PLC and DCS

[mohdalharbi]

In control system of small plants (power or water ....etc)
we will use PLC to control and monitoring Data, while in big plants we will use DCS so the control will be direct through the DCS in the main control room.
I read that we can divide plant to many parts each control and monitoring with one PLC in the site, the sum of PLCs will connect to the control room it may call DCS. what is the differece between PLC and DCS and can we connect many PLCs together in control room how and through what can we say master PLC and sleeve PLCs and can we call master and sleeve PLCs a DCS system or DCS is another type of techniq.

 

[stevemcd]

Over the past three or four years PLc's communication systems have become available which allow them to act in the same way as a DCS (Distributed control system). Now there is really no great difference between a DCS and a group of PLC's with a SCADA for display. You will find that the big DCS manufacturers latest offerings are more like a PLC/SCADA structure.
Each system has it's advantages, for eaxmaple Siemens make the S7/WinCC combination or the PCS7 system. The first for mainly digital systems the second for mainly analogue systems. ABB have moved away from their Freelance platform to the Industrial IT range.
Traditional DCS have advantages still but it is now very difficult to see a sharp division between the two types of system. The last time we had to decide was on an oil platform which needed a system to run the power management with four GT's and a diesel black start. The decision was to install an Allen Bradley ControlLogix PLc system with an RSView SCADA. This would traditionally have been a DCS application and would tradiationally have cost at least five times as much
Hope this helps

 

[FrancisL]

Just a small point
When you say "Over the past three or four years PLc's communication systems have become available...."
I was using networked PLC's in the early 1980's

 

[WVB]

I also see that the differences between PLCs and Distributed Control Systems are getting smaller. However, The DCS does provide a much more robust capability to provide "enterprise system" functions. The DCS is designed primarily to coordinate large system applications such as an entire power plant or water/waste water facility.

In response to some of your direct questions, the DCS can be used to coordinate a large number of PLCs. This can be done as strictly a monitoring function or the PLCs can be slaved to the DCS. The combinations are quite flexible depending upon your needs.

The most important aspect I believe is to perform a good needs analysis early on to determine what you really need or want to accomplish. Then start moving towards a system that will meet those needs.

 

[roby74]

As far as I know the biggest difference between DCS and PLC's network is the capability to exchange data. On a DCS the Data Base is usually distributed and each of the controllers is able to call DB point located on other controllers without the need to call the other controller itself but just the point. Viceversa a PLC need to call the PLC where the DB point is located and then call the point. This leads to an highere bus load for a PLC network that sometimes implies overload of the network that could even collapse under the PLC's demands.
But it is true, new DCS architecture are very similar to PLC's network. This is now possible (but not recommended)because of the increased capacity of the ethernet.
As far as I know PLC's network applications are very limited for oil and gas plants.

 

[GerH]

From my experience where you will see the biggest difference between DCS and PLC/SCADA is when you are configuring. PLC's are generally configured as individual controllers. If an I/O on one PLC is needed for control in another generally you have to configure a separate data transfer. On a DCS you could expect to reference the "remote" point with the DCS auto configuring the data link.

PLC's are moving towards having configuration tools that treat the networked group of PLC's as one system e.g. Siemens SIMATIC MANAGER but not every PLC manufacturer is equal e.g. you cannot simultaneoulsy "open" two PLC configurations at the one time using Schneider Quantum.

 

[Mike6158]

Personally I'm not sold on DCS technology. Distributed Control System architecture, in my opinion (and I admit that I could be completely off base), arose because of the lack of a processor that could handle all of the tasks that a fully integrated control system needed to be able to handle. So, the mfg "distributed" the control application across more than one processor. Redundancy = lack of reliability of electronic components "back in the day". There are exceptions to the last statement. My background is in gas processing and we, again, in my opinion, are far more tolerant of processor faults, rare though they may be, than say a petrochemical facility. We keep a spare processor on the shelf but to my knowledge have never needed to use it. If you look at the new systems that are coming out (Matrix) and even some of the "older" new stuff (Delta V) they are all PC based and the IO looks a lot like PLC IO.

My office is located at one of our 200 mmscfd gas processing facilites. The primary PLC has over 200 analog alarm blocks, 70 PID loops, 1,100 lines of ladder logic, 21 Special Function programs (language similar to structured text) including one that calculates gas orifice flow using an AGA7 routine for 15 orifice plates, and 6 remote IO racks and 3 remote Profibus IO racks. There are 13 other PLC's that monitor and control fired heaters and natural gas engine driven compressors and pumps. The backbone is 100 Mb/s ethernet. I've got the system linked to our WAN and can make programming changes and troubleshoot the system from anywhere that I can connect to the WAN with my laptop. We use Wonderware for our HMI. Actually there are 5 separate WW nodes (no single point of failure). We set them up on NAD and use one machine (separate from the control room) for a historian / configuration machine. The system has been very reliable and very easy to maintain and expand.

Most people that see a block diagram of our system or even the graphics assume that the system is a DCS. We have implemented numerous advanced control strategies in the system. While it would admittedly have been easier to do in a DCS because of the blocks that come prepackaged, not having the overhead and cost associated with a DCS installation and maintenance out weighed the slight benefit that a DCS may have given us. After all, we implemented the strategies and they work flawlessly and that is the desired result.

 

[Instruite]

Nowadays The difference between the PLC and DCS doesnot lie at Hardware level. You can have a system consisting of PLC/SCADA looking similar in hardware configuration to DCS system. The main difference in PLC and DCS is at the functional level. DCS is much more flexible then any PLC/SCADA system. For an example, in case of an icecream factory, where it is required to change the configurations of the controllers for different flavours of the icecream. Normally all DCS provides Receipe management systems for batch controls which will automatically configure the controllers for the specific receipe and also maintain the history of all the events. I have not found this kind of feature in any of the PLC/SCADA systems.