than100,000 I/O points
• Reduced risk through integrated process safety and comprehensive IT security
• Protection of automation investments through a step-wise Modernization strategy
• A Global network of experts and Solution Partners providing service and support locally

Hmmm, and you thought I was being selfish. I wonder who this Guest works for????

For a large-scale system such as multi-boiler electric generating facilities, I prefer Invensys Foxboro I/A series. It is infinitely scaleable and easily configurable. It can also be readily set up for control and monitoring of multiple independent processes. The fieldbus is very robust and requires no climate control other than dust protection. It will also interface with a variety of PLC's for sub-systems where an independent controller comes with the equipment. The latest operator interface uses WonderWare on either Windows or Linux PC's.

For smaller systems I normally make my own DCS using GE-Fanuc 9030 PLC's. For a complex system I will use WonderWare on a PC for the operator interface. For simpler processes a touchscreen from Automation Direct saves programming and training time.

Unfortunately you cannot compare control systems in as simplistic a manner

as you imply.Control systems are evaluated in accordance with the degree with

which they articulate the control function at hand.Names alone are not enough to

base your comparison on.Control stsytems are phyiscal models of a mathematical

solution so at the end of it all you can simply ask yourself the simple question

(Does it do what I want it to do consistently and without causing me headaches?If

your answer is YES then that is the system for you otherwise you have to do it the complicated way.You will have to first study your system,locate other similar sytems

and compare experiences or derive the control equations and functions mathematically)

E Dube

Dear friend

The application is what accounts.

For instance: if you have a transmission system, the most important thing is the protection. and it is important that it be able to communicate with your control system.

You have mentioned some manufacturers, but, particularly if you are in USA, why not thinking in SCHWEITZER or GE?

SCHWEITZER has, by far, one of the equipments with the best ruggedness. and, by the way, if you need a switch, why not using RUGGEDCOM?

Both equipments work in the rage bracketed -40º C up to +85ºC without any need for special environements requirements.

In my opinion, you should commercially analyse tenders from ABB, SIEMENS, GE and SCHWEITZER.

Regards

I've been around several DeltaV installations that perform well (in continuous and sort-of-batch processes, i.e. sequential function charts), but I would imagine that each system has various strengths and weaknesses. I'm not a big fan of DeltaV's historian, I liked it better when they bundled PI into their DCS as the historian (although it made managing the PI system a bear). The Diagnostics tool is nice and the auto-tuner has some moments of brilliance as well (although it struggles at times too). DeltaV is really easy to navigate through and at least at the module level, the code seems straight-forward so troubleshooting is pretty easy as well and forcing values is fairly simple to do. Of course my only other comparison is PLC 5 so take that for what it is worth.

I would say regarding control systems, regardless of the control system you install, everything will depend on the engineering team installing that system. Find a team that will be thorough and willing to work with you and understand your process/unit and install their system.

The only DCS I'm familiar with is the Emerson Ovation DCS, only because that is what is installed at a power plant where I work as a construction consultant.

The only thing I know about the system is that it is used to control two 175 MW gas turbine driven electric generators and two 5mlb/hr HRSGs heated by the exhaust gas from the GTs and supplemented by gas burners.

I'm not aware of any problems encountered with that particular system that would be different from any other system. To the best of my knowledege, the customer is satisfied with the system as evidenced by the fact that they have since purchased other similar systems from Emerson.

Hmmm,

Interesting Question.

I have interfaced to Honeywell, Delta V, Siemens Equipment etc., With some early "DCS" PLC based on Reliance Automax, and recently AB ControlLogix.

The AB ControlLogix platform actually meets most of the requirements of the larger DCS systems.

My experience has been the large DCS systems easily handle the slow process control, set points, and a lot of the "process" logic.

However, it has always been way too slow for high response control where the updates are in the milliseconds. That control is better done in the drive or the local drive controler. Then things start to blur, because the profiles and set points may change way too fast for the DCS to provide. Imagine a veneer lathe that peels out a log in 6 seconds. The carriage position has to be accurate to several thou, spindle rpm goes from 200 to 2000 rpm and back to rest, stopping of spindle occurs in .3 seconds etc. A DCS can't provide those profiles, but it can provide a more supervisory layer: the peel is to be 0.2" thick.

I have always found the "DCS" IT types think the DCS is the end-all be-all, and it isn't. The PLC (large ones like the AB ControlLogix) can duplicate almost all the functions of the DCS. The drive suppliers get frustrated interfacing with the DCS (and IT people) and would prefer to keep all their functions local to their controls.

Done right it is a dream to work with. Done wrong it is hellish.

For local high capability DCS / PLC type control ( not the mill wide DCS control) I have preference for Reliance Automax or AB ControlLogix due to familiarity.

Interfacing to Honeywell, Delta V or what ever has always been more dependent on the capabilities and vision of the people involved, and how up-to-date the system is. (What, no ethernet connections? You want to give me contact closures for speed increase decrease or an analog reference that is 10 bits but you want it accurate to 0.1 fpm in 8000fpm? What do you mean the nominal gear ratio is 4.9, what is the exact tooth count?)

The background for this comes from largely drive controls, Pulp & Paper, Wood products, and other process lines. Experience mostly with Reliance and AB/Rockwell product, 33 years in the industry. (So the bias.)

I completed an extensive analysis of five DCS systems (Honeywell Experion, Emerson DeltaV, Foxboro, ABB 800xa, Yokogawa Centum 3000) recently. Of course, a lot can change in time but this is where I ended up.

The analyses started by creating a list of requirements from the project specifications, dividing these requirements into 'Must Have', Commercial, Technical, and maintenance. A rating system was devised (Scale from 1 to 5). Ultimately we ended up with over 300 items. Due to the quantity the high priority items were picked and we scaled the total down to approximately 100. I started rating the systems using written documentation supplied by the vendors and then completed the rating with live demonstrations from the vendors.

There are two important things to note:

1) Project specified requirements were used as the basis - ultimately, it depends on what you're looking for.

2) The vendors are upgrading their software continually. What may have been the lowest rated system a year ago could be on top today.

The system with the overall highest score and that satisfied the largest number of requirements was the ABB 800xa system. It actually ended up in a category by itself. This was due to the shear number of control algorithms and libraries, programming languages, test abilities, expansion abilities, operator interface options, flexibility (controller, I/O, and network), trending (seamless historian and local trending) and openness. Farther down the list and in second place was the DeltaV, followed by Experion/Foxboro and Yokogowa (the last three being pretty close). Of course, there's two sides to everything and the drawback to the 800xa was its complexity. I had to throw a lot of my past experience out the window and start again to program/configure it. In the end, however, I felt it was well worth the effort.

The DeltaV system had a lot of characterization algorithms but fewer control and logic algorithms. The online manuals were very good and the programming software was intuitive and easy to use. Its drawback was programming flexibility. Code had to be imported to text files versus the 'drag and drop' ability of the 800xa. Licensing was also a concern as project licences had to be purchased or borrowed for a limited period of time.

The benefit of the Experion system was that the controllers were based on HPM parameters. The HPM was a great controller. The drawback to the Experion system is that they removed the best parts of the HPM controller during the process. I think that they are improving on this as time goes on. Structured Text code is imperative for serious control system design. The C300 controller had no where near the number of algorithms as the 800xa system.

The Foxboro system had some hardware issues at that time (controller battery backup). The software was kind of strange as logic was forced into ladder diagrams located in the I/O cards. As well, there were a great deal of building blocks that had to go into creating a typical scheme that left the configuration pages cluttered and hard to look at. Tag naming was an issue as well as every tag was prefixed by a reference number. I remember the hardware being impressive to look at however. Looked like you could drive a tank over it.

It was tough to get information out of Yokogawa and therefore we could not complete thorough analyses. It seemed to me that it was a new GUI on an old control system – the hardware was very large and required a great deal of cooling and the network seemed quite closed and restricted. They were very pleased with their simulation software which could be used to test the configuration. Although it was good it was a separate package which required an imported configuration. The 800xa configuration testing could be done immediately within the configuration software.

This is a small sample of the items looked at and found. Although the situation may be different today I thought they may interest you.