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Old 09-27-2011, 03:27 PM
vks_foe vks_foe is offline
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Default Advantages of PLC

What are the advantages of PLC that they are still preferred over microcontroller based circuits in industry??
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Old 09-27-2011, 03:42 PM
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I would think the ruggedness of PLC systems is it's main advantage. Industrial systems are subject to harsh environmental conditions and electrical disturbances.

However, there are probably some situations where PLC's are being used where a PC system or another solution would work fine. But for the majority of industrial applications, PLC's offer less down time.

Another advantage with using PLC's is that it is easier for a technician to understand, service, and troubleshoot. Ladder logic is easier for a technician to use. If they had to know how to program using a language such as C, then the vast majority would not be capable of doing so without proper training, schooling, etc. So the company would have to hire another individual to handle the programming side of things...all in all it is a money issue. Less down time and easier to troubleshoot means less money the company is losing.

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Old 09-27-2011, 03:47 PM
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This might help answer it a little bit.

I have developed with both PLCs and microcontrollers, and the biggest difference I can speak to is the ease of programming. Especially when you consider the settings that PLCs are used in (industrial controls for example), the amount of time spent programming and the ease of being able to make changes AT the installation location give PLCs a big edge in this area.

That being said, microcontrollers have their advantages too, one of which being size. You wouldn't see a PLC being used in a coffee pot, but a microcontroller could do the job easily.

In the end, it really comes down to the application.
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Old 09-27-2011, 04:52 PM
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physical interfacing and powerful processing, in an 'off the shelf' package. Micro's make a great subset for specific tasks.
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Old 09-27-2011, 08:07 PM
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A PLC already HAS a microcontroller embedded in it that does all the work, all it does is run your compiled code, so basically someone already did all the coding for the microcontroller, with a bare microcontroller, you still have to develop the application, and user interfaces....
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Old 09-27-2011, 09:06 PM
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I am an industrial controls engineer, I program PLCs on a daily basis. I also build embedded control boards based on picmicro chips, primarily using the 18F series because of the ease floating point calculations.

PLCs offer ease of installation, ease of programming and ease of communications to other systems. However, they are not very good at high speed high level calculations even using high speed counter cards. High speed counter cards generally do not have the ability to do logic level processing like simple IF/THEN statements. They work great for "count to this position and do this" type of logic. But not with, "count this until this happens and then do that" Because of these limitations, many of the controls systems that I build at work have both installed. The PLC serves as the operator interface thru a touch screen (typically) and the process recipe can be generated thru that interface. When the operator has selected all of the variables, then data is sent serially to the pic control board and the process is generally controlled that way. This gives the best of both worlds. If no high speed input based calculations are needed, then a PLC is really the only way to go. Look into Automation Directs product line. Using the Click series of controllers, one can do a variety of normal machine tasks with very little money and no cost software. More powerful PLCs are available if you need more horsepower.

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Old 09-27-2011, 10:32 PM
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I think ultimately the reason PLCs are preferred in industry, as already been touched on, is ruggedness. PLCs (from well established manufacturers, Siemens, Allen Bradley, et. al.) go through numerous rigorous tests and are RATED for the conditions they will be subjected to in industry. That's also why they're so expensive. You could order a microcontroller and a handful of relays, and make your own "PLC" for probably under 10$, but nobody would buy it because you haven't spent years and millions of dollars in R&D subjecting it to all kinds of tortures and making it prove it's worth. When they build things in industry, they build them to last. That's where the frequently over-used term "Industrial strength" comes from. Regular consumers will always look to buy the cheapest thing possible, despite the fact it it's a piece of crap and won't last a year. That's not the way they think in Industry; it's "take the bite and buy it once" I have PLCs here in my plant that are over 30 years old. One has been sitting in a panel bolted to a hot vibrating machine since the early '80s, and still funtions just is it did on day one; let's see an Ipod do that.
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