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 it has rugged computer controllers, however, the DCS contains multiple autonomous controllers that are distributed throughout a system, also used for automating processes.
A Little History
The PLC was actually born more than forty years ago when an antsy design engineer named Dick Morley quit his job over a dispute with his boss (he wanted to work on Saturdays instead of Fridays, so he could ski more…his boss said no) and formed a company with a friend called Bedford Associates.
At Bedford, Morley and his friend wrote proposal after proposal, mostly for local tool firms hoping to evolve into the new solid state manufacturing arena with the help of small computers. The proposals didn’t exactly meet Morley’s creative nature—they were so similar and repetitive that, being the antsy type, he eventually grew bored.
That’s when Morley began to think outside the box, literally. He wondered if he could create an automated programmable controller that could handle his clients’ everyday jobs.
Morley got to work drafting an idea for a programmable controller. He shared it with his team who enthusiastically embraced the design. After finding financial support, Morley’s new enterprise, Modicon, was created.
As good fortune would have it, during the design phase of Morley’s creation, an executive at GM presented a request for a solid state controller that would make plants more reliable and durable, one that could also replace the hardwired and unreliable relay systems that were pervasive in manufacturing.
Boom…GM heard about Morley’s work and eventually contracted with Modicon to purchase over $1 million worth of their innovative programmable controllers (it was several years later, when personal computers or PCs were invented and the “logic” part of the name was added to create the term, PLC).
Modicon was now in business, big time. The company name stuck through a couple of acquisitions, then morphed into what is now Schneider Electric.
In the beginning, the PLC was used primarily for discrete controls. The programming of the PLCs was primarily in ladder logic, a format very similar to a schematic.
The PLC received device information from the field, solved the logic, and then energized the outputs to produce the desired effect, which generally meant performing repetitive tasks in a reliable and durable manner.
DCS Enters the Picture
The DCS appeared around 1975, out of necessity. Basically, the creation of a DCS system came about because of the increasing use of microcomputers. There had been other computer-based systems in the industry since the late 1950s, but these had limited scopes for scalability, robustness, and security.
There were many benefits to the DCS, but one of the primary advantages was that an entire plant could be connected via proprietary communications and controlled by a distributed system.
For instance, say you had a plant that made an ice cream-filled cookie sandwich. The plant would have a production line for the ice cream, and one of the autonomous controllers would process the batch of ice cream.
After the ice cream batch was complete, another autonomous controller would process the freezing of that ice cream. Yet another controller might process the cookie batch, while another might supervise the baking process. With several autonomous controllers, if a controller failed, it would impact only that process and not all of the others, which led to a robust system that virtually eliminated entire plant failure.
Bottom line? The DCS was really good at autonomously controlling single or multiple processes.
Another major benefit of the DCS was the integrated monitoring and control system similar to today’s SCADA systems. The entire tag base is right there, already created for the process control, and available to use on the monitoring and control screens.
Also beneficial was that the DCS had function block programming. Function block programming, if you aren’t familiar with it, is a section or several lines of code behind a single interface. That interface may do something like handling the manual and automatic operation of a valve. Function block programming saved a lot of time and redundant programming.
So, What the Heck Makes Them Different?
Essentially, the difference between PLC and DCS forty years ago was considerable, and if you owned a large plant with continuous processes, you likely would have chosen a DCS.
In today’s industries, however, the DCS and PLC are quite similar, save for the integrated monitoring and control.
With open source communications, fiber optics, Ethernet and the like, many PLCs can now communicate with each other and behave as autonomous PLCs that communicate over the network to other autonomous controllers.
That wide communication allows for single or multiple processes being controlled by one PLC to communicate with another PLC. A connected PLC system can have nearly the same security and robustness as a DCS, while a single PLC is a single point of failure, so you wouldn’t want to control an entire plant with a single PLC.
Take the ice cream sandwich example. PLC-A could process the ice cream batch. When the batch is complete, PLC-A communicates with PLC-B that the process was complete, and PLC-B then launches the freezing process.
You can see that with today’s technologies, a wide and robust PLC system could do virtually the same thing that the DCS can do.
Yet another advantage of the DCS is installation cost savings. This advantage occurs because of the autonomous controller can be located in close proximity to the process versus pulling long runs of I/O wire across a plant. The DCS’ onboard monitoring and control system is also a plus.
Speaking of advantages, today’s PLC systems offer nearly the same ones as the DCS, excluding the supervisory control and data acquisition (SCADA).
With a PLC system (multiple PLCs in a plant structure), you still need to create the supervisory and control system. The entire DCS database would be available for the creation of the monitoring and system, the PLC systems individual PLC databases would need to be created in the SCADA system software.
One of the drawbacks to the DCS has been the scarcity of programmers that have some experience with it.
Most plant floor technicians are familiar with ladder logic programming however, DCS programmers and technicians typically need more specialized experience in database functions, as well as IT-related networking knowledge.
Because of the specialized training, DCS programmers are a bit harder to come by.
In contrast, there are more programmers available for hire in the PLC arena and with the new programming languages such as function block, sequential function, etc., the advantage of function block programming is no longer exclusive to the DCS. This saves in development time when programming a PLC.
The takeaway from this comparison is that, with today’s technologies, either system can control an entire plant. One needs to weigh the system costs and the advantages/disadvantages to make the best decision for the situation at hand.