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rizwan
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Need to Know 'Basic' Difference Between a PLC and a Normal Controller

10/11/2010 10:27 AM

I want to know the basic difference between a PLC and a normal controller in context to the scenario written below.

I work in a gas processing plant where we have two Caterpillar's G3412 genset, each equipped with CAT's own EMCP II panel (containing both an LCD screen and pushbuttons). The technicians call it a PLC panel. Now I know it's not a PLC panel but a simple controller. The difficulty I face is in proving to them 'Why it's a normal controller panel' and 'Not a PLC panel.'

Similarly, the same genset are started and put on load with a DeepSea Plc panel (Model: DSE 550 http://www.deepseaplc.com/product/view/dse550) in the control room. Technicians call it a PLC panel (only because they see 'plc' written/ printed on the panel) which it is not as nowhere in its manual it is written that this is a PLC panel.

I try to defend myself by saying that PLC is logic controller with a microprocessor, runs on ladder logic etc, use for automation of different processes, and is configurable through PC blah blah. But then they ask 'then what is a normal controller', that contains a logic too as well as set-points, what logic is behind that, how does it process etc. that is where I am found wanting – both controller seem similar in purpose.

With this background I would like to know the basic differences between PLC controllers and normal control panels. I hope you'll help me in this.

ThanX
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RDGRNR
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#1

Re: Need to Know 'Basic' Difference Between a PLC and a Normal Controller

10/11/2010 11:17 AM

Technically, the controller is a PLC, albeit a small one, with it's own proprietary software and limited accessible parameters. It makes decisions based on inputs and outputs data to it's HMI. All of this conveniently programmed for you by the manufacturer. So, your buddies are right to a degree, that degree being the level of functionality you consider "programmable".

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JRaef
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#2

Re: Need to Know 'Basic' Difference Between a PLC and a Normal Controller

10/11/2010 11:38 AM

Here's the acid test for whether it is a "PLC"

  1. Is it Programmable?
  2. Does it solve Logic?
  3. Does it preform the functions of a Controller?

In other words, PLC is a descriptive term so a PLC is hat you make of it, it is not a specific list of features and functions. Given that most microprocesser based devices like those are using logic to preform control, 2 of the 3 criteria are already met.

If the "normal" controllers (an even more ambiguous term) are not programmable in the field, then they are usually referred to as a "proprietary controller" because only the OEM can look at and alter the programming. But if they allow you to do so, it's a PLC. It may not be a COTS (Commercial Off The Shelf) PLC, but it is a PLC.

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rizwan
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#3

Re: Need to Know 'Basic' Difference Between a PLC and a Normal Controller

10/11/2010 2:00 PM

My 'buddies' may be right to a certain degree but still the're incorrect because they dont use the term PLC in the BROADER sense, but believe that these controllers are the same PLCs that are used in automation process. To them every control panel, even TERBERG panel (www.terberg-besturing.nl) is a PLC.

Thanks for guiding me. More help welcome!
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NotUrOrdinaryJoe
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#4
In reply to #3

Re: Need to Know 'Basic' Difference Between a PLC and a Normal Controller

10/12/2010 8:23 AM

The panels your guys are working with are likely just dumb terminals also known as an HMI (Human Interface) terminal.

All computers have a processor and an operating system of some kind. Even dumb terminals have processors and boot up separately from the controller.

Just to make it confusing, there are devices which have embedded computers in them so that all you see is screen. Apple computers are a good example of how compact your computer can be.

So, the HMI is nothing more than an extension of the I/O of some controller, dedicated or programmable device.

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PWSlack
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#5
In reply to #3

Re: Need to Know 'Basic' Difference Between a PLC and a Normal Controller

10/13/2010 3:40 AM

So is this thread really about semantics?

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