PLC and DCS Technology: How to choose the one you need

For many people, the difference between a PLC and DCS is not clear cut.  Many businesses therefore don’t know which one to use for their day to day operations.  Here is a look at how you can know which process suits your business most.

“A PLC means a Programmable Logic Controller.  This system controls your process or machine with the aid of a custom program that is written most of the time in ladder logic. It may also be written in structured text as well as function blocks” says Dave, expert at All Drives and Controls, one of the biggest names in Unitronics implementation. “DCS on the other hand stands for Distributed Control Systems. This is a system that covers an entire process and can also cover an entire plant with ease”.

A DCS generally brings together one or more PLCs with a Human Machine Interface (HMI). This means the integrator can build a DCS and PLC together. In building such a project, focus is on the entire DCS and this means all constituents of the system are developed together as against, developing the PLC first, the HMI, historian and so on.  PLC solutions are often combined with HMIs. Read more on the PLC and HMI combination.

To determine if you need a PLC or DCS, there are vital questions you need to answer. They are covered below.

What plans do you have in the long term for the facility?

If you have any plans of someday bringing the control processes together into one DCS and the processes currently taking place in your facility will not be affected negatively, you are better off going the DCS route at an early stage.

Are you running a standalone, skid system or supporting process?

If you are running a standalone process then you don’t need anything more than a PLC and HMI combination.  In future when you decide to migrate fully to a DCS, there won’t be any need to dismantle the PLC and HMI combination.

Is the configuration, building and commissioning of the system to be done entirely in-house?

If the system is to be entirely designed in house and you don’t have DCS training a PLC and HMI combo is better for you. This is because majority of DCS packages are extremely complex thus it is almost impossible to avoid getting into trouble without formal training.

Will your processes be affected by occasional downtime?

If so, you should consider a DCS. A DCS makes it possible for you to have redundancy for your controllers as well as operator system servers.

Do you need trend package, batch system, historian, etc.?

A DCS will serve you best.  There are PLC and HMI combinations that provide all of these features but a DCS can offer them right out of the box.

So in summary, PLC will serve you if you are running a medium sized process, while DCS will serve you best, and save you more money, if you are running a large process that needs to be running continually.

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Osho Garg

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Osho is Tech blogger. He contributes to the Blogging, Gadgets, Social Media and Tech News section on TecheHow.