PLC versus PC-BASED?

Hello friends.IN our factory we have some simatic s5 plcs.Now we have

decided to upgrade.But,the question is upgrade to plc again(for example to s7) or try to install PC-BASED systems.I welcome any information or comments aboat the comparison of these two systems(advantages,disadvantages,limitations.....).

Thanks all
Enric Boom

This are some of my considerations (only my opinion - who am I to set rules):

I for one wouldn't rely upon any MicroSoft operating system for production until it has proven to be as stable as the old hard-PLC, which is (in my opinion) far more trustworthy. But I think I will be retired by then (I've only 23 years to go).

Translating a program from S5 to S7 is no hard job at all. The conversion utility does a lot of the work for you. Of course you'll still need some fine-tuning, but you won't have to re-write the complete program as I think would be the case for PC-based automation.

Another consideration is the speed of I/O refreshes. With a conventional PLC you have the choice between a parallel bus to the I/O cards (with a lot of wiring to the outside world) and a field-bus (which is slower to refresh the I/O' because it's serial). With PC-based (and also with slot-PLC's) you're stuck to a field-bus.

Kind regards and good luck with the upgrade,

Generally a PC based system will be less reliable, and more expensive than a PLC based system.
Unless there is some special reasons for it, you should stay with PLCs also in the future.
The most obvious for you is to convert from S5 to S7. But it does not HAVE to be S7.

If you could provide some more details about your current system and wishes for the future, then we could give you better advice.

If you have the chance to leave Bill Gates out of your control system, leave him out. Put him in charge og nothing.

You'd be better off with magnetic controls than some "windows" thing.


http://www.plctalk.net/qanda/showthread.php?s=&threadid=6839

See the above thread--it is about outputs but the controller is also robust.

One of the problems you might run into with PC based control is the PC hardware lifecycle. To the best of my knowledge Siemens still actively supports the S5. I know you can still by a new PLC-2 from Allen-Bradley, and that platform is all of 15 years old. In either of these cases you can get up an running again simply by purchasing a new processor of the same model and dumping in the program. I don't know if you can even buy a PC with a 486 in it anymore. This means if you need to replace something that is even 5 years old you will go through hardware and driver h**l trying to get it operating again since everything that used to support the old hardware won't support the new hardware.
Yes, PC based control is lightning quick and the enterprise connectivity is pretty nice. But for my dollar, I'm sticking with PLCs.

Keith

This might help make up your mind.

Try setting up a race. Get a PC and a PLC. Load the PC with some HMI software, then shut both units down.

NOW, turn the PC and the PLC on at the same time. Which one is booted and controlling your process first? Did you need to log into one of them first? What would happen if the power went out when no one is watching? Would the PC come on again if nobody hit the power button?

You could run a second race, too. Run them as long as possible and see which one fails first.

In my opinion, it's not a good idea to put all of your control in the hands of a machine that also plays Solitaire and downloads pornograghy. I don't mind HMI packages for datalogging and top-level control, but you still need something out there that doesn't take breaks or catch cold.

AK

Several years ago a project was done in-house to install and program a small (8 lines) sortation system. Steeplechase, a flowchart based programming software residing on a PC was chosen for the control (don't ask why, I wasn't involved in that part). It has run flawlessly since installation.

One time another tech and myself had to trouble shoot it, work wasn't going down one of the lines, as I remember. Knowing virtually nothing about the design, we managed in a few minutes to narrow down and isolate the problem, a switch turned off. I'm familiar with ladder logic and so I'm sure we'd have been at it several times longer if we'd had to have waded through all the timers, and latches and permissives, jumping around following cross references, etc. It may not be a perfect system, nothing is, but, I was impressed with the relative ease with which novices were able to use the features and navigate to locate a problem area.

As always, your mileage may vary.

One thing you may consider is the Omron CS1 PLC on a PCI card. It is a full PLC that runs communication lines to the I/O racks/blocks. It requires a separate 24VDC power supply. The PC can be turned off/re-booted when Bill Gates does his usual and the PLC card will continue to run.
beerchug

There is NO right or wrong answer to this question. Rockwell for example sell their Logix system in both hardware and software platforms...ControlLogix and SoftLogix....and for most users there is hardly any apparent difference in the programming.

The main point of distinction is this. With the current state of Windows and the PC industry there is no doubt that the soft PLC is a higher risk option over the entire lifecycle of the project, for a number of reasons.... stability, robustness and support being the main ones.

On the other hand if you are informed of these risks, and are able to manage or mitigate them, AND if you are able to make use of the higher performance of the soft PLC, then going soft is a sensible option.

At the end of the day it is an engineering choice, and over all automation applications, at this point in time, about 95% of the time most people are going to stick with the lower risk hardware PLC.

As time goes by I would guess that this percentage will drop in favour of the soft PLC, and ulitmately the two technologies may merge into one...but I would guess that the hardware PLC as we all know and love it has a lot of life left in it for at least a decade.

My mileage has varied. Last weekend I got to look in on another Forum member's retrofit of a major distribution center that ran on Visual Logic Controller (Steeplechase's product).

Another vendor had built the system with A-B 1771 and 1794 I/O for field device reliability... but the VLC was so unstable that they're pulling it out and putting in two PLC-5's.

I, too, have a PLC-5 gravestone from SoftPLC's marketing campaign. I got it the week before I started making my living with PLC-5's.... seven years ago.

WIN AC

Ask your Siemens sales engineer about WIN AC. Its a Siemens soft PLC but its programmed with Step 7. If you're unhappy with PC based control, just purchase an S7 PLC and download the software. You don't lose all of your development time.

As you can see from my first post I'm not the world's biggest PC-based control proponent. However, Ken's comment does surprise me some. I guess I need to find out for sure what he means by 'unstable'.
A previous employer used a Steeplechase system on a machine and it did what I considered a pretty amazing thing. Because of an HMI problem NT would occasionally lock. I believe we also got the occasional blue screen. However, the process running on the VLC continued to operate as if nothing happened. Granted, since the HMI disappeared it was a bit tough to control. But the machine at least had hardware start/stop buttons going into the fieldbus I/O for the VLC so we could stop the machine gracefully before rebooting the computer the VLC was running on. In our case we got the HMI software corrected and everything was cool. From what I'm led to believe this is because the VLC logic engine loads before Windows and actually calls Windows as a process. So Windows can't affect it's operation.

This may be a question of semantics. Is it possible that the VLC could cause Windows to lock? Yes, I suppose so. But on the flip side, at least in my case, the VLC still operated correctly without Windows. So was the VLC really unstable? I guess if you're the one who's PC is locking up all the time it really doesn't matter. It really is a system question and if the system is unstable it doesn't matter who's at fault if it isn't fixed.

Keith

Simple Rule of Thumb

I adopt a very simple rule of thumb...Use PC's for supervisory control and leave regulatory control to either PLC's or DCS's.

Kamenges wrote:
Because of an HMI problem NT would occasionally lock .... Granted, since the HMI disappeared it was a bit tough to control.

Click to expand...

You just put your finger on a very important point.
Neither control nor HMI functions should be handled by a Windows-dependant system.
When I do setup a system with a PC HMI/SCADA, I allways include a simple operator panel as a fallback.

About SoftPLCs (Win AC, SoftLogix, etc.): The best option for control on PCs. You get PLC functionality. And usually the PLC engine will continue to operate as a service.
But for all the trouble, all you get is PC converted into a PLC. It will have higher CPU power, but will never be as reliable as a hard PLC.

About Windows: The automation business is very small compared to the entertainment and office bussiness segments. Microsoft will develop Windows to cater for these. Also, Microsoft has to create "revolutionary" changes every 5 years or so to make it necessary for its customers to keep purchasing its upgrades. If Windows was "finished" so would Microsofts business.
Anyone heard of Longhorn

Hi there!

I have a number of production lines, the control of which is split between 75/25 between PLCs (Allen Bradley ControlLogix) and PCs. Our lines run 24/7 for 360 days of the year and I can tell you from looking back over our records that all our problems have been with the PCs.

The first thing that fails is the power supply. Now, because the power supply usually starts to die a few days before it's actually dead it usually means that a few weeks down the road after changing out the power supply we have to change out a hard disk. Flutuating power going to the hard disk??

If the power supply doesn't fail, the hard disk will! This is the second biggest fault in our plant.

Last but not least is the operating system. Now I run Windows 2000 Pro on most of the PCs which is fine except when Microsoft find a bug which results in me having to install a patch which may or may not affect my production software...oh joy oh bliss.

To summarise, what I try to achieve is in every control system is:-

'Have a PLC control the production line
'Have a PC gather line information
'Have a Panelview on the line for manual operations
'Ensure that PC failure will not stop line production