why use many PLC's instead of few PC's

I went to a tour in a brand new factory and I saw that they use so many PLC's
They use mainly AB and siemens.

why not concentrate the whole thing to 2 or 3 powerfull computers
or even 4 computers if you go on the safe side.
it is much cheaper this way.
dump the length limited copper wire and use fiber optics instead.

If you don't trust windows, there are many other OS's
much more reliable than windows and even than the OS's that
the PLC runs.for instance the OS's that runs the banks transaction.

furthermore,sometimes they use PLC+scada while the scada
is not just for monitoring but for reporting and storing data
and without storing the data the whole batch is worthless.
So why use PLC in the first place?

One simple comment to your post.
Do you want the company IT guys
even partially involved in your
project?
Do they know how to use multimeter?
Oscilliscope? AC & DC inverter drives?
I hope you see where I am going. Cut
costs the IT guys can handle the
machine problems ( Because it is a PC )
.... YEAH RIGHT..

Your question(s) are not in order.

Why do you need "computers" at all? This site is about using PLC's.

SCADA means Supervisory Control and Data Acquistion, it works in conjunction with a PLC that performs and collect the data.

GEEZ, you are suppose to be an engineer, these comment surprise me. Every situation has it own needs.

You can use PC's on Plant floor

This is what PC Based Automation is.

Your PLC's can exist on PC's.

Good luck with your projects... You ARE going to need it.

What sort of tour, must of been very quick. I am sure any engineer on site would of explained the reasoning behind their architecture.
No offense but how long have you been in this industry?
Regards
Alan case

I believe you already know the answer
But here is my attempt at an answer

PC's are good for entering data from a keyboard / mouse with people as the input output source.
Everything you see is entered by someone who has done the self checking

Or in SCADA case automatically collects Data from data tables that also were collected for it by magic.

With a few PC's you still have no inputs or outputs so to keep wiring distances reasonable many locations of inputs and outputs are required.

Now to get the real world inputs and outputs not human key strokes, you look to have a hardenened industrial envioronment creating isolation self checking etc etc.
PLC's run in a loop program, several times a second checking the same inputs and outputs automatically, sec by sec hour by hour year in year out.

What the PLC does well, is no easy task for any PC,
PC's tend to hate loops in there programs.

Briana.

Is this flame bait ?
You are stacking several suggestions together.

Why PLCs or PCs ?
PLCs are MUCH more reliable than PCs. Industrially hardened PCs are approaching but not reaching the reliability of PLCs.
Industrially hardened PCs (from reputable vendors) are more expensive than PLCs.
PLCs have a product life cycle of several decades, typically with migration paths between generations.

The OS on a PC is an open topic by itself.
Windows XP or Vista ? You have to add 'realtime' extensions to apprach PLC reliabilty and determinism. Even so, what Microsoft OS will there be in 5 years or 10 years or 15 years from now ? I have seen machines with DOS and/or Windows 3.11. Guess what a nightmare it is to support those.

Use some sort of RTOS ? Is doable, but then what advantage have you achieved over a PLC ? Your PC is then proprietary and expensive, and still not as reliable as a PLC.

Copper or fiber ?
This has nothing to do with PLCs vs PCs.
Actually, there can be good reasons to use copper even today.
But putting everything together in one big network is become quite common, and then you typically use fiber for the backbone.

Central or distributed ?
This has nothing to do with PLCs vs PCs.
Distributed has the advantage that a 'cell' isnt interrupted by problems elsewhere. If the machines are from different vendors or 'OEMs', then they typically come as single integral unit or 'cell'.
Central has the advantage that you cabn easily coordinate large interacting systems, i.e. processes rather than machines.

Finally.
The cost of automation hardware today is so small compared to the costs of what they control. It is the wrong place to save a penny.
PCs in control may have their place, but dont chose PCs to save money.

Briana:

I built an X-Y table for cutting metal.
I first used the parallel port of a PC. This required
a lot of extra programming (try and make a counter in
GeeWiz Basic > "PC's tend to hate loops"). It also
required extra outside electronics to interface (i.e.
buffer transistors, opto-isolators, relays, etc.)
But now I got a PLC. Why? Because they are made to
control machines. They have counters, timers, screws to
wire switches directly. Much more reliable. The PLC
is it's own 'cell'. Our network can go down but it
won't. They are hardened for the industrial enviorment.
And I am confident it will work for the next 20 years.
Just my own experience.

the basic question seam to be use of centralized vs. distributed
system. there is already plenty of answers but think of this:

- if you plan on running one machine, does the whole plant need to be powered on?
- if one modification (replacing faulty component, firmware change etc.) has to be done on one piece of equipment, do you want to shut down or reboot whole plant? nowdays it is not incommon to see systems featuring hot-plugging but this wasn't always so and it has limitations too.
- troubleshooting - it's so much easier if you have smaller system (and local).
- if control is centralized, single fault can shut the whole plant down. try explaining cost advantage of centralized system when hundreds of people are sitting and rolling their thumbs and production is not running because of some glitch on a computer (drivers, viruses, corrupted data, new programs, servicepacks and whatever people should not be doing at at work etc.).

OK. I get it.

I am a software engineer.I worked for several years
in developing software,and i found PLC's quite interesting, although i have only basic knowledge in electricity.

sometimes i find it quite difficult to implement in ladder logic
things that in so called "regular programming" are obvious
like implementing queue, stack and so forth.

a lot of extra programming (try and make a counter in
GeeWiz Basic > "PC's tend to hate loops"). It also
required extra outside electronics to interface (i.e.
buffer transistors, opto-isolators, relays, etc.)
But now I got a PLC.

Click to expand...

any GUI is infinite loop based.

when i was at high school I developed a rugged PC controlled motor including ethernet monitoring.
is cost me around 1K US$ including the motor( 1300$ today).

a year ago client paid me to do the same job with an AB PLC

4000 US$ !! NOT including the time for developing the program
and RSView + RSlinx.

[groan]

not this old chestnut again.....

the PLC vs PC thread has been running over on control.com for more than 7 years...

http://control.com/thread.php?id=941648158

...anyone for a quick round of "Why should I pay for my PLC programming software?"

briana banks said:

sometimes i find it quite difficult to implement in ladder logic
things that in so called "regular programming" are obvious
like implementing queue, stack and so forth.

Click to expand...

Then don't program this in ladder. Program it in Structured Text. This will be much more similar to what you are accustomed to on the PC with "regular" progamming, that be pascal, VB, or C.
One thing you will never get on a PLC is dynamic memory allocation. So a stack will have to be defined in a fixed manner.

About that "rugged PC controlled motor" that you did in high school: Did you factor the reliability of OS and PC hardware in the "ruggedness" ?
Anyway, without knowing the details, we cannot know if 4000 USD is cheap or expensive for the task given.

krk said:

[groan]

not this old chestnut again.....

the PLC vs PC thread has been running over on control.com for more than 7 years...

http://control.com/thread.php?id=941648158

...anyone for a quick round of "Why should I pay for my PLC programming software?"

Click to expand...

That has been here too http://www.plctalk.net/qanda/showthread.php?t=275

rsdoran said:

That has been here too http://www.plctalk.net/qanda/showthread.php?t=275

Click to expand...

bah...doesn't even stretch across 4 years....just a baby compared to the control.com offering!