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Plc Vs Hmi

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Guys. Here in this plant, we use Ti-555 PLC's with a Wonderware Intouch 7.11 / 8 HMI screen to operate the Manufacturing lines. This was originally Ti-555 PLC's with a CVU HMI and CVU was used as a Window to the PLC where EVERYTHING took place. Calculations, Calibrations, Production counts, control, alarming, Analisys Data, Machine Control, cycle time processing, EVERYTHING. This was upgraded to intouch 7.1 a few years ago and since is in the process of being upgraded to Intouch 8. We've an InSQL Server oveseeing the plasnt production numbers and are currently looking into Intouch with Archestra on Allen Bradley PLC's (RSLogix-5000 I think) on New Manufacturing lines. This has raised a few questions and debates recently, mainly about all that's still going on in the Ti-555. Some are stating that Production counts, Yield analyis, basically most of the Non Control processes within the PLC be removed and placed on the Wonderware System to allow the PLC to function as a controller only. Others - Like myself - Are opting to Place As much as we can within the PLC as I believe the Purpose of a HMI is to simply act as a Window into the PLC and to pose as a Smart Switch system where necessary. What I would like to see is what do you think? Am i wrong? Should as much as possible be placed on the HMI? Should the Load be shared? What should the role of a PLC be compared to that of a HMI or is the Term HMI Irrelevant, Should the system operate as more than just an Interface? What do you guys think? Am I right? If I'm not, try to convince me and others that think like me Otherwise.

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IMHO keep it in the PLC, use the SCADA to collect and historize data, and provide an interface to the plc. Windows based PC's are not reliable enough for process control. If Windows becomes more reliable, then you can re-evaluate.

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HMI = box of crayons. Great for pretty and colorful displays.

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I've seen similar problem. There was a system which uses Citect SCADA to get parameters from PLC, do a bit of claculation, and sent it back to PLC. So in the end, we get 2 platform that must not fail. The PLC or the Citect, must not fail. I think this is ridiculous. A good control system would allow the system to function safely when the SCADA is down.

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I don't see any problem with this. Unless those items are absolutely required to run the process. Your SCADA should be able to be unplugged and the process should still run. A SCADA is actually better at computing yields, etc. than a PLC. The PLC does have to keep certain counts, such as part counts, etc. then pass those values to the SCADA, then let the SCADA log, and compute final statistics.

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My personal opinion is that anything critical should be handled in the PLC. However, the emerging trend seems to be toward enterprise data systems. PLCs do not have the memory for, and are not computationally powerful enough, to handle growing demands. If a PLC can't perform the necessary statistical analysis of what's going on with a process over an annual period, or cannot store enough batching information to be useful, then you need a computer to do this. I promote a product called FactorySQL that effectively connects PLCs with SQL databases. So my opinion is to keep critical control functionality in the PLC, but expand it's memory and computing power where necessary with RDBMS (database) systems. ---- Nathan Boeger Integrator, Microsoft Certified Systems Engineer Inductive Automation

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It may be just a rehash of the opinions already expressed, but I think you need to differentiate your discussion three ways not two. PLC vs HMI vs SCADA. Let the PLC do the control and perform the base short period counts {Last minute, Last Hour or Last shootf}. Let the HMI provide the pretty pictures and operator adjustment controls to tweak the process as it runs. Always having some backup means to run if the HMI goes belly up. Let the SCADA or Data Accumulation and Manipulation occur in a controlooer with the power to di it efficiently. Don't risk losing data by demanding that your SCADA and PLC be always connected, but allow them to synch up when reconnected after a service period. Hope this clarifies how I feel on the issue after 25+ years in Industrial Controls.

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Bob, I understand your theoretical distinction between HMI and SCADA, however, I don't see how your SCADA system can be sufficiently powerful and run on dedicated hardware. That is, you eventually want to get the data to standard SQL databases, which are typically off site or in an air conditioned room. If you're worried about those servers going "belly up", you would probably want to implement a redundant configuration. If you really need to cache the data locally, you can use a dedicated machine to do that. My classification would be to put it in the PLC category, referring to hardware devices in the field on the rack with the PLC, especially since this is often done in PLC memory anyway. I think we may be in total agreement here, differing only on semantics. You're distinguishing between SCADA and PLC whereas I'm referring to both as "PLC". We both seem to agree that the PC falls in the HMI category. ---- Nathan Boeger Integrator, Microsoft Certified Systems Engineer "Design Simplicity Cures Engineered Complexity"

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Nathan: We are for teh most part in agreement, but I would use a PC for the SCADA function since the SQl database is there. I would use an intelligent display {panelview, XYCOM} for the HMI function and a PLC for the PLC. Since network stability in an industrial setting is less than ideal. Forklifts do crash into switch cabinets. I want the "local" plc data storage to allow me to reboot teh scada pc and not lose data.

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Sounds like a solid layout to me. ---- Nathan

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