In a recent blog post we paid tribute to Dick Morley and his pivotal contribution to the process automation industry: the Programmable Logic Controller (PLC). Since the PLC and Distributed Control System (DCS) are both instrumental in controlling complex production processes, people occasionally use the two terms interchangeably. While the two are related, their applications are notably different.
As shared in our Dick Morley tribute, the PLC is essentially an industrial computer used to control from one or a few production processes. It can be applied to common tasks such as controlling the rate of airflow in an HVAC system, maintaining liquid levels across a series of tanks, or any number of similar responsibilities associated with regulating industrial production processes. Historically, the one job that the PLC couldn’t fulfill was coordinate the control of an entire plant. That’s the task for which the DCS was first developed. Indeed the DCS doesn’t regulate one single process. Rather, it supervises and coordinates each of the many controllers that are deployed across a plant. The core capability of a DCS to orchestrate plant-wide operations has become increasingly important to manufactures as they pursue greater efficiency from their existing production assets.
It’s worth considering what a DCS is, how it’s used, and why it has become so important:
What is a DCS?
© Daniele Pugliesi
As the name implies, the DCS is a system of sensors, controllers, and associated computers that are distributed throughout a plant. Each of these elements serves a unique purpose such as data acquisition, process control, as well as data storage and graphical display. These individual elements communicate with a centralized computer through the plant’s local area network – often referred to as a control network. As the ‘central brain’ of the plant the DCS makes automated decisions based on production trends it sees them in real-time throughout a plant. As an example the DCS at a power plant might automatically increase steam generation capacity of multiple turbines in order to keep up with changing demand for electricity during hot Summer days and then decrease it as outdoor temperatures cool overnight and demand subsides. Whereas a PLC could adjust a single unit operation, the DCS can make adjustments to each of a plant’s many interacting unit operations.
How is a DCS Used?
While DCSs are used across the process control industries to supervise complex production processes, they are more widely deployed at large, continuous manufacturing plants such as those in the petrochemical industry. With the help of a DCS these and other manufactures can efficiently coordinate adjustments in a top-down fashion using a centralized network of computers. Instructions from the DCS are deployed throughout a plant and fed to individual controllers. When configured appropriately the DCS can improve safety while also enhancing production efficiency.
What Makes the DCS so Important?
A comparison of two plants may help to illustrate the importance of the DCS. First, imagine a small municipal waste water treatment facility that employs a dozen control loops. The plant’s engineering staff can easily keep a mindful eye on the performance of such a limited number of controllers. Next, imagine a large refinery that operates 10,000 highly dynamic and interacting control loops. While coordinating control at the treatment facility is relatively easy, the task of orchestrating control at the refinery can be overwhelming without the use of a DCS.
Over the years, the line between DCS and PLC has blurred. While the DCS has traditionally been the only solution for achieving safe and efficient plant-wide control, advancement in processing capabilities has allowed the PLC to rapidly take on greater responsibility. In the years to come it can be expected that the PLC and the DCS will become more and more interchangeable.