PC based versus PLC

I know this has been beat around before, but I really haven't found much good information on it. So, what I'd like to see in responses in this post is information and or opinions on cost and reliability of systems that are PC driven and PLC driven. I DON'T want to hear from people who are PLC system strong and PC system weak (that would describe me) bash PC based systems. I'd like informative answers from people with experience with both. Basically, I'd just like to see how PC based systems stack up and why you would choose one over the other.

This whole post was driven by a tour I took of another facility in town. 90% of the facility was driven using PC based systems programmed in mainly VB code, using remote I/O over ethernet. Their claim was that systems were cheaper to create, maintain, and readily setup for redundancy. My inclination, naturally being a PLC guy was to think, "I doubt all that. I'll bet you use this type of setup because it's what you know best." However, now having had some time to think, perhaps they looked at all options and opted to go that route for a reason, and I'd just like to know what those reasons may be, just to satisfy my curiousity. Thanks in advance to those who add useful input.

Unless the question is constrained a little bit the distinctions get real fuzzy. In the original example we have a PC running VB, obviously PC based control. But what about a PC running AB SoftLogix? This would the the Logix engine running on a PC. Or what about the CoDesys engine? Does where it runs define whether it is PC based or is it always considered a plc?

I don't keep my finger on the pulse of PC based control as closely as I would like. With the ControlLogix and the S7 family running as fast as they do now most of our customers don't even want to think about PC based control.

If by PC based control you are talking about any given logic solver running on a commercial PC, in the cases I have seen it has worked well. You may need to be a little cautious with environment but that's not too hard in most cases. And you have to love the available power. I have yet to see my first 'plc' running at 2GHz.

On the flip side, it's hard to argue with a plc's closed hardware and operating system. They really are pretty hard to screw up. Yes, you can mess up the application. But it is pretty difficult to hurt the operating system. That is not necessarily true of a PC based solution.

Keith

I have looked at WinAC a few times as a possible alternative to S7 PLCs.
From a cost benifit point of view, we get a break even at around the 414-2DP range CPU if we use a normal, commercial PC.
For an industrially hardened Siemens embedded PC, the break even is about the 414-3DP range. So far, I don't think the PC can unseat the low end PLC.
At the high end, there could be a payback, but it is not big enough yet for us to consider the risk.
From a performance point of view, the PC seems to be better, but generally also not good enough for us to change over.
From the reliability and crash point of view, if WinAC is the only thing running, then the PC is generally almost as good as the PLC. Using embedded PCs rather than the off the shelf standard type, can be good for minimising the risk of games or special movies being run on the machine.
I think Soft PLCs are at the viable stage, but are not clearly superior to PLCs at the moment.

I should mention that I have used WinAC as a PLC simulator in the past for testing code, and it is very good in that role.

Disadvantages PCs

At the moment we are using soft-PLCs this is a mixture between a PC and PLC. Years ago we also used IPCs with Windows XP and software written in VB. One of the mayor problems we had is that you don’t always get a fast response (real-time behavior) from the PC, even if you change the priority of the program. An other disadvantage when you use a PC is that you have to write all the software you need for example the communication between an input and the software to handle the sequence. Most (all) PCs run with a hard disk and this is a weak point (hard disk failures), most (all) PLC have some kind of flash memory.

Before we used Windows XP with VB we used PCs with Solaris (Unix with real-time extensions) and software written in C this had all the disadvantages of the Windows XP with VB and more (Most of the time you had to write your own device drivers and it was a lot of work to make an acceptable HMI). For PCs you have to take care about power failures (UPS). When you switch of the power before shutting down the OS sometimes it will not boot up.

Thanks guys.

Thanks to those of you who have replied. Your input is appreciated.

I am using both PLC & PC. I am using WinAC from Siemens on the PC side and AD for my small PLC stuff. One of the main things that we looked at was what type of HMI do you need. On my plc stuff I don't need a HMI but on the PC side I get a lot of extras with Windows. As for cost here in the States I am getting real good Siemens pricing. I can even get parts cheaper here in the states than my friends in Germany. I think Siemens is wanting some of the AB market here in the states.

CharlesM said:

I am using both PLC & PC. I am using WinAC from Siemens on the PC side and AD for my small PLC stuff. One of the main things that we looked at was what type of HMI do you need. On my plc stuff I don't need a HMI but on the PC side I get a lot of extras with Windows. As for cost here in the States I am getting real good Siemens pricing. I can even get parts cheaper here in the states than my friends in Germany. I think Siemens is wanting some of the AB market here in the states.

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As u here mention here that ur using PC as HMI. So i think u can help me out.

I have also developed pc based scada system using wincc flexible advance 2008. I am using adam adapter which converts 485 to 232, I am using PC as HMI, and there is Profibus interface so it requires to convert 485 to 232 for PC, thats y i am using Adam RS 485 to RS232 isolated converter.

Now in PC/PG properties, there are many access points i.e. S7Online, MPI, PROFIBUS DP. And the funny thing is that i have tried all the access point but wasnt able to take my runtime online.

Waiting for ur reply

Thanks

Ernitin.

Posting in multiple threads about the same subject is no-no.
Hijacking posts is no-no (your problem is not related to the discussion in this thread).
Resurrecting old threads without a very good reason is no-no.

JesperMP said:

Ernitin.

Posting in multiple threads about the same subject is no-no.
Hijacking posts is no-no (your problem is not related to the discussion in this thread).
Resurrecting old threads without a very good reason is no-no.

Click to expand...

Ohhhoo come on, I am in deep **** just got few more days to complete this project, require immediate help and as i found that one who have success fully installed the system thats y i asked

And thankU for ur quick replies.

Ernitin, consider yourself added to my hall of fame list.

JesperMP said:

Ernitin, consider yourself added to my hall of fame list.

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I'd love to see that list! You got to love people who need your help, but only on their terms!

Lol - Nice one Jesper!

JesperMP said:

Ernitin, consider yourself added to my hall of fame list.

Click to expand...

JesperMP said:

Ernitin.

Posting in multiple threads about the same subject is no-no.
Hijacking posts is no-no (your problem is not related to the discussion in this thread).
Resurrecting old threads without a very good reason is no-no.

Click to expand...

All right dear Jesper,

I apologies for that. I understand. I hope u can under stand.


Thanks