PC based Control .vs. PLC

No, hardware architecture is much less prone to bugs than software architecture. In part, simply because it is much, much larger.

I think PLCs are best suited for relay replacements. They are best at high speed single bit ON/OFF control logic. When using them for slower analog control then the resolution and accuracy is usually limited to 12 bits.

PCs are best at high resolution computations (64 bit), recipes, HMI displays, data logging, networking.

When properly selected and installed in a clean cool environment, and have not been loaded with all kinds of junky unstable software, then they are just as reliable as PLCs.

The hard drive is the lowest MTBF component, but this could be replaced with a Flash drive. This also depends upon the software. If the program only uses the HD to load the program at startup and then does not use the HD, then the HD should last a long time. If you have a Norton Ghosted backup HD available, a HD crash can usually be repaired in a few minutes. And the PC hardware is cheap and readily available at you local PC store.

We have been using PCs to control temperatures in plastic extruders for over 5 years with few problems. This is a high resolution - high accuracy - high computation application where millisecond response is not needed. This also provides HMI, recipes and data logging in the same software package.

Warren
http://www.pc-pid.com
"the PC-based Temperature Controller people"

Hello,

This rivalry between PC based control and PLC is good.

Let me tell u that both the technologies have advantages. Yes the electronics have reached to the point that there is no difference of advantages or failure in the components of both technologies and are reliable. Yes, all electronics componenets suffer the failure reliablity fatigue (after running continuously, there are failure rates) and the components have to be changed. PLC components I/O cards are more expensive as compared to the PC cards They are expensive if the PC is Industrial PC. PLC is like program and forget and the monitoring by SCADA. PC based control is always connected to system all times but can be customised for SCADA.

As regards the operating systems, Yes NT has been discontinued but WIN 2000 is still fine. If someone asks the Blue screen problem, PLCs have the same problem like red LED that means program has stopped.

The distinct advantages are the price and economics. I give you my examples.

Our system Mantra by ControlSoft Inc. (US based company--a Pioneer in Loop tuning technolgies and OEM vendor of these technolgies to many top brads in automation) is available both in Hardware and Soft form (PC based). both offer connectivity to OPC, DDE or drivers. But we found that customers are more worried about the Hardware rather than the technolgies being offered. In this hardware form we are offering unlimited loops and FBD while in PC based control we are offering no of Loops and FBD usage based. We never had a problem in PC based control but we had in hardware. The prices are also the matter. Rather than the harware modules cost, there is loop cost and FBD usage.

Then the PLC is programmed by its own software rather than by third party vendor But PC based control can be programmed with interfacing with SCADA/C++ etc.

It all depends upon customer choice. he will prefer that in which he has less hasels.

Manmeet

An interesting property that I haven't seen mentioned is the fact that PLCs program can be edited in runtime. This is equivalent to recompiling a running program on a PC, which is not possible to my knowledge.

For example, in a PLC based system, you can edit a running manufacturing process, commit the change, and seamlessly run a new one. I don't see why this wouldn't be possible on some emulation layer on a PC based control, but in all the systems that I've ever seen PLCs handle this well while PCs don't

Hope this helps,

--
Nathan Boeger
http://www.inductiveautomation.com

KW-Software's MultiProg (PC-Based) will let you perform an online edit. Conversely, a low-end PLC, like an AB MicroLogix will not.

A good (read "stable") PC-based control runs under it's own kernel and uses Windows (or KDE or <insert favorite GUI here>) as a nice clean interface for those of us who don't have time or inclination to do command-line type programming. Can I do it? Sure! Do I want to? Not really!

For what it's worth, this opinion and a dollar might get you a can of soda.

Regards,

Jordan

Yeah, but.
PLCs are usually made with close to mil spec components.
PCs on the other hand...

If AB or Modicon puts out a PLC with a code defect that costs someone their life, they are responsible.

No one is responsible for open source.

This is the first time I've been to this site and I'm quite amazed how engineers are comparing a PLC and PC-based Control System.

I'm not sure of what you mean by PC-based system. Guys, correct me if I'm wrong, the PC-based system is the DCS or the Distributed Control System.

It is a fact that a PC cannot and should not control a process. It is only used for configuration, Man-machine interface (MMI), historian or event loggers, etc.

In both PLC and DCS systems both uses PCs or programmers to configure the operation and download it to the controllers. PCs for MMIs, etc.

Both have advantages and disadvantages i.e. trade-offs.

The main point of the discourse now is reliability against functinality and flexibility. (Not to consider the $$$ for this functionality.)

PLCs have the reputation of reliability, robustness and even life-cycle because once programmed it is already there in the controller.
With respect to system crash, comparing it to PC-based, it is true that the PLC quickly reboots with just a click of a button called system reset.
The problem does the process quickly resets. It has to start-up again!

DCS has the reputation of being fairly reliable but is also flexible. New functionalities can be added to the system easily using open-system configuration, i.e. OS platform like NT, XP, LINUX.
The problem with open-systems is, it prone to attacks intentional and unintentional. Life-cycle with respect to hardware is comparable to the PLC but with the PC, every now and then MS is releasing new OS versions. NT is no longer supported by MS. So users will have to upgrade to XP, then to VISTA, then who knows!

There is really no comparison with PLC and PC-based or DCS.

It is up to user on what he wants on his process!
There will always be trade-offs.

It's interesting that one could even generalize that PCs are unsuitable for PLC tasks. These days they are more similar than ever and the ultimate PLC, when they finally gain all that they are lacking now, will _be_ a PC. With the right software and a few hardware selections to eliminate the hdd, etc., there is no reason a PC can't be a Super PLC. You are confusing the machine with the crappy software that comes loaded on it. I have several PCs that have no idea that they aren't PLCs. And the applications don't know the difference either.

Regards
cww

Your assumption is incorrect. PC control is not the same as DCS.

PC based control refers to systems like AutomationX, LabView, or SoftPLC.

Once you understand the definitions better, I leave the comparisons to the previous posts.....

--Joe Jansen

Personally I am never touching a traditional DCS or PLC system again. Seriously it gives me a headache to work on that stuff.

Over the past decade, we've been doing installations of fully PC based "PLC/DCS-like" systems. We run entire plants on a PC server pair, or complete process areas in larger plants. The PC's are throttled direct to the I/O without PLC processors, controller modules etc.

Advantages are: less hardware, more reliable much less wiring, as we are mainly using Profibus efficient setup and maintenance easy upgrades Great ROI with long life cycles and protection against obsolescence. Servers and/or OS are commodity items and are swapped out ~ 5-7 years. Software upgrades typically we do annually for sites under contract.

Speaking from experience - The PC servers doing the job have never been hacked, disrupted or tinkered with in anyway by anyone. Only respected and appreciated. With the redundancy, PC system uptime has been remarkable, close to perfection and on par with the DCS and PLC systems that they run alongside.

A lot of IT missions these days are as or more critical than traditional industrial, in terms of impact of downtime. So the IT products reflect that.

Cheers,
Paul Jager

It all depends on what you are doing. Windows is not deterministic in the "Hard-Realtime" sense of the word, and Profibus is fast, but not as fast as a backplane of a PLC (I'm sure there are exceptions to that, but you get my point). PLCs also have the advantage that the popular ones are supported for decades and not in single digit years. The hardware of most good and popular PLCs will outlive just about any computer with a standard PC power supply and a mechanical hard disk. And yes, you can make operating systems more robust, get rid of hard drives, and do all sorts of things with redundant power supplies, but most people don't do that because it is more work to maintain and set up.

I'd like to sum it up and say that if all of your control is slow enough and not dependent on things happening in a certain time (seconds) you can use a distributed PC based system and be very happy with it.

If on the other hand you are running fairly high speed equipment (1ms response) that needs to run the same day after day, don't use a DCS system.

~Ken

On February 25, 2007, Paul Jager wrote:
> Personally I am never touching a traditional DCS or PLC system again.
> Seriously it gives me a headache to work on that stuff.
>
> Over the past decade, we've been doing installations of fully
> PC based "PLC/DCS-like" systems. We run entire plants on a PC
> server pair, or complete process areas in larger plants. The
> PC's are throttled direct to the I/O without PLC processors,
> controller modules etc.
>
> Advantages are:
> less hardware, more reliable
> much less wiring, as we are mainly using Profibus efficient
> setup and maintenance easy upgrades Great ROI with long life
> cycles and protection against obsolescence. Servers and/or OS
> are commodity items and are swapped out ~ 5-7 years. Software
> upgrades typically we do annually for sites under contract.
>
> Speaking from experience - The PC servers doing the job have
> never been hacked, disrupted or tinkered with in anyway by
> anyone. Only respected and appreciated. With the redundancy,
> PC system uptime has been remarkable, close to perfection and
> on par with the DCS and PLC systems that they run alongside.
>
> A lot of IT missions these days are as or more critical than
> traditional industrial, in terms of impact of downtime. So
> the IT products reflect that. <


( Complete thread: http://www.control.com/thread/941648158 )

Reply to PC is not so reliable. Cycle time of the PLC is the big advantage. Also reliability. The hard disk in the PC is the weak part in a PC (vulnerable to vibrations). Also programming is a point.

However when it comes to cost in particular for small applications (PLC with graphical interface) an industrial PC with bus system to i/o modules is more cost effective and has more possibilities for programming a user interface. If you want a PLC with graphical user interface (or graphical screen with interface to PLC) the cost rises considerably compared to an industrial PC. A beatiful example is the Teamster (Swedish company) glue application where an complete gluing application (gluing gun and control for robot) is controlled by a industrial PC (graphics and user interface are just magic). If you would want to this with a PLC the cost would be 3 to 5 fold. Also if you depart from a PC you have more possibilities for logging (hard disk) networking (standard), etc.

When it comes to maintenance: keyboard 6€, screen 200€, hd 50€ is much cheaper then PLC accessories. I think overall the industrial PC will make its entrance (more cost effective). With the PC also the way is open for companies to make the steering of your machine more open for low-skilled workers by working with a graphical interface. PLC manufacturers are also going this way but at a very high price for the customer. The only but is the software at the moment. If you start with PC steerings you are on your own to program it. Not only the steering but your interface also. But for manufacturers of machines that always perform the same process, I think it will make them more competitive and enable them to offer a higher grade of service to the customer at a lower price.

Here's what we are doing for installations:

Major process industry: Paper, Steel, Oil and Gas, Nuclear Power, Food,
Mining, Waste treatment
Major manufacturing: Auto sector, Consumer goods, Electronics, Equipment
Transportation - Roadway Tunnel management/cotnrol, Big ships, Railways
Large Building and Facility Automation
Electrical Utility Systems

I think the record for one server pair is over 6000 I/O. That's huge and I mean REAL I/O - analog and digital wired inputs and outputs. Typical
installations are anywhere from a low of 100 I/O to 4000 I/O with about 4-12 client stations. Clients can be anything - remote view only to full
engineering stations - all depends on setup and login.

The SERVER scan times we are running are ~ 20mS but typically set at 200 mS. System response is in the PLC range and superior to every DCS I've used. For faster scanning or nesting of functions the engineers divert to a distributed "device" arrangement. Using the Profibus coupler module as a control CPU, or a small compact industrial PC unit managed from the server.

Client response or what the people use and the display update is almost instant because everything is on the server ported via X-windows.

For most operations, the priority is not about working the details of hardware. In most cases, the priority in designing a system for operation is about the efficiency of the people and process using the system. If the in-between can be as open, efficient, effective, transparent and simple as possible (but allowing the very complex to take place) with the least amount of hardware and other barriers, then that's progress.

Personally, my hands will never touch a keyboard related to a traditional PLC or DCS. Too long to spec, setup, & cumbersome work with, pain to commission - just wretched. OK if you like OT I guess.

Paul Jager

I have just built a system with a PLC and touch screen for my company. It a packaging check that checks a part with a seq. number through 4 stations. It is to make sure that the part went through all the stations. I also capture all the variable data in a database in the screen (leakage rate, date time, part number). The screen runs javascript so the sky is the limit. All over ethernet.

2 months earlier I built a similar system with
an industrial PC. My finding was there was a lot less coding in the PLC system than in the VB one I did. It was also much easier to handle the data transfer from the different stations to the screens database than with the PC. When something went wrong in the VB app (runtime errors for no reason), the maintenance crew could not fault find it. The PLC system on the other hand is still running with no "run time errors".

Regards,
Craig

PCs have to come a long way to be as easy to integrate as PLCs.

PLC vs PC? Wow, hasn't anyone seen the Xycom computers used to provide FASTER control, more reliable control, and are even integrated into a PCI interface with an AB plc backplane.

this literally is the best of both worlds, easy to troubleshoot, and program editing is a sinch through Wonderware.

Climb out of your holes....

Well, in the old days, we always concerned the blue screen and long boot up time from PC. However, now in market, there are seveal good PAC (Programmable Autoamtion Controller). It over came these PC's issues including blue screen and long boot up time because these PC based controllers use Windows CE.net. Check these two companies.

ICP DAS USA, Inc.
http://www.icpdas-usa.com/pac_programmable_automation_controllers_.html
Their PAC can run logic control and HMI all in one with standard VGA monitor port. You can program them in ladder logic and many Microsoft software.

NI, http://www.ni.com/pac/ You can program them in Labview. Most of everyone knows Labview.
Both of these controllers belong to PC based control but with PLC robustness.

If you want robust control use a tank.
If you want to distribute large amounts of data use a truck.

If you want both, use both: a PLC and SCADA or an advanced MMI.

MATLAB is also offering real time interface solution... "dspace" is that one. Using this you can compute highly complex algorithms for real time solution with ease.

PLC code runs smoothly, it is so old, and cannot do complex algorithms. And it takes too much time to code it.

On the other hand, PC is more friendly with a full-screen SCADA (PC-monitor), with a very cheap price.

In my opinion, one should use both. With least memory PLC (for uninterruptable processes like motor controls), a Modbus connection, and with a Java enabled PC. There you can get most robust, most clever, and cheapest system one can ever have.

PC vs. PLC: both have strengths and weaknesses BUT I don't think there is much of a debate if one is worried about mean time between failures (MTBF).