DCS..HMI..SCADA whats the difference??

Preeya

Member
Join Date
Nov 2003
Posts
112
Hi everyone,

Being in the process automation for some years now.. I have noticed different people (and of course me!) interchage the terms DCS, SCADA and HMI.

Correct me if I am wrong but this is what I think is the difference

HMI (human machine interface) - Any type of interface between a "blind" controller and operator.

DCS (distributed control system) - A centralised control for inputs distributed geographically. (is it necessary it has to be a computer controlled?!)

SCADA (Supervisory control & Data Acquisition) - More similar to DCS (distributed geographically) but having some sort of operator interface to bring the data from various sections into a more sensible report?! This has to have a centralized monitoring system like a computer interface?!

I am sure I may be missing something.. Please correct me!
Thanks!
 
That's pretty much how I see it, too.

HMI (human machine interface) - Any type of interface between a "blind" controller and operator.
That's a good technical description. But, I find that the term is typically used for PC-based applications. Anything else is usually identified as a touchscreen or operator interface.

DCS (distributed control system) - A centralised control for inputs distributed geographically. (is it necessary it has to be a computer controlled?!)
Interesting isn't it? Your control is actually processed at a central location(s), but it's called "distributed" because the IO points are all over the map. And, no I don't think it has to be computer controlled. Technically, you could throw everything into automatic and unplug all the operator stations. But, how would you monitor or make program changes? Because so much of a DCS system is proprietary, it doesn't make much sense to plug your laptop in for programming. So, the DCS catalog will have PCs, UPSs, software, printers, desks, and even CHAIRS (I found the part number for a Bailey workstation chair).

SCADA (Supervisory control & Data Acquisition) - More similar to DCS (distributed geographically) but having some sort of operator interface to bring the data from various sections into a more sensible report?! This has to have a centralized monitoring system like a computer interface?!
Right on. The SCADA systems I've seen are like a patchwork DCS. They span a larger geographical area (city-wide vs. plant-wide), and are less strict on architecture (you'll see radio, fiber, copper, AB, Koyo, & Wonderware all playing happily together). The idea is to get data. How it gets there, how fast, and how reliably tend to vary by project and even by IO point within the same project.
If something is controlled by a SCADA system, I bet $10 that there's an operator sitting there who'll tell you "Yeah, that alarm light's been on for 10 years now." Aside from the 3 signals that get reported to the EPA on five minute intervals, most of the data will get ingored.

AK
 
At the facility I work at we have both DCS(Foxboro) and Plc systems with SCADA software (TI-565, AB PLC-5 and SLC, Intellution Scada).

I work on both types of systems, and here are the differences I'm aware of:
1. DCS and PLC hardware cost are about the same. However, you have to purchase a costly license to use the DCS hardware.

2. DCS system is much more robust and fault tolerance is built in.
Processors can run in redundant pairs. Processors communicate over a redundant network, and I/O communicates over a different redundant network. Discrete I/O card run very basic ladder logic within the I/O card itself. You can program for loss communication, to ensure the process is safed.

3. DCS system has a much steeper learning curve for programming, but once learned, it's easier to program difficult/complex logic. Has had , structured text, function blocks, and basic ladder since the 1980's. Majority of programming done with function blocks, and there is a vast library to choose from.


4. DCS systems speed compared to PLC's is very slow. Fastest possible
I/O update is 1/2 a second.

5. Plc system is usually much easier to program and trouble shoot. Don't have to purchase a license for the hardware. Lots of support available. Much faster I/O updates.

Typically DCS systems are used when down time cannot be tolerated, such as power plants and chemical industry.

I believe there is a place for both, but the PLC can be used for about 90% of all applications.


With the new line of PLC's on the market such as S7 and Control Logix
the line is becoming somewhat blurred. They now too support all the different types of programming. However, I don't think the PLC's are quite there yet.


Just my two cents worth.


Ken
 
I guess it depends where your roots lay. I do not like DCS systems as I find they are very restrictive in many ways. Also far too slow for most of my applications. They are fine where scan times and fast collection of data are not important. One of the leading DCS companies, for example, promote their DCS system as something special. At the end of the day it is only a PLC with cheap analogues that probably would not stand up in most noisy indutrial environment. I have seen one used on machinery and it was pulled out and replaced with a Schneider Premium PLC within 6 months. The analogue cards did not like the noise generated around the machine and false readings were a common occurrence. They are fine for building air conditioning control.

For example, an air conditioning system and motorised valves would not care if the response time was 10 seconds as they move very slowly.

Fast positioning of packaging machinery, packaging lines, shutting down a generator that is going into overspeed are far more critical and require fast response times. A DCS would be absolutely useless in these situations. So would Modbus RTU, ASI, Device Net, Profibus etc. You need a fast processor, in many cases, high speed counter cards, motion control cards etc etc.

Some of the networks that are used by DCS systems are certainly not amongst my favourites. BACnet, for example, I find a pain in the tail. It is so slow compared to what I normally use it is not funny. Sit there and wait for 5 minutes to see things happen. But, once again, it is fine for air conditioning.

I do not necessaily agree that DCS systems are more robust and fault tolerant. Dual processor and communications systems are quite common in PLCs, and have been for many years. I quoted, and then withdrew because the consultant was an absolute pain, a hot standby system recently. An option in the quotation was redundant communications between PLCs. I quoted Omron CS1 hot standby PLCs with redundant fibre optic Controller Link cards. You will not get anything more robust or fault tolerant than that.

SCADA systems are very versatile and most can communicate with almost anything, in one way or another. I have installed Citect SCADA communicating with several brands of PLC, fire panels on Modbus RTU, BACnet (that is why I do not like it, have had a bad experience), a modem etc. Most DCS systems have the ability to use BACnet or Lon, Modbus RTU and or ASCII and not much else, unless you roll your own drivers. Citect, for example, has about 150 different drivers available. They are virtually all included with the package at no extra charge. There are a few exceptions, such as RSLinx where AB require a licence fee for each purchase of Citect. Most SCADA manufacturers want to charge you for each driver. OUCH!!!

I might add that PLCs, generally, are so reliable these days that replacement due to failure is a rarity. Most failures are caused by incorrect wiring, a 240VAC voltage being applied to a 24VDC input etc. These situations occur just the same with a DCS system. I hardly ever replace any PLC components.

In most cases, ladder logic is fine and is fairly easily understood by plant electricians. They do have difficulties with function block, structured text, SFC etc. However, many PLCs have some or all of the IEC languages available these days. Probably all manufacturers have all IEC languages under development, if not already implemented. Once gain, depends what you are doing. Function block is great for building control where you write a block once and copy it from zone to zone and job to job. I do not have many uses for it as even my alarm routines vary from job to job and even rung to rung. Sometimes wish I could use it more as it would save me a lot of time.

There are things that can be accomplished better in ladder, structured text, FB, SFC etc. Unforunately there are also a lot of things that cannot be transferred/converted from one programming language to another. Horses for courses.

I think some PLCs are there right now if you require the type of "advantages" that are available in DCS systems. Just about all others will have all IEC languages available in the next few years.

The blurring has been happening for years. The big advantage that PLCs have is that they do not have to run on Bill Gates, they do not suffer from the failure of cheap Chinese computer power supplies, they are industrially hardened, they are designed to work in noisy industrial environments, etc. DCS system are not designed for that.
 

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