Generally, Distributed Control Systems are far more feature laden than Programmable Logic Controllers (PLC). The definition of a PLC
is that it controls your process or machine via a custom program written most often in ladder logic, but can also be written in structured text or function blocks.
It’s very hard to talk about PLCs without also talking about HMIs, or Human Machine Interfaces, which displays a graphical representation of your process or machine for operators to monitor and control. Whilst an electrical engineer or programmer might be able to easily use a PLC without an HMI, your average operator wouldn’t, so if the process or machine is going to be regularly monitored or controlled by operators, then a Human Machine Interface is essential.
A Distributed Control System (DCS) typically covers an entire process, and is capable of covering an entire manufacturing plant. A DCS might have several PLCs together controlled by an HMI, and all are integrated. These are usually made with an overall outcome in mind and designed from the ground up so that a very bespoke end product can be successfully created which encompasses all of a given plant’s automation needs.
As well as having several PLCs being fed to an HMI, there are several other features that are often integrated, such as batch management, historical data logging, synchronised alarms and security, and servers and clients. The servers are there to gather tag data from the PLCs and serve graphics and tag data to clients for operators to use.
Why Go with a DCS When I Can Add All Those Features to a PLC/HMI Combo?
It’s easy to just look at the costs and decide against a Distributed Control System. After all, with some customisation, you can do a lot of the same things a DCS does in a PLC/HMI combo.
You might find though that whilst you’re making savings upfront, you’ll be paying for it in the future. If you’re planning to expand your process, it would be best to look at having a higher cost upfront for a DCS which will save you money down the line. There’s going to be less engineering time because of the alarming, security, tag logging and trending, face plates and in some cases the logic blocks have already been developed and tested and used in plants around the world.
Where would you use a Programmable Logic Controller
PLCs are often used in:
- Machine automation (quicker processing time).
- Skids, stand-alone systems (doesn’t need to be part of plant-wide system, or is developed by an OEM).
- Utilities (lower in cost).
Where would you use a Distributed Control System?
DCSs are often used in:
- Large, plant-wide processes.
- Batch processes
- Chemical plants