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This might be one of the most common questions between automation engineers and technicians that are starting their way into the industry: “What is the best option for my project? A PLC or a DCS?” Well, as with everything else in the field of automation, it depends. But in the case of PLCs and DCSs, the line might be blurrier than in any other comparison of automation equipment.
And this is a common fact around all the comparatives where PLCs are involved. They have improved so much in the last couple of decade that they have become more alike to their previously more advanced brothers, like PACs and DCSs. But that doesn’t mean that PACs or DCSs are useless now. In fact, they might be more useful than ever, since that progression has come without losing the advantages that they have had since the beginning.
To be able to better understand the features of each system, and how they differentiate from each other, it is necessary to take a closer look at the beginning of their history in the industry, when their differences were more obvious than they are right now.
PLCs were born in the 1960s, when there was a great necessity for a more efficient way to design discrete control system. At the time, they were designed with electromechanical relay systems. These relay systems were such a big problem, that General Electric started to pull strings to stablish a new and better standard in the industry, the answer to this were the PLCs.
At that time PLCs were not that complicated, they were a simple device made to replace a functionally simple system. This, and the increasing availability of microprocessors, is why the industry was eager to get a more complex and integrated system to fix some problems, and fill some gaps in the continuous evolution of automation. That is when DCSs came to be, as a much larger and more scalable solution to what PLCs of the time had on offer.
Industrial devices based in microprocessors started to appear in the early 1960s, but it was in 1975 when the first device known as a DCS was introduced to the industry. These devices had integrated many functions that contemporary PLCs were only capable of with external addition modules. Not only that, DCSs were capable of handling more I/O points, and of carrying much more complex tasks, like PID, thanks to the inclusion of function blocks, which was primarily used to emulate analog hardware.
As was mentioned before, PLCs have greatly improved over the years, to the point that the difference between a PLC and a DCS is now often considered largely academic. Of course, the DCSs have improved as well, just not in the same way the PLCs have. Specifically, DCSs have improved mostly in added functions and software development, rather than in processing power.
PLC systems are the most common in the industry, this is not a surprise since it is not really difficult to enter and stay in the world of PLC development. They are easy to use, modular, and have been improving greatly while also having much better prices than other systems. At the same time, PLC development has been confusing, some might actually say that PLC terminology is going to be taken by PACs, and vice versa, because of how similar these are.
Because of this, PLC manufacturers have started to focus their efforts to develop new devices with another approach, mostly concentrating on integrating many functions into just one device, while maintaining a good level of processing power and other common advantages of PLCs like compact form factors.
Maybe, the biggest advantage of a PLC system is the fact that it is much faster than a DCS in terms of scan time, which is almost instant inside a PLC. This means that they are much better in situations when timing is critical, whether for safety reasons or just a process that needs some real-time action. PLCs usually enjoy of a scan time of less than one-tenth of a second.
Though the word scalability is always on PLC brochures, this is not actually one of the strengths of theses systems nowadays, this doesn’t mean that they are not scalable, just that there are better options in term of scalability, especially in the face of a DCS. In this case, PLCs shine in the opposite direction by being capable of functioning without the necessity of buying a complete system, for instance, PLCs are great for occasions where HMI are not required, buying a DCS in this case would be an expensive bit of overkill.
On the other hand, DCS are known to be the best option for complex operations and continuous processes, of the likes of oil and gas, which require a high number of complex functions and analog emulation to be able to automate their processes. This is possible thanks to the way they are programmed. Of course this comes with drawbacks in the many layers of software that they need to function, making for considerably longer scan times.
One of the biggest advantages of the DCS is, without a doubt, redundancy- a feature that is integrated in every instance of a DCS. As you know doubt already know, redundancy is incredibly important for system reliability, this ensures that the system will keep working even if there is a failure in the process. PLCs are certainly capable of doing this, but the increased time, complexity and cost mean that if redundancy is a high priority, a DCS is generally a better choice.
PLCs are highly scalable, they have the option to handle a few thousands of I/O, but this doesn’t come even close to the number of I/O that DCSs are capable of handling. This gives a huge advantage over PLCs, giving the ability to add new equipment and enhancements is why DCS are usually used as a center of operation to many other devices, including subsidiary systems of PLCs.
In the last few years, DCS manufacturers have been adding more advanced functions to their systems, from wireless protocols to the addition of embedded web servers, but this has been a slow adaptation since, along to the integration with mobile interfaces, these adaptations can pose a security risk if not properly integrated.
After going a bit deeper into the advantages of each device, it becomes a much easier task to choose between a DCS or a PLC system, since they clearly have their secured spots in the industry. DCSs are great for bigger and more advanced solutions where continuous processes are a thing, not only that, if the process is planned to keep growing, they give the opportunity of easy integration with new equipment, thanks to the great number of I/O points, and how the software layers ease the problem of software development.
On the other hand, PLCs bring great advantages in solutions where discrete processes are the rule. Not only they are robust and very reliable, they have a much faster reaction time to the software and hardware conditions programmed into the system. PLCs also have the advantage when considering how easy it is to find PLC developers to design the systems of a plant, which means development costs are significantly lower. An important consideration in any application.
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