Soft PLC or Hard PLC?

[TimothyMoulder]

Simon wrote:

>>Which soft plc system have you experienced problems with lately ?
Soft PLC's run 'underneath' windows and are real time<<

VLC, formerly known as Steeplechase. I've wrestled with this turkey for 5 years. I know where-of I speak.

I will use a PC to balance my checkbook or play a video game - and that is it. It may be owing to the choice of software - we've had good success with a Beckhoff TwinCat these past several years - but for the sheer pain, hate and discontent of these VLC machines, they are a misery and an albatross around my neck.

Archie:

>>But can you REALLY do as much with a regular PLC? How would you write your historical data do a database on a remote server? <<

Unitronics. Built-in Database. OPC over Ethernet. 8 installs running merrily as we speak.

>>Program the PLC without getting out a laptop or other PC? <<

If I wished, I could hook up a PC to our Data Lan and program any of these Unitronics V280 controllers remotely, and operate them that way (not that I'd want to...)

>>Use DeviceNET, Profibus, Ethernet, ControlNet, and Interbus IO all on the same controller? <<

Why would I? Modbus covers 99% of my needs, 2 serial, one TCP. With the new AD servo drives, I've got a motion controller going on now. And practically, who needs to talk to that many protocols from one controller anyway?

>>Even the most advanced PLC's eventually become dependent on a PC in these days of data requirements. In many cases, it's easier and lower cost to go ahead and use a soft PLC when the system needs a PC anyway for some of its functionality.<<

In a word - hooey. Anything you thought you saved in hardware will be devoured shortly in frequent repeated crashes and upgrades when the hardware you bought six months ago is rendered obsolete. We spent a fortune to upgrade these VLC PCs to XP - I could have simply redone the control system for less, but didn't have the time.

>>PLC's have their place in stand alone and less complex systems. PC based control can be a very good solution for more complex systems where a PC is required anyway. <<

I have been at this 14 years now. I have not yet EVER seen a process that a PC could have done, but a PLC could not. As PhillipW said earlier, even with my little M91 controllers, I can go with a distributed architecture to share the load. With Beckhoff IO and AD servos, I can create an entire system with multiple motion axes and hundreds of IO points, on a PLC costing less than $600.

PCs have their place - on the manager's desk, playing SOCOM, not on the production floor, playing with your livelihood.


TM

 

[Archie]

I'm not sure quite what you mean here. There have always been tools such as RSSql that do an excellent job of that.

Sounds like you just introduced a PC into the process. What I meant was the fact that a PLC could not perform the task without the help of a PC.

If I use RSLinx as the OPC server for both the HMI and RSLogix5000, then I can happily program without having to get out another laptop..

Once again, a PC was needed to accompany the PLC.

In the real world it just hasn;t worked out that way. The imminent death of the HardPLC has been confidently predicted for as long as I can recall...and yet sales of SoftPLC's is still less than 2% of HardPLC's.

I would dare say that PLC's are still dominant because of comfort level more than capability. Think about it this way, do you think a die hard AB user would be readily willing to convert to Siemens PLC's if the Siemens have more capabilties? Most likely not because they would be right back at the bottom of the learning curve. The same applies to the transition from PLC to PC based.



I would never say that PC based is the universal solution to replace all PLC's because it is not. But I have seen many PLC systems that would have been much better off in a PC based solution.

If your system is dependent on a PC anyway for its HMI, programming, or SCADA, then a PC based control should be considered as a possible solution. I know a well designed system will not depend on its HMI to continue to operate, but it will need it eventually. Otherwise why would you bother putting it there.

I know this is coming, so I'll cover it now.....
Reliability always comes up in a discussion about PC based control. Many of the bad experiences come from the use of a white box computer. Would you use your household microwave oven to run production? I would hope not, so you should not use an office computer to run PC based control. Another issue arises when the control PC is used as a general purpose PC. Do you run your word processor or games on your PLC?

 

[PhilipW]

Archie,

I take your point that as soon as you want to do HMI, databasing, programming, MES connectivity etc, then you are introducing PCs into the system. I cannot recall the last PLC I did that was completely stand-alone with no HMI, and even simplest systems require a programming tool running on a PC. So I would have thought that was a given in this discussion that PC's are very much part of the automation world.

But just the presence of a PC (regardless of why it is there) does not justify ditching the HardPLC. Nor am I arguing that SoftPLC's have no place, but experience has shown that there are some specific conditions that must be met.

1. It must be well qualified hardware.
2. The PC must be isolated from any non-Control LAN's.
3. It must be locked down from unauthorised access of any kind, keyboard, USB, LAN, Wireless, etc.
4. You must be able to demonstrate long-term support for all the hardware and OS aspects. What happens in 10 years time when the "video card craps out" type scenarios need to be handled.
5. There must be operational windows of opportunity for system maintenace, updates, re-boots etc.

This is just not how most PC's are operated, they exist in a relatively open and dynamic environment. For instance, just last night for instance Adobe Acrobat did an automatic version update and committed my PC to a re-boot. Not cool if that PC had been controlling a process.

The point is that PC's and PLC's are complementary technologies. The former are optimised for running large complex applications, relatively few (although complex) IO devices and can only offer non-deterministic performance. By contrast PLC's run very small compact programs, a lot of relatively simple IO devices, and have very deterministic real-time OS's. In any real system there is a place for BOTH, but almost always it is best to partition them onto separate platforms....it is usually a mistake to break that boundary down.

Looked at from this point of view it makes no more sense to try and perform real-time control on a PC, than it does to try and run Excel on a PLC.

 

[monkeyhead]

The point is that PC's and PLC's are complementary technologies. The former are optimised for running large complex applications, relatively few (although complex) IO devices and can only offer non-deterministic performance. By contrast PLC's run very small compact programs, a lot of relatively simple IO devices, and have very deterministic real-time OS's. In any real system there is a place for BOTH, but almost always it is best to partition them onto separate platforms....it is usually a mistake to break that boundary down.

Good stuff PhillipW.

Just wanted to point out the only thing holding back the PC from real time deterministic behaviour (hard real time even) is the operating system. There are projects out there for acheiving this both with a linux kernel extension (RTIA i think?), and I remember reading something about XP embedded offering real time capabilities. Plus I'm sure there are OS's out there that were designed from the ground up for real time.

 

[TConnolly]

Plus I'm sure there are OS's out there that were designed from the ground up for real time.

Back in the early 90's I played with a system that used QNX - which is a ground up real time operating system.
(Actually, you've probably use QNX youself, just didn't know it - many bank ATMs use QNX) The job was a small system. But I haven't seen too much of QNX in the industrial controls world - I'm sure its out there, I just haven't seen it.

 

[Gerry]

Back in the early 90's I played with a system that used QNX - which is a ground up real time operating system.
(Actually, you've probably use QNX youself, just didn't know it - many bank ATMs use QNX) The job was a small system. But I haven't seen too much of QNX in the industrial controls world - I'm sure its out there, I just haven't seen it.

I've come across a few systems in the past:

• an HMI system called "RealFlex"
• simulation software "PICS" from SST
• control/HMI from AlterSys (Montreal)
• control/HMI from Cogent (Ontario)

I think PICS has since migrated to windows. Altersys have also and may have acquired IsaGraf (there is an IsaGraf engine for QNX).

 

[Peter Nachtwey]

Are you guys done yet?

Not covered.
1. Environmental testing.
2. Program changes on-thel-fly
3. Non-voltatile memory

 

[Gerry]

Not covered.
2. Program changes on-thel-fly

That was something I was going to mention. I know some (many?)of the soft PLC systems do not support on-line editing and that is a MAJOR handicap.

The only soft system I've worked with is AB's SoftLogix 5, which does support on-line editing using RSLogix 5.

 

[PhilipW]

Gerry,

Shows you what a sheltered life I've led. Having only ever used SoftLogix 5/5000 myself, I kinda assumed they must ALL support Online Editing. Sheesh!

 

[TConnolly]

4. Boot up time.

 

[Archie]

Just wanted to point out the only thing holding back the PC from real time deterministic behaviour (hard real time even) is the operating system. I remember reading something about XP embedded offering real time capabilities. Plus I'm sure there are OS's out there that were designed from the ground up for real time.

This brings up a very interesting point. Windows services can be developed with priority boosting to give a very deterministic behavior. Jitter times of scans can be brought down to less than 15 microseconds. That is better than PLC's that base their scan time on how fast it can scan the program in the PLC.

The real real point is....Windows CE is a Microsoft operating system with real time operating capabilities and it can run on a large variety of devices, including a standard PC. Would that make a control system using Windows CE a softPLC? What is the true definition of PC based/softPLC?

 

[HenryLamboo]

On this forum you will not get an good picture of the advantages of softplcs. The reason is that on this forum more than 95 percent of the members are diehard ‘Hard PLC’ users. And they will never be positive over softplcs.

May be it is better to open a new thread “Who has used a softplc and what do you think about it”.

 

[elevmike]

Henry,

Maybe you can tip the balance to 94%?? Have you ever used a softplc? What are the advantages?

The reality is the answer to the question is totally dependent on the application, so the entire argument dosnt make sense without knowing the spicific application is being considered.

 

[Steve Bailey]

When PLCs were first introduced, there were plenty of diehard relay users in the controls business. They were quick to see the advantages of PLCs over relay based controls and enthusiastically started using PLCs.

Whatever advantages soft PLCs offer over hard PLCs are no quite so obvious, so implementation has been slower.

To claim that soft PLCs have not been given a fair chance because of 'diehard ‘Hard PLC’ users' might be considered an insult. Ultimately, the marketplace makes the decision. Up to now, soft PLCs have been unable to convince a large percentage of users that they offer a better solution.

 

[HenryLamboo]

Mike,

I find it always difficult to say what I use it is more a softplc than a ‘hard plc’.
I think a softplc is cheaper than a ‘Hard PLC’. For example I pay 3000 EURO for a C-IPC and a 15inch touchscreen. It can handle almost endless IO and theoretically it can control 90 servo’s. All big suppliers can do that but what are the cost? (Siemens, AB etc).
The smaller PLC suppliers can only do that when you use more processors (for example Unitronics).
For me the development software is free. One point mentioned in this thread that it is expensive that’s not true.
An other point ‘Environmental testing’ it is there.
Programs change on the fly is possible but limited, but for also a lot of ‘Hard PLCs’ this is not possible.
Non volatile memory, no problem it is there.
Boot up time, the C-IPC I use is started in about 25 seconds.
Ethernet is present for almost nothing, what does it cost for a ‘hard plc’?
I admit that a ‘HARD PLC’ is more reliable than what I use, but for the machines we build it is not a problem.
I think that softplc can be used for many machines without a problem.