DCS versus PLC

20th May 2016

Posted By Paul Boughton

Mark Proctor gives his analysis of DCSs and PLCs in the modern plant

The difference between distributed control systems (DCSs) and programmable logic controllers (PLCs) can be boiled down to a simple football metaphor. Your DCS is your captain. The first name on the team sheet, your DCS is dependable, hardworking and controls the whole outfit. Your PLC is more like a utility player - he's nippy and doesn't mind where he plays, but don't expect him to be as reliable as your captain.

Traditionally, a DCS system was expensive, large, complex to implement and only seen as a control solution for continuous or complex batch process industries.

On the other hand, PLCs evolved from solid state relays and, as opposed to DCS, were used where process restarts were not a major concern, but processing speed was important.

The best way to think about DCS and PLC is like this: in the majority of cases, a PLC controls a machine and a DCS controls a plant.

A DCS is used when the value of the products manufactured is high, the production is continuous and failures in the system result in damage to process equipment - for example, if a glass kiln dropped below a certain temperature. This is because a DCS normally has built-in redundancy, ensuring a higher level of system insurance. All upgrades are made online while the system runs continuously.

Conversely, PLCs are often used when the value of the product is relatively low and production needs to be flexible. Systems can be shut down for maintenance, troubleshooting or upgrades without damage to equipment or significant downtime costs.

Diagnostics in a PLC system will alert an engineer when something is broken. Whereas in a DCS, asset management software provides alerts of what might break before it does, so a fitting substitution can be made. This kind of predictive analysis is particularly important for obsolete part replacement and avoiding costly downtime.

As a utility player, PLCs are highly customisable. They have standard libraries and routines built in, but also have the capability to be specially programmed using custom code from scratch. On the other hand, engineers expect a DCS to offer an out-of-the-box control system with features such as - historian, sophisticated alarms and logic from pre-existing function blocks. Like a good captain, the highest priority of a DCS is to deliver reliability and availability. DCS designs often trade high levels of functionality for repeatability and dependability.

The speed of logic execution is another key differentiator between DCS and PLC. PLCs are designed to meet the needs of applications that require scan rates of ten milliseconds or less. This allows them to accurately control motors and drives running at high speeds.

However, DCSs do not need to be this quick because they control systems rather than individual devices. A DCS's regulatory control loops generally scan in the 100 to 500 millisecond range.

For all their differences, PLC and DCS are becoming more alike. DCS was originally developed for analogue control. However, the latest generations of PLCs are increasingly capable of delivering simple to complex proportional integral derivative (PID) control. In addition, today's DCS hardware is not as expensive as it was a couple of decades ago and is less difficult to implement. It is also no longer as cumbersome either - modern DCS hardware resembles a PLC in size. This is why DCS is now used in smaller applications, whereby it is not spread across the whole plant, but rather a complex subsection that needs reliable control. For example, take the server halls of data centres: these types of control systems are often seen as hybrids of both DCS and PLC.

Modern industrial applications are demanding the reliability of a DCS system but the flexibility of a PLC. This change has brought about a certain level of technological convergence between PLCs and DCSs that defies traditional ideas of the two. In essence, some industrial plants are now employing more than one captain to ensure critical systems are well managed. Game on.

Mark Proctor is managing director of industrial spares supplier EU Automation.

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