PLC Vs PC Control

chavak

Member
Join Date
Jul 2002
Posts
750
Any body have experience with PC based controls, It was aksed by my boss to look into the feasibility of PC based system and come up with a comparison of both PLC and PC control in terms of Cost, maintenance, development time, data acquisation and storage and of course end user training + learning curve, etc.

What is the general comment about it. Any links where I can find more info turorials etc. I am totally new to PC based automation.

Thank You 🔨
 
My experience with PC based control is - alot of potential, little of it realized.

Sure, it's got lots of memory, data collection and processing, ethernet -- all the goodies. But it doesn't mean much if your computer won't run your process. The essential problem is Winblows, which likes to crash. Alot.

Like the old joke says, if Gates had built cars instead of computers:

1. Your Chevy would throw itself in a ditch twice a day for no apparent reason.

2. If you changed the stereo, the engine wouldn't turn over anymore.

3. Every other year, you would have to replace the engine and drive train - to accomodate the changes in Microsoft-brand motor oil.

TM
 
DONT DO IT!!!

PC based systems are definately NOT the way to go. Yes you get some advantages but as was said previously, Windows crashes a lot and PCs whether industrial or not, are never very user friendly on a machine no matter how good the user interface looks.
You also need to be a bit of a wiz with high level languages like C (or VB if you want to use a beginners language!) Anyway, stick with PLCs - they are a recognised standard, faults can be traced with a laptop by most people and you can make quick changes if required.
 
Re: DONT DO IT!!!

Steve Wood said:

You also need to be a bit of a wiz with high level languages like C (or VB if you want to use a beginners language

Steve, please don't underrate VB, it is the best application language what I know. I have done solutions by MS-VisualC++, but it is not for application development and it needs very much of knowledlge, what it can accidentaly do if programmer don't know all gimmicks.
VB-compiler do almost similar exe-code than c++, it debends compiler only.
 
And so another can is opened!
I have been using PC based control products for some time now, mainly Entivity Studio (formerly Think&Do) and I will have to agree with what TM has said in that it can only be as reliable as the OS that it is running on. I can also say that in the time I have been using it I have found the occasional quirks that PC base control is subject to, to be outweighed by the enormous potential and added benefits that it has to offer. I still use PLC's to control critical equipment and processes. Until some one comes up with a crash proof OS PLC's will remain to be the only truly dependable control solution.
 
And so another can is opened!

Good call, RDay. This is always an entertaining one if people come out and play.

The PLC vs. PC debate is similar to the US political structure. You have two relativey small groups at the tow extremes with extremely polarized views. Then you have the rest of the people somewhere in the middle with shifting feelings on the subject. So it's really hard to feel confident with any information you hear. You just don't know the mindset of the people presenting the info.

That said, I've only done one PC based solution in my time. This was with Steeplechase, which is part of the Entivity group now. I must say I was impressed with the performance. I got sub-millicecond scans on a process that would have been way over 30 msec on a PLC5 family processor. The menu and data handling capacity was pretty sweet, too. I don't know if Steeplechase is still like this or not but the realtime operating system loaded before NT. So if NT locked the control application still kept on running. We had an operator interface qwirk that was locking NT regularly and the control engine didn't care one bit. That was nice!!

But you have risks too. The biggest in my opinion is the mutation of PC hardware. I can buy a PLC5 today that will take the exact program off an EEPROM that I wrote 15 years ago (OK, maybe 10). Try that with ANY PC you buy today. Things just change too fast with commercial PCs for quick replacement of failed parts. We went through the technicality of making an installable image CD to help recovery. But since none of the drivers are likely to work whan they replace the PC that isn't much help. Also, you will be taking on more responsibility for getting all the parts to fit together than you would with a PLC solution. You will end up with multiple hardware and software vendors all with their little piece of the action and you in the middle to piece it together.

So, yes, PC based control has many advantages that cannot be discounted. Just research what you are getting into before you leap. The biggest question to ask is are you going PC based just because it's cool. Granted, I don't do anything REALLY big. But I have yet to run into something I can't accomplish with either one plc or a few plcs networked together. Most machines simply aren't that big and don't move that fast.

I hope I didn't muddy that waters for you.

Happy Hunting,
Keith
 
Other os options

We have three large product sorting, stacking and palletizing robots running on QNX. AB flex i/o is used for the realworld side, but all of the data handling and control is pc based.

Pros: It is generally quite reliable and error free save when there is a hardware problem. Inexpensive control hardware.

Cons: The controls are written in C++ and compiled. This reduces the possibility for improvements/quick troubleshooting by the techs. Hardware problems(flickering sensors, bad ethernet cables, bad scsi cables) can be annoying and difficult to trace.

It seems a very tidy solution, but then it's not Micro$oft based.
 
Thank You for all the replies.

I do understand the flaws in windows operating system. It is a big disadvantage when comparing PLC and PC.

But the argument is, it has come a long way sinse old W95 days. With NT, W2000 etc, proves it is much reliable, than their old counterparts. Today the expectation from customers is not merely make the machine run and churn out parts. Every single data is considered important, so they can improve the performance and thus the OEE. There is a lot of limitations for PLC to store historical data by itself.

I also understand data can be acquired from the PLC by stand alone applications developed by any SCADA system or VB applications (yet to explore more on VB). SCADA system = $$$$ for the customer, if it is VB applications, there is a limitation of linking it to the PLC, is depends on whether you have a good driver ($$$), activeX controls ($$$)etc.

The extreme goal is, the management wants everything with minimal cost, thus the standard cost of their product is lower (also to sustain in this competing world)
 
It's getting to be more and more of a toss-up, but my personal preference is to strongly urge my customers against using any PC-based control platform for a process.

I have used some 'pseudo-PC' platforms, the Siemens 'SlotPLC' for example, that will continue to run even if the host computer is turned off.

I much prefer to use a dedicated PLC to control a process, and, if required, a seperate PC for Data-logging, generally through either a DDE or preferably OPC server. If an HMI is required, I again use a PC, but one of my design criteria is that the HMI PC should NOT be required to run the process. Yes, setpoints can be entered, recipe's used, lots of fancy graphics and controls on the screens... but you can unplug any one of my systems PC's, and the process will continue running. Restarting a PC should not affect the process either.

If cost is an issue, I'd still go with a dedicated PLC, and roll a data-logger / HMI PC using low-cost OPC servers (Kep for one), and Microsoft Office for the logging and control. You can do a heck of a lot with just Excel and it's VBA.

For databases, I prefer SQL always, and a very low cost yet robust SQL database is MySQL. Automation Direct + AD's DDE/OPC Server + MySQL + Excel + budget PC = a fairly robust control system.

More and More PC's are ending up on plant floors, but the failure rate is still pretty high. Not just OS shutdowns, but hardware failures too. Hard drives die, motherboards burn up, power supplies decide to go on extended vacations... even with 'Industrial' PC's. To buy a truly rugged, high performance PC system, you are looking at a lot of money just for the box.

One other thing is that while the PC platform for process control is becoming more and more attractive, someone up above mentioned the real biggy... I can download a program into a new PLC CPU in about 5 minutes. I can tell whoever answers the phone at a company how to do it in about 20 minutes. I can program a CPU at my office and ship it to a customer.

Compare that with trying to explain how to install and configure an operating system, load the application executables, configure any special hardware and drivers, install the control program, set up logon's, and get it running. Not 5 or 20 minutes.. Not by a long shot.

Just my opinions though.
 
I did MS-VisualC++ application where was DeviceNet-Master card in PC and MS-SQL-Server 7.0 and small HMI by MFC.
In D-Net was abs.-encoder, light-curtain and PhotoCell. System scans plate product profiles, count their cross-section-areas, put them to SQL-database piece by piece with work shift, line-speed, error-sql-links to error descripiron table, time-stamp by SQL-Server etc. 1000 pieces per day.
For MS-Excel I did SQL-inquiry where final result was salary-count by work shift.
OS NT4.0 have worked 3 years without Windows-errors and whole production is in database memory.
With PLC PC is necessary in every case, so for what I need PLC ?
 
Seppo - To you, Congratulations on having the machine running that long with zero downtime. But your application as described sounds like data acquisition, and not actual process control.

Even my best enterprise PC Server has had to be shut down, or has failed in the past year. (Shutdown required for a @(*%&@) up print spooler, it got confused requiring a maintenance reboot to clear the queue, then a normal reboot to serve again, Failure due to extended power outage which caused the UPS to fail out on low battery. Then it took an administrator to come back in, hook up a monitor, and find out it wouldn't continue the boot process because there was no keyboard attached (Bios battery failed, wiping the 'Ignore Keyboard Error' suppression).

I won't trust a process to that. Or operator safety.

Even on some of my Data Acquisition systems, I have gotten transaction fail's (timeouts) to SQL server, that means that some process kept the CPU busy for more than 5 seconds. 5 Seconds when dealing with a control PID, or state logic is an eternity.
 
I used to use PC control in MS DOS days, which was at least as stable as many of the Windows variants. At that time I felt that the advantages of lower cost analog I/O (usually at least half my point list) easy implementation of math functions, and fairly straight forward operator interface (using Q-BASIC no less) made it a good choice.

I now use PLC based control exclusively, for a variety of reasons.

First, you need I/O anyway to tie your PC to the real world. A series small micro PLCs is at least as economical as serial or ethernet I/O, and you get the ability to provide local control logic "free".

Second, graphic operator interface panels for PLCs have gotten inexpensive, very capable, and configuration is faster and simpler than writing code.

Third, PLC math functions have gotten very good and reasonably easy to use.

Fourth, if I want data in a PC it is simple and economical to use a SCADA type system and a communications link.

Fifth, analog I/O cost per point on PLCs has gotten very low, and is no longer much of a consideration.

Finally, the de-bugging capabilities in most programming software are easier to use to those of higher level languages, although some functions may still be lacking.

It will take a significant shift in either technical capabilities or economics to get me back to PC based control - I don't see it in the near future.
 
Alot of people don't quite understand that PC control has changed alot since the past good of days of softare running ON windows...DOS was a simple and great working platform, but is now ignored by most sadly.

Most GOOD products utilize a microkernal of some type to ensure their machine control Real time operating system runs independantly from a client system, like good 'ol winblows. This is nothing new, Old school kids like me know about thinks like machine virtualization, IBM has been doing it for a long, long time on things like mainframes, running hundreds of vm instances as all independant 'machines' running on one big peice of mainframe iron. The advantages of doing so are huge.

Why also did microsoft snap up Connetix recently, after their VirtualPC product. VMware and others have shown VM'ing is great even now on PC hardware.

What i'm trying to say here is that many of the products out there will run a micro kernal on the system, which in turn will host two child kernals, one being windows and one being their machine control RTOS. SO windows crashing and being the peice of **** it *can* be bares little direct harm on PC based control. Windows can crash, and the main kernal and RTOS keep on chugg'n.

Of course alternatives like Linux and other Unix's and RTOS like QNX provide a much more stable base, and in many of those examples above are very low cost, or no cost. This is why the embeded market is hot over linux and open source. These will creep into all forms of control idustry. I'm doing my part >:)


Another thing to keep in mind is that x86 pc hardware bases do keep very stable in their spec and design. Embeded 486 and now pentium systems are still widely used. A simple PC104 base system with compact flash memory and some I/O serve tasks very well, and have been around for a long, long time. You don't NEED full fleged pc's. And remember kids, if you write portable code under something as fluid as linux, well you can port to any platform easily ;)

Don't associate microsofts pathetic products and performances as the benchmark for general computing devices used in industrial control applications.

Remember Microsoft is not the reality the rest of the non-windows worshipping world knows. There is a promised land, but its not redmond washington ;)


Entirely clear from your minds Microsoft and all your experiences with them. Only then will you being to see think and understand your choices more clearly. Of course, to do so you've got to drop windows. Its nice on the linux / mac side now (both unix boys).
 

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