Soft PLC or Hard PLC?

[Hack]

Which one does most people prefer. Also which one is more reliable and stable.

 

[TConnolly]

I think we all know from first hand experience about the reliability of PCs, but we can discuss reliability and dependability all day.

Or we can cut to the chase:
The IO is a wash - either way you are going to buy IO. Generally a soft PLC program costs just as much or more as a hard PLC (more if you factor in annual software support contracts) and since you already have an IO rack you may as well use a hardware PLC.

 

[CroCop]

Soft PLCs are great for specific applications.
However, with the CLX line, I can do just as much with a regular PLC as I could with a soft PLC.

 

[Ken M]

There is absolutely no doubt when you compare the different installed bases that most people prefer hard PLCs. As far as denting the traditional hard PLC market is concerned the soft PLC may as well not exist.

Ken

 

[PhilipW]

I agree with Ken 100%.

Rockwell for example have both the "Hard" ControlLogix PLC platform, and the SoftLogix PC platform, that are very similar and both program with RSLogix5000. SoftLogix in fact is rather more powerful than it's ControlLogix cousin, and on the few occasions I have used it, seems stable and robust enough...but I can only think of two real-life applications where I have seen it used. For every one SoftLogix in the real world, there must be 50-100 CLX processors.

SoftLogix does have some significant performance advantages. For all intents the program size is almost unilimited, the number of files and programs that can be created is enormous, and the FP and Motion instructions are blazingly fast simply because a 2.4Ghz P4 CPU has the sheer horsepower to beat even the latest 1756-L6x series of HardPLC.

But there really are not any significant cost savings to the Soft PLC for most systems, and the advent of high speed Ethernet has so tightly coupled most PLC's with their HMI's that even that advantage of the SoftPLC has largely evaporated. And most larger applications can be readily broken down into smaller functionally independent sections, thus allowing mulitple HardPLC CPU's to spread the load. At this level of system size and integration the nature of the application being controlled usually demands a high level of secure control, which counts against the SoftPLC.

In a nutshell, except for a relatively few applications that demand very largest and fastest performance integrated into one CPU, there is not sufficient benefit to justify all the other disadvantages that are inherent in using a Windows PC platform running SoftPLC's for real-time control.

 

[SimonGoldsworthy]

Any system that has a Windows based HMI (for example embedded XP, no hard disks) may as well use a soft PLC. Why buy another CPU ?
This type of system will be more suited to (relatively) high volume products that are not modified by end users.

 

[jstolaruk]

Any system that has a Windows based HMI (for example embedded XP, no hard disks) may as well use a soft PLC. Why buy another CPU ?
This type of system will be more suited to (relatively) high volume products that are not modified by end users.

Why not? Because Windows is not a stable platform to launch a PLC control system on. Viruses, non-real time OS, no control of interrupts that create latency, etc. are many of the reasons.

This controvery has been going on for more than 20 years and thankfully the PC based PLC is still rare in the field.

 

[SimonGoldsworthy]

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

 

[Archie]

However, with the CLX line, I can do just as much with a regular PLC as I could with a soft PLC.

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? Program the PLC without getting out a laptop or other PC? Use DeviceNET, Profibus, Ethernet, ControlNet, and Interbus IO all on the same controller?

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.

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.

 

[PhilipW]

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?

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

Program the PLC without getting out a laptop or other 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.

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

Yup, just add the modules to CLX to your hearts content, and have as many different networks as you like. eg: SST Scanners

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 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.

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 agree on this, but it has to be a very large and demanding process that requires an unusual combination of a very large program, high speeds and complex data integration with other databases to make it all worthwhile. The mere presence of an HMI does not qualify.

 

[HLeap]

Personally, I would not let a soft PLC control anything more than the sprinkler system at my home. I don't think there is anyone who hasn't experienced the ocasional glitch, unrecoverable fault, blue screen of death, etc. in the windows operating environment. I have been working as a contract programmer in the petrochemical industry in Houston for 35 years and I have never seen an application of a soft PLC. IMHO, it is just the reliability factor and I could never recommend a soft PLC to a customer. Having a palletizer shutbown for a few minutes and having to reboot the PC is one thing. Having a batch or continous chemical process shutdown unexpectedly is a completely different situation which could have disasterous consequences. As for PC based HMIs (RSView, WonderWare, etc.), I always try to design their interface so when (not if)they fail there is no adverse effect on the process. In other words, the control is not dependent on 100% up time for the PC.

 

[DGeller]

Soft Plc's

I have to agree the soft PLC is usless on anything that requires a high safety level or up time. Tried one once and will never use one again. One blue screen of death was enough. Nice toy but not practical for the real industrial world.

 

[S7Guy]

Any system that has a Windows based HMI (for example embedded XP, no hard disks) may as well use a soft PLC. Why buy another CPU ?
This type of system will be more suited to (relatively) high volume products that are not modified by end users.

Well, in the controls systems I've designed, I always use a Windows PC for the HMI, but I design it so the HMI doesn't stop stop the process if it crashes or needs to be rebooted. I can reboot the PC and the process still continues to run in the PLC, and the operators don't know the difference. Looking back, if I had gone with SoftPLCs, I would have lost my shirt on those projects just in the commissioning phase.

I can still feed endless amounts of data back to a server for archiving, trending, reports, etc, so the customer gets the best of both worlds: A PLC that keeps his process going no matter what, and a PC that provides an effective interface for his operators and IT department for reports.

I have heard just too many horror stories from first person accounts. SoftPlcs just don't offer enough of an anvantage to go prime time.

 

[elevmike]

I think that when you compile all the previous responses you come up with one answer. It depends on the requirements of the application. In my opinion, if the project can be done with a hard PLC then it should be done with a hard PLC.

 

[TConnolly]

Why buy another CPU ?

Because the Soft PLC software costs as much as a PLC CPU. Add annual software contracts, and it costs more.