DCS vs. PLC: Choosing a Control System

Relay walls and complex switchboards were once an essential part of a sophisticated manufacturing operation. Technicians and electrical engineers adjusted systems by hand, switching touchy connections and crafting elaborate circuits.

Today’s digital control systems look and feel very different: compact, rugged computer modules now manage vast arrays of inputs and outputs in real time, and can be easily managed with simple coding and adept supervisory control. Reliable and high value, these computerized solutions have truly changed the way we work. But which style is best for your operation?

Digital Control Systems, Defined

As you evaluate computerized controls to determine the ideal fit for a particular application, bear in mind that there are three major types:

Programmable Controllers (PLCs) offer digital computer performance for the most rugged conditions. Explicitly designed as an industrial computer, PLC modules can be readily adapted to suit both simple and complex operations, including assembly lines, machinery, automation, and robotics.
Distributed Control Systems (DCS) offer plant or process control with a large number of control loops. This set-up typically maintains a central operating control with autonomous controllers distributed throughout a system, and is more than capable of acting as the center of operations for an entire plant.
Supervisory Control and Data Acquisition (SCADA) is another style of control system architecture for high-level process management. These systems tend to utilized peripheral devices—in addition to computers, data communications, network technology and user interfaces, a SCADA often integrates programmable logic controllers, PID controllers, field sensors, and more.

Key Differences in Industrial Control System Computing

Control systems are designed to be modular. Any industrial setup can be altered and built up with additional components to suit a given application. The technology is inherently variable, and the uses of PLCs and DCS often overlap.

However, a few key differences set these systems apart. Asking the following questions can help you to inform your choices:

Question #1: Are you controlling a plant or a single machine?

PLC technology evolved from solid state relays to create a fast, efficient control system for isolated machines or individual processes. These tools offer unquestioned speed with the convenience of modular versatility. Custom code can make a PLC perform to exact specifications, and a PLC-based system can be built out with HMIs and additional network capabilities as needed. They’re easy to manipulate and troubleshoot in-house, on-demand.

A DCS, on the other hand, comes with highly sophisticated software right out of the box. It’s a more significant investment with more performance capability. While a DCS may not have the rapid processing of a PLC, it is infinitely more scalable, built to accommodate many thousands of I/O points and new equipment, data integration, and process enhancements as needed. This technology is designed to be the brains of a complete operation.

Question #2: What are your operational priorities?

For continuous production, a DCS offers dependability and up-to-date operation without interruption—it is designed with built-in redundancy and the associated production insurance. A PLC, on the other hand, offers flexibility. It can be readily shut down for maintenance, improvement, or reworking without significant difficulty or downtime impact.

The most important offering of a DCS system is reliability, even at the cost of high-speed processing or enhanced function. DCS systems are built specifically to accommodate highly complex operations, high-value products, and zero-error demands.

Question #3: What will it cost?

Certain operations demand the power and reliability of a full DCS—water treatment, chemical processing, and oil and gas all depend upon tremendous data handling and continuous production that a PLC-based system can’t provide. However, a DCS is a more significant up-front investment and may come with features that are unnecessary for a facility’s work.

A rugged, easily programmed PLC is less expensive, simpler to install, and straightforward to manage without vendor-reliant integration and process change implementation. Most applications require the addition of an HMI for graphics display, but that may be all your system needs — you’ll pay for exactly what you need, and nothing you don’t.

Controller Technology in Modern Manufacturing

While PLCs emerged as a standard relay replacement and the DCS was a direct answer to PID controllers for batch processing, time has seen both become vastly more complicated and equally integral to manufacturing. Some PLC-based control systems work like a small-scale DCS; some DCS systems integrate PLCs directly into their network.

Despite the differences and subtleties of modern digital controls, one thing is certain: Manufacturers would be lost without them.

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