PLC vs. PC based

Excuse me folks if this seems like an elementary question.

I would like to know people's opinions reagrading choosing a PLC vs. PC based measurement and control system.

We are a manufacturer of embedded controls and I am responsible for testing SW. We recently had started on developing a generic PC based automated test system using Measurement Computing PCI cards. This system as configured is around $4000. It has 192 Digital I/O , 48 Analog Input , 2 x 40 kHz Analog Outputs. We are writing the code to control the system in VC++.

Our system must be able to execute scripts that can control outputs in real time and read inputs based upon interrupts in addition to polling. Our actual I/O need are around 48 Dig. in and maybe 48 digital out, 12 analog in and 2 analog out.

One of our development groups has been using a Crouzet XT20 which, although very economical (~ $250), is limited in I/O and does not support in-use editing I believe.

Is there a middle ground where we could use a PLC? Does anyone have a suggestion of a particular PLC to look at?

Thanks,
Mike

Would you trust your life, or even a finger, to Windows?

Well , no, I wouldn't but this system doesn't have any saftey related issues ( beyond software bugs leaking out into released code...).

The entire system is simulated by the I/O and I monitor the response to the simulated inputs.

We make PC based retrofits for punch presses and roll formers.
Running Win NT4.0 and XP.
10 years, 105 customers, 135 machines and NO physical injuries.

We use a dedicated motion control card and a home grown frontend written in VB.

Enuff about safety-

It sounds like you could use a PLC with the Crouzet - just a guess.

If it were me, I'd keep researching I/O cards and stick with PC based system.

Rod (The CNC Dude)

I have two PC based systems in my facility:

Steeplechase (VLC) - sucks, pain hate and discontent without end. Terrible. Crashes out constantly, slower than mud, a host of performance issues, and that abysmal flow-chart programming software.

Beckhoff TwinCAT - has run now for over three years WITHOUT ONE CRASH. I just realized this a few weeks ago, and am now giving serious thought to retrofitting the VLC machines with Beckhoff controls (the machines are too tightly designed around their fieldbuses to move away from the PC altogether).

Programs in ladder, ST, function block, what-have-you, and unlike VLC, which is nothing but software, these guys offer a whole lineup of hardware, preconfigured PCs, and lots of add-ons and goodies. Pretty decent tech support too, from what I've worked with.

So I'm reserving judgement for now - but generally, I prefer a PLC to a PC running my process, where I have a say in the matter.

TM

Alaric said:

Would you trust your life, or even a finger, to Windows?

Click to expand...

A properly designed system where personnel safety is a concern has absolutely no bearing on the method of control, be it PC PLC or Homemade microprocessor. The integrity of the required safety system should be completely independent of the control method.

That said, it is up to the engineer to select the best system for his application. I still prefer the PLC!!

This topic was discussed here many times:

http://www.plctalk.net/qanda/showthread.php?t=3113&highlight=PLC+PC

http://www.plctalk.net/qanda/showthread.php?t=9235&highlight=PLC+PC

TimothyMoulder said:

Steeplechase (VLC) - sucks, pain hate and discontent without end. Terrible. Crashes out constantly, slower than mud, a host of performance issues, and that abysmal flow-chart programming software.
TM

Click to expand...

TM, you are the first person that I have run across that has a Steeplechase system.

It is refreshing to see that you have the same fondness for it as I do!

I notice that when Windows crashes, it is the blue screen of death, not the screen of LIFE.

very best regards.....casey

Ayman thanks for the links. I did try and search under the very topic of PLC vs. PC and did not get those links.


My bottom line here is the bottom line, price. I am not controlling any manufacturing process or assmebly line or any safety critical thing at all.

What I would like to know from the PLC community is can I purchase the capabilities discussed before for under $4000? The PLC needs to employ some type of active editing to control output and read inputs. We would be developiing scripts that would most likely be larger than most PLCs.

For what its worth, I too have used the Steeplechase VLC system!!! It put me off pc based systems for life!
I tend to use Q series Mitsubishi's where I get a free say in the job, However will use most plc's if asked.
If Im after raw speed I use B&R, Although they are a bit of a pain they are very quick and not badly priced.

eldocko said:

What I would like to know from the PLC community is can I purchase the capabilities discussed before for under $4000?.

Click to expand...

192 DIO, 48 AI ... etc
No, To purchase a PLC equipped with this number of I/Os, You 'll pay more than $4000 in most cases (in ALL cases?).

Visit this www.automationdirect.com and check it your self.
The reasons for giving the address of this particular manufacturer are:
1- They mention the prices of all their hardware (you can buy online)
2- Their PLC's are very good and you can buy it with a fraction of the price of other equivelant PLC systems from other manufacturers (e.g. Allen Bradlley, Siemens, Mitsubishi ..)

So pick an intermediate model (DL 205 for example) and start counting I/O modules and it will be a good comparison for you.

Anyway, Although I did many PC-based data acquistion systems, But I do prefer PLC's more than other systems to be used in conrol applications, butI just wanted to help you.

We recently had started on developing a generic PC based automated test system using Measurement Computing PCI cards.

Click to expand...

Once again it depends on the application. You stated that you are making a test system, and would like to use scripts. To me that means a PC system. And, if its a test system I'll bet you are collecting test data. Which is much easier with a PC and database. I think your original measurement computing PCI card method is probably a good choice.

Another option for you is to look into National Instruments Labview software. I've never used it, but our R&D Lab technicians like it.

As far as using Steeplechase. I have one installation. The steeplechase flowcharting I really like, my steeplechase program has never crashed. However, my hmi built on top of it crashes once per quarter. I believe it has something to do with bad values coming through the OPC server. I will not be using it again.

I have also used Think'n'Do. Some winplcs and some Nt machines. It works ok for test applications. Once again, I like flow charts. Initially I had no problems, but now I lose OPC communications once in a while with the WinPLCS. When V 6.x came out it totally sucked and I will no longer use it. Version 7.x is too expensive.

Overall, I don't reccommend either Steeplechase or Think'n'Do, or as I now call it Think'n'Don't. My best app's for test have been a Automationdirect PLC and Vb.net communicating with OPC.

TM

I'm with you 100% with the Beckhoff PC based control. My first Beckhoff system was put in 5 years ago and only suffered one failure. That failure was caused when lighting hit the building and got into the computer zapping the ethernet and DeviceNet (but what system would stand up to that anyway).

The flexibility of the TwinCAT programming system allows me to develop programs a lot faster than if I were bound to ladder logic and in a lot of cases probably not even possible with a common PLC. It also allows me to give a low cost system a lot of capabilities at about 1/3 the cost of lets say a typical AB system.

I think most people that fear PC based control miss 2 points

1) Most windows system do not start out unstable, they get that way because of continuous changes in the system software. Operators can not load software or change settings on PLC's, so keep them of the PC system. If PC systems are left alone once in operation, they will run stable potentially forever.

2) A typical PC is made with cost in mind, not reliability. A typical PC should not be used on a system that is expected to be reliable. Look in the IT department and see what computers they use for servers that must be reliable. I bet its not a typical PC bought at Best Buy.


That's my 2 cents in the debate of PC vs PLC.

Gerry M said:

EDITED QUOTE: As far as using Steeplechase. I have one installation. The steeplechase flowcharting I really like, my steeplechase program has never crashed.

I will not be using it again.

Overall, I don't reccommend either Steeplechase.

Click to expand...

I'm impressed. I figured someone had to have it running somewhere. For the price they get for SteepleChase, it should never crash. And it should serve you coffee every morning.

regards.....casey

I'm working at a Daimler-Chrysler plant that has a couple of old presses my employer sold them and at the time the plant specified Steeplechase. What a pain in the buttocks. The software crashes several times a day and the maintenance people have to power down/up the computer every time. The most recent one we sold them has a 5/04 and runs without a hitch. It there was a cost savings when it was purchased, its most certainly been eaten up by downtime and lost production.