Why PLC is preffered over Industrial PC's and controllers for automation?

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Thread Starter

M Divakar

I wanted to know why PLC's are preffered over industrial PC's for industrial automation....
 
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marc sinclair

Hi,

I recently went to a factory and witnessed a PLC I programmed 12 years ago, It runs 24/7/365 (a siemens s5, but it could just have easily been AB, etc.) At the same factory they run a sterilising system recently installed, it runs under XP on good brand name industrial PCs - it
crashes every week - the systems company almost live on-site. They recently added a small plc which cycles power on a watchdog check and this allows a return to on-line in just a few minutes unless the crash is during a critical measuring time, when the restart time can be 45
minutes. Incidentally I have quoted for a PLC version of this system, I'll likely get it too.

I'm not an oss zealot, I would never try to play tomb raider on a PLC, or trust a critical process to windows. Yes curt and jiri, Linux, (my Linux box has been crash free for 17 months) but I've looked at all the offerings and they are nowhere near usability yet - I'm experimenting at the moment with an S7-200 and IT module connected to a Linux PC running Firefox, all the processing still done in the PLC - it's as far as I'll go at the moment, because if I make a mistake, 20,000 litres of milk could end up as yogurt!!

marc
 
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Curt Wuollet

Because Microsoft poisoned the well with instability and this has caused many people to think PC's are unreliable. Also, suitable IO and software have been hard to come by until recently. But the point is becoming moot as many recent PLCs are essentially a PC under the covers. Not that PLCs aren't a good solution, it's just that PCs get a bad rap fron the Windows experience. Many large automation systems ran and still run on UNIX hardware and a PC with
Linux is as reliable and vastly more powerful than say, a VAX. So, it's mostly perception and the fact that vendors have made PLCs very convenient, if somewhat pricey. I expect a trend towards more PCs as Windows fades. They can do a lot more with higher bang/buck and far greater flexibility.

Regards

cww
 
It's because a PLC is a dedicated PC for Industrial control. It got a number of digital and analog Inputs/0utputs and a sytem to handle them.

Simply where u need a truck u will not prefer a mercedes benz.

Hope U got the difference

A PLC programmer
 
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Robert Scott

Here are two reasons.

1. PLCs are the direct descendents of the pre-solid-state relay control panels that were used in all control applications. So by inertia, PLCs have an advantage.

2. The architecture of a PLC is especially well suited to handling a large number of simulataneous parallel functions (the kind that are so common in industial control applications) with a high degree of reliability. Even though the PLC internally is implemented as a sequential processing device, it appears to the user as a highly parallel processor.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting
 
I think it's simply because PLC is really mean for process and automation control. With its IO and special function modules, it takes less time in doing programming because one only need to concern about seq and loop control. Commercially, PLC is more expensive, but the way it has been designed makes it more realible in the long run. Just think of what a PLC does be passed to a PC, the PC will be overloaded with rapid processing works which is required for industrial control.
 
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ScienceOfficer

M----

Because PLCs are designed for automatic control (relay replacement) and PCs are designed for human interface (video, keyboard, pointing device) and data storage (rotating media).

Hope this helps!

Larry Lawver
Rexel / Central Florida
 
It is simply because PLCs can operate 20 years without any major problems! What PC can do that?

ASL/Finland
 
My experience is that a PLC is a much tougher platform and will hold up with industrial apps.

Additionally, Microsoft constantly changing the platform causes nothing but long term problems.

We have PC's on the floor that had DOS 2.2 running on them, when the PC failed the replacement was very tough and can be very expensive.

Most newer hardware i.e. temperature controllers etc. do not communicate with old PCs and software, well that is a major pain.

On units that I have replaced, we use a PC for data collection and PLCs to perform the work. This has been very successful and very cost friendly.
 
Proven reliability is the main reason. Another is that the main operating systems by MS have had problems and the reliability of PCs is not near as good as a PLC.
 
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James Ingraham

1) PLCs have rugged housing, broader temperature range, and no cooling fans.

2) PLCs have I/O capabilities. Adding I/O to a PC means integrating a third-party solution. (Also, it typically means having to open the case of PC; it's much easier to add a card to a PLC rack)

3) PLCs don't have harddrives, and do have battery backed and/or non-volatile memory.

4) Historical reasons. Why change? This also encompasses training issues, e.g. your maintenance people all know Allen-Bradley but can't change a video driver.

5) Security. Turn the key on a PLC to "Run", then pull the key out. Now nobody can change anything going on in the PLC without the key. PCs tend to be harder to lock down.

6) Stability. This is a perception problem as much as anything; I've seen plenty of rock-solid PCs on the factory floor. Still, if you buy a PC and it screws up, your bosses will ask you why you didn't use a PLC.

7) Boot time. PLCs come to life much quicker than PCs.

Naturally, there are people who will disagree with this list, and some who will say "You forgot reason X!" Still, this gives you an idea.

Incidentally, I'm a big fan of PC-based control, and my company has been doing it for almost ten years now.

-James Ingraham
Sage Automation, Inc.
 
probably because the PLCs arrived first in the automation business. But now days PC are being preferred for automation task due the fact that they can keep a lot of acquired data and they can export that data to any PC (using the internet) or to any software program. Besides is a lot easier to develop a program in a PC than on a PLC.

Best regards
Rosco
 
D
Having used Windows platforms for automation since Win
NT 3.51, I would have to take issue with Curt's
statement about Windows instability. When properly
set up and administered (i.e. lock the operators out
of everything except for the absolute minimum of
required apps) I have found Windows to be more stable
with each iteration - and XP Pro is the most stable of
the bunch. If the system adminstrator and programmer
do not have the requisite level of competence, then
there can be significant stability issues. The
biggest problem I have seen in 13 years of Wintel
automation programming is that there are far too many
people out there who think that since they can log on
to AOL and chat at home, and maybe install a USB
camera driver, they have the knowledge required to set
up and administer a factory system.

I have also programmed and administered HP-UX and
Solaris based automation systems - and I found both to
be little more stable than the Wintel systems
including and post Win 2000 Pro. Not to imply that
they were problematic, but only that they were not
much better than a properly set up Wintel system. I
find it unlikely that Linux is any more stable than a
properly set up XP Pro system.

Davis Gentry
Applications Engineer
Delta Tau Data Systems
 
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A PLC has been optimized for use in an environment
where there is limited sophistication in maintenance
and setup. There is typically very little
configuration involved with PLC cards - most of them
are plug and play - something easy to do if you are
using cards which all come from a single vendor for a
single hardware platform. So very seldom are there
issues with bad drivers.

From a programming standpoint, RLL is a simple
language which has a huge population of maintenance
personnel who are competent - so unless you need the
flexibility of an open system, or want a lot of power
for not a lot of money, why retrain your maintenance
guys?

Davis Gentry
Delta Tau Data Systems
 
C
Hi Marc

Indeed, that's my whole point. People won't feel good about PC reliability until 5 years _after_ they dump Windows. That gives the PLC folks a pretty good edge. Of course Windows infelicities cause them a good deal of grief as well. The first PLC vendor to support Linux will have quite a competitive advantage where reliability matters. What I think is hiliarious are the folks who argue to the death about the reliablity of PLCs and then build systems dependant on Windows or Windows technology. In a lot of applications, it doesn't matter if the PLCs keep on truckin' if your people can't see what the PLCs are doing. Obviously, reliability isn't very important yet. I think there will be tremendous synergy when you install Linux and PLCs together and which you choose for what role will depend on what needs to be done and IO count. I have already seen the extrordinary power of this approach. It makes you want to tell people about it.

Regards

cww
 
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Lynn at alist

One simple reason - open a PC you bought just 6 months ago and see how many of the EXACT same cards and componants you can buy today. I doubt many of them are still available; everything is "new and improved". Functionally, a new Ethernet card or CD-ROM drive works the same, but if
you start experiencing subtle problems in 1 of 20 Industrial PC, it will be hard to rule out the diverse hardware. True "Industrial PC" suppliers hold componant supply constant, but ultimately their price will be higher so users will decide buying a "new" CD-ROM for $29 is better than an exact replacement for $169.

I remember working with a compressor company in Singapore using Industrial PC for some realtime "call-for-service" systems and the field techs even complained how different models needed different screw drivers to service. And the modem cards - some had the "line" RJ11 as the top of 2, while others had it the bottom of 2. ;^) These things sound small, but when you are supporting hundreds of sites - and may have to DRIVE BACK to one if you goof up reconnectling cables - these kinds of inconsistancies can be costly.

- LynnL, www.digi.com
 
H
Just a few that I can think of right now:

1. The PLC was invented as a modular control solution right at the onset (this has to do with what it was meant to replace). The inputs are in modules, the outputs are in modules, the rack is another module and so is the power supply. Each PLC maker usually has a variety of CPU (and
communications) modules to suit a wide range of application. The programming in modern PLC's also allows modular development.
2. There is a long term hardware manufacturing commitment by the maker, usually in the range of 7 to 10 years.
3. The hardware is tested to rigorous standards, the better manufacturers modules withstand serious abuse (electrical & mechanical).
4. Components of the more recent PLC systems are hot pluggable.
5. There are fully developed on-line programming & diagnostic tools available.
6. There is a ready supply of competent electrician/programmers available. The big item that matters here is that the person standing at the PLC
programming terminal more likely thoroughly understands the electrical & control system that he PLC is interacting with.
7. While there are lots of people that like to argue about it, it's easy to do decent process control with PLC's, and it's done with the same hardware/software that is there for the rest of the system.
8. Decent PLC makers work on a continuum when they bring out new lines of hardware, building on previous models and capabilities. This allows gradual upgrading of the workforce and retaining values of installed interfacing
hardware.
9. There are more likely to be local training opportunities for getting into PLC programming. I know I have taken lots of electricians, techs and engineers (even mechanics) to competent PLC programming in a one week
training course. I have not seen any comparable course that would take one of those guys to a competent computer programmer in a week.
10. There had to be ten at least, so lets make up another reason: Competent sales staff. Where I buy my PLC hardware I can get some help with applications problems (whch is fortunately quite rare - but valuable when needed). Is your local computer dealer going to be of help (or the operating system vendor, or the application software vendor)?

Enough already.

Hugo
Rockwell Solution Provider
http://www.plc-control.com
 
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