What is the Difference Between PLC and DCS?

==========================

In a nutshell, a PLC or programmable logic controller is a ruggedized computer used for automating processes.

A DCS or distributed control system is similar to a PLC in that is has rugged computer controllers however the DCS contains multiple autonomous controllers that are distributed throughout a system, also used for automating processes.

As you can tell, there are likely advantages and disadvantages in both systems. The take away is that with today’s technologies, either system can control an entire plant. Which system is chosen will likely take the advantages and disadvantages into account as well as system costs.

In summation, the DCS has autonomous controllers dispersed throughout the entire plant. If a controller fails, the entire plant doesn’t necessarily get impacted. It also has the onboard monitoring and control that saves development time. A single PLC is a single point of failure. You surely wouldn’t want to control an entire plant with a single PLC, however; a connected PLC system can have nearly the same security and robustness as a DCS.

==========================

Missed our most recent videos? Watch them here:

=============================

To stay up to date with our last videos and more lessons, make sure to subscribe to this YouTube channel:

=============================

=============================

#RealPars #PLC #DCS

[isaacpardue]

I am taking a mechatronics class and the teacher uses a lot of your videos. I always love them because it’s a break from the lectures.

[bubbamc]

Nice video. One thing I might add is the DCS is usually designed with a narrower focus on process control. For example, implementing a complex cascaded multiple PID control scheme with constraints on a DCS can be done much more quickly and easily than a PLC. PLCs are more general purpose - you can implement the same control scheme but it will take longer, functions like tracking, reset limiting, signal selection, filtering and clamping sometimes need to be explicitly programmed so you really need to know the low level details (it could be argued PLCs are more powerful since you can make them do exactly what you want).

DCS usually has a standard library of function blocks which cover pretty much everything you would ever need in process control. The PLC library is often more limited, extra functionality may be available at a cost however. Btw the function block diagram you've shown looks like modern ladder logic, quite a bit different to the typical DCS function block control drawing layout.

The tight integration between the controllers, HMI (which you mentioned, but worth repeating) is a big deal. This can save many thousands of man hours on a large project if a DCS is chosen.

Other notable differences:
-Alarm configuration is typically more powerful in a DCS than a PLC/SCADA combination.
-Fieldbus configuration is often tightly integrated with a DCS development environment, in fact some Fieldbus protocols common in continuous processing applications are not available or have only recently become available on PLCs.
-System health assessment (via system alarms) in particular is tightly integrated within a DCS. You can see at a glance how the system is performing.
-Operator keyboards providing key process control interaction funcitionality are standard with a DCS installation.

Not to mention the seamless integration between DCS and SIS from the same vendor, but that's another story :)

[stevelockett]

Great series from what I've seen so far and I'm thinking of sharing this with my non-I&C clients so that they have a better appreciation for what it is they're paying for and why we seem to ask a million questions when they ask for quotes :)

One thing about the DCS is that historically, it was an evolution of traditional process automation, that is, everything that wasn't covered by relay logic, and it didn't just "invent itself" from trying to find something that's traditionally recognised a computer (i.e. a beige box or mainframe) would do.

I realise that following that the following history would have taken the video off on a tangent and possibly lost some people but for the benefit of those who may be wondering "so what was before DCSs", here's some context for you ;)

Whereas relay logic was predominantly the domain of sequential switching control, i.e. motor and solenoid on-off control, traditional process control - variable control - started back at the turn of the 20th century with mechanical controls. This evolved through the '40s and '50s into pneumatic controls with more complex mathematical functions, transitioning into analog electronic equivalents. All of these required external analogue computers to perform additional simple mathematic functions such as square root, addition, subtraction, multiplication, division, high-low select, and so on.

When microprocessors hit the scene, initially these were employed predominantly in early DCSs due to cost. Whilst Distributed Control System is a clever marketing term, in brown-fields plants, these microprocessor-based controllers effectively replaced the area instrument panels ( a collection of single loop controllers and indicators), so in a sense, it could be countered that they actually integrated more than distributed process control. Controllers still existed in the same physical plant locations in most cases, but the control room could now be centralised - that was the kicker.
Meanwhile, for the end-users who could not justify the expense of a DCS, microprocessors made their way into single-loop panel mount controllers, which initially didn't allow much in the way of user functionality over the analogue and discrete digital electronic version which they replaced.

But when users had access to programmable microprocessor-based controllers, that's when configuration/programming of microprocessor-based single loop controllers really shone. As with the DCS, it was now possible to bring in up to four analog inputs and perform the mathematics which once required additional external devices. Complex control was becoming cheaper but you still needed panel space.
Eventually, communications pathways were developed so that such single-loop controllers could be integrated with PLCs or DCSs and presented to the operator as a homogenous control system which was more DCS like than PLC. The falling cost of DCSs and PLCs - the latter in particular - has pretty much seen the single-loop controller relegated to simple local control panel duties in vendor equipment or commercial applications.

Generally speaking, DCSs and PLCs (SCADA is effectively a PLC with telemetry tacked onto it) can do the job of each other in general applications but you'll see more PLCs in operation than DCSs for a reason and it's not because they're necessarily better, it's more about the application.

For heavily sequential operations such as an assembly line or a mine-site, a PLC is cost-effective as it's fundamental design focus is high-speed switching with minimal complex computational functionality.
For critical complex process automation (chemical, petrochem, etc.) the DCS remains the BPCS (Basic Programmable Control System) of choice since the design focus is on performing complex mathematical computations for a multitude of interactive complex control functions at high-speed.
I wouldn't choose a PLC to control a mineral or petrochem facility, but likewise, I wouldn't select a DCS (purely on cost and wasted functionality) for a mine-to-shipping operation.

[vanguard]

Great video. In addition to the architecture explained in the video there is a bit more that shows each's benefit.
In a DCS, it is extremely tough to program/maintain/trouble a sequential process (especially the ice cream example used). A DCS is better in a continuous process such a refinery where, for example, oil/gas is always flowing in out of the process. Pumps/valves rarely stop/close except during outages. This is because of the programming of function block diagrams. There is little to no need to dig into a function block diagram to see the code in a continuous process.
A PLC is handled better for sequential processes. Being able to see all ladder logic helps to program/maintain/troubleshoot this process. If you used nothing but function blocks and AOIs, and a line went down, it would be tough to troubleshoot. You could have a hard time finding out what is causing an output from being energized. It could be power, a valve stuck in manual, or bit that got tripped up.

[albakhati]

Thanks for informative videos and tutorials

[muhammadkamran]

Every video adds up my knowledge. Good work. Keep it up...

[akmalzia]

Very simple and excellent way of explanation. Thank you

[mattcassle]

@7:59 so true - used to work at a plant that had an old DCS that was slowly being made obsolete via ABB acquisition (to push there DCS platforms). One contractor with specific programming/hardware experience had 3rd party near monopoly for a good chunk of West coast US, made a career out of it.

[paulwhite]

Excellent video will definitely recommend this channel to others!!

[alexward]

I am working as a Controls & Automation recruiter and trying to learn my market. Please keep up this amazing content as you're helping so much to learn about the things that Automation and Control engineers do everyday.

If anyone can suggest anything else I should be trying to understand, i'd be most grateful :)

[pragyeshagarwal]

i just suscribed your youtube channel, followed on instagram facebook and downloaded your app. cant get enough of you

[jsrrrmg]

I like that you touched on how using function blocks gives more capabilities and flexibility for uses of a PLC. However, I was a slight bit disappointed that you didn't touch on Script at all. This gives even greater flexibility to the programmer, and in some aspects even easier to program.

[ElvisCoverca]

You could also add that a PLC system can execute the logic at a much faster rate than the DCS does. Great video!

[rayfollis]

Very clear and concise explanation - well done.

[curiouscavin]

Thank you for creating such a wonderful video

[ardentdfender]

A PLC could have a Primary Processor fail and take out a whole system as compared to one system in a DCS that goes down. But I guess if you maybe build the PLC system right and add redundancy with a Primary & Secondary Processor network, if that system ever going down it should be far less an issue. It should switch and barely skip a beat.

[mnazar]

Simple explaination and comparison. Why dont you start showing the process of programming it from the beginning until the end on a certain equipment . Hope those effort will help thousand of people out there who had lost or currently thinking to create a new project or remodify something for a better purpose.

[alryan]

Great video. That fact that you missed was the DCS was used more in the past in large control systems because early PLC's were not capable of handling analog values. I saw a Honeywell DCS system completely replaced with Rockwell PLC's and a PC based HMI system. The 2 systems were almost identical to the process operator.

[renegadeflower]

Appreciate the free lessons. Ignore those giving a thumbs down. They're drunk.

[gotribe]

Working in controls this is what we consider a DCS system.


Data collection system (DCS) is a computer application that facilitates the process of data collection, allowing specific, structured information to be gathered in a systematic fashion, subsequently enabling data analysis to be performed on the information.