PC based Control .vs. PLC

Z
> 1) You can easily find softPLC software that can be programmed in ladder. 2) There are also industrial PC's. you can find them with or without keyboard, with touchscreens or with only a few function keys so operators can't use them to install other application. 3) If you use a PLC and SCADA software then PC control with remote IO is cheaper. For small applications the PLC will be cheaper. I can think of some applications where a PLC would be better or safer, but in 80% of all applications you can change PLC by PC control. 4) If you use a good quality (industrial) PC, a flash harddisk and a Real time operating system a PC can be as reliable as a PLC. 5) Keep a PC control dedicated, once you have installed all software and hardware correctly you will never have problems with a PC

<--stop for point 1 and 2, your correct. However for 3, Scada information grows taking either softPLC time (this is not wanted) or the scada runs slower, so seperate the Scada and plc. This is not the idea. Also, small applications are better with a HardPLC, and I feel Large applications also. The expandiblity of a PLC is far greater then a PC. Plus you don't have to worry about the driver for the end device. Alone the video card in a PC now. If this dies (ok it happens) this card has to be replaced with a equivalent card and, who wrote the driver and DLL's? which is point 5- at least with us, a control system is yearly worked on. Other's can't leave things alone.
later
zan
 
R

Roland Wagner

Even if PC based control spreads more and more in the automation industry normal PLCs will always be interesting, and if it is just because of the price: Propriatery hardware will in the interesting amount of devices always be cheaper than modular ones. On the other hand if more than just I/O controlling is necessary (for example like engineering, visualizing, motion control etc.), PC based solution are much more interesting even according the cost aspects.

As already mentioned in some of the authors before the technical topics in order to make PCs ready for controlling are completely solved.

Much more interesting is the question whether the programming environment keeps the same no matter if you are working with a PLC or a PC based PLC (SoftPLC). The IEC 61131-3 as an international standard and hardware independent software suppliers for that standard serve the whole range of requests.

Best Regards

Roland Wagner
3S - Smart Software Solutions GmbH
www.3s-software.com
 
PLC's vs. PC based control.
No contest. PLC wins.
PLC's will outperform - but most importantly will
endure over time !

What is the life cycle of a PC ?
I spent $4000 on a PC with 2 floppies and
256 K of memory a long time ago. I do not know where it is now......It had CGA monochrome graphics !
(At that time I was programming A-B PLC/5's.)
Recall those TI435's or was that GE or PLC direct or Koyos ,,,,,,,,, Still need to program one tommorrow...
Let's tell the plant manager that the line stopped due to an application error and it's a Microsoft bug.
But since the PC hardware is so cheap we can use the PC and our boss will be happy and we will make it until the next project....Maybe !
I still see PLC/5 and TI's and all the tried and true ways but that PC is know-where !!!!!!!
Jimmy




 
I am working on a project for school. I am an industrial eng major. The school will not limit me to any resource however i am in a dead-lock. We are trying to create a real-time or near realtime remote monitoring device/network. We have a gould centrafugal pump that is pumping water. . . We want to measure dB(roller bearing life), PSI, Temp(water), etc. . . We have the networking for attachment to remote server via internet. We want to use a laptop PC to monitor the pump station, but not aware of any software availabe that will run on a windows or lynix based OS(keeping in mind we want to do this with out a PLC). Also, what hardware would be recomended to attach our PC to the pump? Basicly we are totaly lost!!! I am in charge of putting the data onto the internet for remote access, but if we can not get the data from the pump we can not put the data on the internet. Please some help!!!!!!
 
K
Phone an instrumentation company up and ask them. National instruments can supply the IO and the hardware you need as well as the software. They can be found at:

"www.natinst.com":http://www.natinst.com

LabVIEW software would be the easiest solution to your problem.

E-Mail or phone them.
 
J

Jesper M. Pedersen

Here is an interesting topic for anyone that considers to use PCs for control applications:

If a control system or industrial plant is manufactured in europe, or is to be installed in europe it must conform to the "CE" regulations.
One important part of this is the EMC directive. Many peoble think that the "CE" mark that is found on almost everything today means that it is OK to use for control applications - unfortunately that is not so. The EMC conformity regulations are split into two "environments": "home/office"
and "industrial".
The industrial environment requires a higher degree of immunity against electrical interference (EN50082-2), but also allows a device to radiate more interference than normally (EN50081-2).

On short: In europe, all standard low-cost PC parts can NOT be used in an industrial environment. If PCs are to be used, they have to be industrialized versions with the appropriate CE/EMC declarations. That also goes for any
expansion cards, monitors, keyboards etc. I do not know if there are equivalent regulations in north america. But I think that the EMC directive is quite sensible in requiring components to perform in a noisy environment.

Greetings all

Jesper M. Pedersen - DISA Industries.

 
PC-Based control is a living fact in manufacturing today. General Motors Powertrain has been using PC-Based control for over 10 years. GMPT began with a DOS product and have moved to the next generation of Microsoft Windows based PC control. They have found the PC to be no more or less reliable than the PLC. The implementation of PC Based control is a part of GMPT Lean Manufacturing Stratige.

The power of the current PC processors allow the PC to run circles aroung the PLC.

The PLC manufacturers have been trying to make PLCs act like PCs for decades, why not jjust use a PC?

PCs are on the plant floor for the HMI and SCADA systems, why not just converge the technology into a single PC with control?


Ken Jones
Nematron Corporation
[email protected]
 
M

Michael T Mellish

1. Hardware Longevity: With a PLC, you can get spare parts which just plug in & work without changing your system for about 15 years. With PCs, you cannot purchase the same video card, harddisk or monitor within 6 months of your purchase. Yes you can find things that work BUT the regression testing for your system can be extreme.

2. Vendor Family maitenance: PLC vendors have entensive testing to ensure that every change fits the current family OR that there is an upgrade path which will carry the user forward. The PC world is filled with dead & dying software and hardware with no path forward.

3. Environment: PLCs systems work by natural convection cooling, across a range of temperature, particulate contaminents, shock and vibration that PCs just don't match. PCs need fans & therefore preventative maintenance to last in many enviroments. Shock is often not considered until the problems show up.

3. System Stabilty: Lets face it, the "Blue Screen of Death" is a fact of life. The sheer size of the operating system is 10x or 100x the size of a PLC operating system. Add the software reliability concerns to the fact that PCs have huge amounts of additional electronics which are NOT required for PLC logic control functions (like video, mmx extensions and the 2 trillion gates in a P4 CPU ..) and you pay for things which just are not used and which reduce reliability.

4. PLCs are also available from 8 I/O to 80,000 I/O so a price point can be found for the most cost effective control. Not many PCs for under $100 but there are PLCs.

PCs have found niches where they deliver a better solution... for example you want to do vision based inspection and reject parts, your vision system will almost certainly add a PC to the project and using it for control makes sense. You want to build to order (like a Dell PC) and want to interact with a computer directed manufacturing system to select parts, deliver kits to work cells, and assembly to order. Needs heavy custom programming, and would likely be better achieved in a PC based system.

But don't expect that an industrial PC, plus software, I/O interface etc. are cheaper than a PLC system sized for the same job...

Regards
Michael
 
> (Originally posted Wed 09/02/1998)
At 11:24 01.09.98 -0400, Lee Brannon wrote:
>I am always amazed at people trying to sell higher and higher tech solutions for non-existent problems. --- Just what is the problem being addressed here. --- Lack of Sales? --- If there is a real need for more than a PLC, then use whatever is required. --- However; make sure that what you use is designed to be out on the plant floor or put it in the control room where it belongs.
> Plcs are so popular because they work. --- No Blue Screens, no reliability issues, they just work. --- They always work. --- <

... until they crash because of software or hardware problems.

>A slight overstatement, maybe, but not by much everything has a few implementation bugs, but after that it should just run and run and run.

> The second reason PLCs are so popular is that they are easy to understand. --- They were designed to replace relay panels and now have developed much more capability. --- Any Maintenance electrician can take a ladder diagram in one hand, look into the mass of red wires of a control cabinet and determine what is making his machine malfunction. <

What a wonderful picture ... but I would take the red wires in one hand and look then to the ladder diagram :))
What is different if there is ‘PC based hardware’ between the red wires and the ladder diagram ???

>--- If any changes are needed to the program, he doesn’t need hours of moving hundreds of red wires, resetting timers, or complex programming languages, he just needs to know how his motor starters (or other outputs) and limit switches (or other sensors) work and apply the standard symbols for them, that he already knows. --- Even his Gets and Puts of values from and to advanced SCADA systems look like common inputs and outputs.--- His program looks like the machine he’s trying to fix.<

I would not trust a machine which looks like thousends of ladder logic lines :cool:

> The first PLC that I actually sold was installed in the US Postal Service. --- It was installed in a relatively Clean (as plant floors go), Air Conditioned Bulk Mail Center (still hot and humid as hell compared to the office where my pc resides).<

AFAIK ... most of the sorting machines of the US Postal Service don’t use PLCs. They are managed by QNX systems.

> My demo was “installed” without a control cabinet, sitting on top of a Motor Control Center, out on the plant floor, with the bundle of wires and cables “hanging in the air” through an open door where it ran flawlessly for 2-weeks attached to the terminal blocks of the new disconnected “Black Box” it would eventually replace. --- Not the ideal installation even for a PLC. --- As I recall, Safety issues were handled by limiting access to the immediate area as this was only a test.

> PLCs are designed to survive in real world control situations. <

Industrial PCs too ...

>--- Have you ever seen what the high heat and humidity of a manufacturing facility will do to your pc? <

I saw a lot of PLCs dieing by high heat and high humidity ...

>--- How about the dirt and grime getting into the floppy, CD ROM, or hard drives? <

Nothing happens if a fan is pressing filtered air into the IPC case ...

>--- How about what the noise/RFI from a motor starter’s coil or contacts will do to for the reliability of these marvelous pc based control systems?<

If the IPC has a marvelous professional power supply ... nothing happens.
Regards
Armin Steinhoff
 
Mr Jesper,

Well, could u write expansions for CE and EMC, which u have stated as industrial safety standards?

thanks,
pran
 
basically plc based controllers and pc based controller are similars. plc based contrllers have also a mini computer with keyboard, any computer program can put as per the requirements.
plc has the limited memory to store the data and ram also. it has the processor is also for certain programmed as per the attached machine.
while in pc based contrller has more memory and fast processor. by pc based controller user can join more machines to operate from the same machine. easy for networking and dcs.
 
E

Eduardo Manuel C. Cipriano

Hi Guys,

It's really very clear that PCs are made for information purpose only, while PLCs are for Control purposes...

thanks
 
PC based control system is also called SCADA: Supervisory Control & Data Aquisition System. It can be considered next generation control system for coming sixty years. PC life cycle for replacement can be extended to 8 years with
spare PC. It can be great Integrated Process Control with data record on CD-ROM for long time. PC based control system can be designed for longer life cycle up to 10 years for PC & wireless hardwares.
 
J
It seems that using PLCs as a base with PCs as operating and programming interfaces seems to be where today's control systems are heading towards. Consider the reliablity, durability, and ability of the PLC, the flexibility of the PC, and the accessibility of both.

 
R
in many cases PCs and PLCs have same charactristics except two important points.

PLCs are very easy for maintain, eg: you can simply add/remove hardware cards to/from PLC in a few seconds. and the 2nd one which is very important to industrial automation, PLCs MUST NOT have a chance to hang. it means that in a PC, if you write an invalid opcode, you will get an exception which may be handled or not, but in some cases (even one) the PC may be crashed. which is a bad thing in industrial automation. in such cases a PLC STOPs and reports all errors by detail.
 
C
But again, that's not the PLC, it's the software run on the PLC. If the same software were ported to a PC motherboard, which is not very far-fetched as some use the same processors, it would have the same features when used with a totally controlled set of peripherals. Again, you confuse your experience with one particular OS as what a PC is capable of and limited to. This is simply not the case. A SBC coupled with a bus that does only automation things could provide all the same attributes.

Regards

cww
 
You try and explain to a production manager why the PC crashed. Bad enough explaining PLC failure. Then again, PLC very rarely crash in my experience from bad software programming.
 
C
And in my experience, you don't have to explain crashes if you don't run crashware. And you _can_ crash PLCs with software, believe me :^) Usually it's right away though. You need 4.5 billions lines of cruft to get the really subtle crash mechanisms. I've found on Linux code that if you run for a week and there's no sign of memory leaks, you're in pretty good shape. Windows leaks core without any user programs running. That says something (bad) for the long term.

Regards

cww
 
B
I would say that is not my experience. At least with SLC5s. Its quite easy for errant application software to cause a CPU fault, and if appropriate action is not taken before the end of the scan, the processor exits run mode.

I see this a LOT, usually with inexperienced or just bad programmers.

Bob Peterson
 
T
Hey Guys and Girls,

I have worked with Soft 5 since it was first introduced by Allen Bradley. With its NT operating system it was not dependable to say the least. I think Soft 5 was designed for PLC applications where there would be large amount of data involved. As you know Windows NT is no longer supported by Microsoft. When I talked with Allen Bradley they could not tell me how to make there computers using Soft 5 and A Win 2000 operating system work.

I have discovered a way to make there computers operate on Win 2000 and I do not have crash problems anymore. It is very dependable now.
 
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