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RedHen
November 22nd, 2004, 05:53 AM
Hi,

I've been given a question at college "Discuss the advantages and disadvantages of using PLC's in industry", my mind has gone blank this morning and the only things I've come up with are..

1, Plc's at the end of the day are computers, and can crash. If your office computer crashes you can't work, If the PLC running your major production line crashes, the production line stops.

2, Its fairly quick and easy to reprogram a PLC, if you want your production plant to run differently, you only need to change a few ladders in your PLC program, whereas without a PLC you have to re-wire major parts of the plant.

Am i on the right track? any more suggestions would be appreciated :)

jvdcande
November 22nd, 2004, 06:23 AM
RedHen,

first of all: it's nice to see a student who has already thought about his/her task. :D

Regarding you answers:

Plc's at the end of the day are computers, and can crash. If your office computer crashes you can't work, If the PLC running your major production line crashes, the production line stops. Yes, you're right. But the crash ratio of a PLC is about 1,000,000 times less than the crash rate of a computer, provided you only take into account those computers running Micro$oft Windows. The operating system of a PLC is a lot smaller and a lot more robust than a PC operating system, especially to prevent production line stops. I would rather look at the complexity (in view of the skills neede by the technicians involved) and price of a PLC to see possible disadvantages.

Its fairly quick and easy to reprogram a PLC, if you want your production plant to run differently, you only need to change a few ladders in your PLC program, whereas without a PLC you have to re-wire major parts of the plant. It's not quite that simple. Changing the workings of a PLC is often a long and difficult task in which not only a few rung have to be changed, but more often a lot of rungs. These rungs have a tendency to influence each other, so you have to look out for these effects too. A big advantage in this is that you can make changes off-line, whereas rewiring is always online. Think at what this means for you production line.

Kind regards,

Redhen
November 22nd, 2004, 06:44 AM
thanks alot :)

mordred
November 22nd, 2004, 07:42 AM
The main advantage of PLCs is that they replace a large quantity of Relays and timers. Imagine trying to tie in 64 inputs and outputs of various voltage levels through the use of relay logic and you'll understand one of there many advantages

Rick Densing
November 22nd, 2004, 07:45 AM
Don't forget, PLC's can crash from a user program not just the OS.

Think math overflow, etc. A PLC is only as good as the person who programmed it.

Sergei Troizky
November 22nd, 2004, 08:06 AM
Originally posted by RedHen
Hi,

I've been given a question at college "Discuss the advantages and disadvantages of using PLC's in industry",

Advantages and disadvantages versus what?

rsdoran
November 22nd, 2004, 08:24 AM
A PLC can crash, an input or output go bad etc but so could a relay, breaker etc. Any component can fail and stop the process.

The PLC offers reduced wiring, less components used, smaller space, reliabilty, less maintenance, visual observation, testing/simulation, math capabilities, and provide a more automated process that reduces human interaction.

At first I didnt like Sergeis' statement:
Advantages and disadvantages versus what?
I see the point to the question now, especially with disadvantages.

chavak
November 22nd, 2004, 08:35 AM
There are many advantages, but disadvantage :scratch: ................. maybe one lolis

You will get addicted to this forum and visit here more often beerchug

marksji
November 22nd, 2004, 08:47 AM
I don't know about anyone else, but as an OEM I know the biggest benifit is diagnostics. My PLC programs can check for all sorts of different problems. Troubleshooting is much easier when a customer calls telling me their text display tells them SSOV1 failed rather than "My heater won't light"...

Another one is reduced assembly time, wiring up all those relays cost alot of time (money) and were much more likely to be wired incorrectly than our PLCs have been.

Doug-P
November 22nd, 2004, 09:03 AM
Originally posted by rsdoran
...visual observation, testing/simulation...

Yes, once you purcha$e the required programming device, whether it be a $150 handheld or a $5000 PC, proprietary software, and necessary cable. In my limited purview, I know of no PLC which provides built in programming/diagnostic capabilities.

One blast-from-the-past exception is the front panel keypad on a PLC-3. You can't program on this but, you can observe and change data values and other machine settings.

TimothyMoulder
November 22nd, 2004, 09:33 AM
Disadvantages?

Well, the introduction of the PLC was probably a major blow to the relay industry at the time. I expect their union was up in arms and lobbying congress to outlaw this new-fangled la-de-da...

Seriously though, there is only one thing I can think of that might be called a genuine disadvantage - proprietary programming.

Prior to the PLC, every hard-wired panel was just a collection of basic components and a host of wires. At the end of the day, anybody with the right background could sort out what had been done, and trace through it with a basic multimeter.

With PLCs, whether by accident (AB does not store rung comments with the program) or design (OEM lock) the designer of the machine can effectively seal you out of the process. You could have the best electrician in the world, but if the ladder code is not accessible, he is faced with a "black box" that he can do little more than guess at. He might be able to look at peripheral sensors and other equipment and sensors and guess at what he thinks should be happening, but he will have no clue as to what is really going on.

In the past, faced with situations like this, I've always pushed for a rebuild of the electronics. In my facility, not a single piece of equipment has been delivered on my watch unless it came with complete code.

My step-son works for a company in Indiana that uses proprietary equipment. When something goes wrong, all they can do is call out the OEM tech and shell out the bucks. The tech makes sure every last person has left the department before he logs on - protecting their investment, the ba$tard$.

That's the only disadvantage I can think of, tho'

TM

Tom Jenkins
November 22nd, 2004, 09:49 AM
Disadvanteges: Think about the two components of total life cycle cost for any technology. They are initial cost and long term cost. Then think about the concept of payback - does your initial investment somehow return money, either by additional revenue or by reducing expenses.

In terms of "compared to what" the guys on this forum tend to think of the comparison in terms of relay logic and single loop controllers, since that is what PLCs were designed to replace. But another basis of comparison should be manual control, because in many cases THAT is what the PLC is replacing.

Think in non-economic terms as well. How many people are capable of manual control? Howm many can use PLCs, and how many can design or program PLC based systems? How is safety affected?

There are pros and cons in all of these areas. I hope I stimulated a little creative thinking - you seem well suited to doing that.

DonsDaMan
November 22nd, 2004, 10:29 AM
A disadvantage of PLC's is the need to learn a new language and syntax for interfacing with it, regardless of proprietary code. Ladder logic is an analog of relay logic, but statement list is a far cry from relay logic, symbolically. The learning curve on PLC's is longer than relay logic. You need to know everything you know for relay logic in ADDITION to whatever you need to know about PLC's.

Another disadvantage is the mental hurdle that "there really isn't a contact there" when one does look at ladder logic. One must be able to separate the idea from its implementation. With relays, one could just look at the relay contact and trace out the wire, but you can no longer do that with PLC's.

Another disadvantage is the need to heavily comment code. The comments become an advantage when done properly, but taking an uploaded (commentless) program and a set of schematics to figure out what MIGHT be the problem is a time-consuming and tedious process.

The reason I'm listing disadvantages is because the advantages are manifest.

One last disadvantage is the lack of support for legacy PLC's. Think about converting programs in 20+ years because the old PLC is no longer made, and the code is no longer usable in its present form. I've not yet encountered this with PLC's, but I've gotten it with enough other (computers, drives, etc) equipment. I think it has probably already happened to many folks who participate in this forum.

DesertDog
November 22nd, 2004, 11:48 AM
Originally posted by TimothyMoulder
<SNIP>The tech makes sure every last person has left the department before he logs on - protecting their investment, the ba$tard$.

That's the only disadvantage I can think of, tho'

TM

I agree with your thoughts about locking the end user out of the code 95%. There is one instance otherwise I can think of though. Where I worked always supplied code and appreciated maintenance help with our notification & help, however there was one situation where several unusual problems kept coming up. After much back and forth finger pointing they were locked out until the end of the warranty. (There was tech support on site in case of major issues as it was a large installation)Once this was done the problems immediately stopped.

To bring this on topic :rolleyes: You can control the accessibility according to your needs.

Ron Beaufort
November 22nd, 2004, 12:12 PM
Greetings to all,

well played, gentlemen ... I am totally impressed ...

the tone of this entire thread (so far) has been both sincere and helpful ... I shudder to think what would have been the result if RedHen had phrased his/her original question along the far more typical lines such as:

Hi,

I've been given a question at college "Discuss the advantages and disadvantages of using PLC's in industry", my mind has gone blank this morning ... any suggestions would be appreciated

I think that I’m going to put this thread in my “favorites” folder and post a link to it whenever we get the typical “do-my-homework-for-me” requests ...

and by the way ... doesn’t the “Little Red Hen” story go something like:

”Alright,” said the Little Red Hen, “I’ll do it myself.” And she did.

akreel
November 22nd, 2004, 01:41 PM
One advantage used to be that the PLC was sort of a mysterious black box. Once you programmed it and it worked, you could be reasonbly assured no one was going to mess with it.

Now, Doofus Dave, the plant electrician has his Dell (DUDE!) laptop. And he's going to monkey with your program, even if the problem is a busted limit switch.

So I guess now the advantage is PLCs are generally accessable and understandable to most technical-minded people. Or, is that a disadvantage?

Is the glass half full, or is it twice as big as it needs to be? :yingyang:

AK