dcs & plc

This subject comes up frequently. Try doing a search on this forum for DCS. If you still have questions, come back with specifics.
 
DCS & PLC

plc can do any work that DCS can do,but DCS offers some advantages,
when it comes to redundancy and hot swap capability.
Higher level integration with MES,ERP and IT is easily done with DCS
than PLC..
 
The following is my personal opinion only

I will state this for the record...I have had very limited experience with DCS.
BUT
The few occasions I have had to deal with DCS...to put in plain words I thought it sucked. It was proprietary, dos based, used plc components without the advantage of using the plc software...etc etc.

As I stated this is personal opinion only. To me the use of a plc for relay type logic control combined with a motion controller (when/if needed) will perform as well if not better at a much lower cost overall.

The KEY to all this to me is UNDERSTANDING...a DCS system is not always easy to understand...no simple method to troubleshoot.

For an explanations:

DCS means Distributed Control System: technically the idea behind this follows a master/slave scenario...actually the DCS in most cases is the master and slave. In most systems the I/O are plc racks with no cpu, the signals are sent directly to the DCS cpu.

The concept behind DCS is to have each aspect of a system have its own "brain" and a "manager brain" control the separate functions.

As far as integration goes, there is no difference.

As Tom said, search the site on this.

As a last note....RTFM or the study guide. Any book that is applicable to your studies.

NOTE: The above is in no way meant to imply that a DCS will not or can not perform efficiently. IN the few cases I have dealt with them they are very efficient just not easily diagnosed when a problem occurs....technically problems were rare.
 
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I maybe way off here, but I think the significant differences between a PLC (Programable Logic Controller) and DCS (Distributive Contol System) are:

1. How the proccessors scan the I/O.
PLC-in scan top to bottom
DCS-scan all I/O ussually based on change.
2. The refresh speed of the data or scan time.
PLC-Depends on the length of the program, loops, Sub routines, etc...
DCS-Typically more real time data.
3. Cost
PLC-More cost efective on a per I/O basis than DCS.

If I am going to apply a new control system to, for example, a chemical proccess that would tend to be easily upset, I would choose the DCS.
If on the other hand I am adding a package conveying system, a PLC would be more that sufficient. Of course this is an over simplification of the problem, but I think you see my point...


Opinions are like bosses, everybody has one.... :eek:)
Daniel Camacho
 
LoL, it's quite easy to tell which guys here started with PLC or DCS. For the record, I started out doing DCS. And yes, I think DCS is much more user friendly than PLC. DCS is usually used in the process (chemical, petrochemcial, power) industry since it was designed for continous process (PID loops, etc). I would share more but this subject has been covered to death here.
 
I'll be nice. But only because I feel like sharing my opinion.

In my mind the distinction is in what the hardware looks like and what gets sold to you with the package. A DCS is usually a cluster(s) of rack-mount cabinets. The guy that sells it to you will want you to use HIS cabinets, HIS software, HIS workstations, HIS sensors, and HIS desk chairs. Then, he'll want to do all the design and programming, provide a few hours of training, and turn the system over to you. Merry Christmas.

A PLC system tends to be much more modular, more piece-meal (lots of different vendors, and more hands tend to be involved in the design/install). A large scale (DCS substitute) PLC system has the controllers actually DISTRUBUTED more than they would be in a DCS. The control cabinets in a PLC system become wall-mounted panels or free-standing multi-door cabinets (typically) because the space requirements and mounting options for the PLCs are different (PLC-5 being an exception). I guess the "distributed" part of "DCS" is because the IO is all over the place, and you run a spiderweb of mile-long twisted pair cable everywhere.

AK
 
Sorry to hijack the thread, but.....

Has anybody spent a significant amount of time with any PLC/DCS hybrid systems? The new facility I'm working at is running a combination of CLX and Honeywell Plantscape system.

I find it very confusing being that I'm not all that experienced with DCS, and even less experienced with RS5K.

We also have a AMS system that is tied into that, meaning I can communicate with any smart device in the field with HART protocol, at a computer terminal without having to go to the actual instrument. Nice system!!
 
Voltimus said:
Has anybody spent a significant amount of time with any PLC/DCS hybrid systems? The new facility I'm working at is running a combination of CLX and Honeywell Plantscape system.

Did they get a best-of-both worlds approach, or an ineffective implementation of both?

I could see how this type of system could work very well, or very poorly. Is the control in the PLCs, with datalogging and supervisory in the DCS? That would make some sense to me. DCS systems are built for data.

AK
 
Did they get a best-of-both worlds approach, or an ineffective implementation of both?

I believe they put together a very strong system, DCS does handle all of our data along the lines of trending and PID loops, while the PLC system handles safety process and automated valve controls. Tie that in with the AMS (assett management system) and you have a place where a tech can get spoiled very easily. We have a client in our shop that enables us to check trending on any instrument in the field, so, we know what that instrument has been doing for the past month, great for solving those operator error work orders that techs always receive. And if it's a calibration issue 9 times out of ten we can attempt to reset our trim (from our shop), if it doesn't respond we know the instrument is no good, we go change it. Almost sounds like bragging, right? I know.

My only problem is the time it will take to completely comprehend the system integration.
We have everything from fiber optic communication to rs232.
It boggle me, make head hurt.
 
while the PLC system handles safety process and automated valve controls.

I suppose that's why the engineer's chose to do it this way. My guess is they got your memo concerning DCS, but, didn't read the one concerning PLC and DCS being dead terms. I'll forward a email and straighten them out. đź““
 
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Not all DCS runs in a crappy old DOS environemt. The Yokogawa Centum CS we use at my plant runs on a Unix environment. We use FreeBSD unix for our 50 odd operator panels (because its free!) and to engineer the control drawings we Telnet into the EWS (Engineering Work Station) that is a HP Unix machine (costs $50000).

We currently looking at upgrading to the CS3000 platform that runs on XP and is very, very user friendly
 

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