Speed / tension calculations execution

JeffKiper

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How do you execute speed or tension calculations in a PLC? Do you use a STI (Periodic task) to make sure the calculations are executed at the same intervals every time?
I am thinking of using an Event driven task to make sure that speed and tension loops are updated every xx.x feet of the line.
We have several machines in house that just run continuous task and don’t make any efforts to handle things consistently. Does it really matter? I don’t know if it does but in the theory I think it should.
 
Thanks Peter
I am looking at using a 1769-HSC card and using one of the internal counters and have it as my trigger.
I don't remember what the RPI of the Compact I/O is because it is grayed out at 5ms.

I am designing this system at 500FPM the fastest I have every seen this line run is 265 FPM. So if I design for the worst case scenario of 500FPM = 0.1388 FPMs and a 10ms update would only be 1.388’ I am thinking of 5’ tension loop updates. Does that sound correct on the math side of things?
 
Thanks Peter
I am looking at using a 1769-HSC card and using one of the internal counters and have it as my trigger.
I don't remember what the RPI of the Compact I/O is because it is grayed out at 5ms.
Hopefully you can get better than 5 ms updates from the counter card.

It looks like you are planning on about a 40 ms update at the 500 fpm rate and 5 ms is 12.5% of that. When running at normal speed 265 FPM 5 ms would still be about 6.25%. I see the updates from the counter card being a limiting problem but it may not make much difference if you are simply using PI control
 
I am researching the update time of that card in the Compact chassis.
Do you know how the RPI is handled in the Compact I/O? I am not aware of how to set this up yet. I am downloading the manual now and doing some searches.
 
I am researching the update time of that card in the Compact chassis.
Do you know how the RPI is handled in the Compact I/O? I am not aware of how to set this up yet. I am downloading the manual now and doing some searches.
No, I have a Compact Logix though. I just haven't played with it enough to tell you an answer. It is setup for a training class about 40' from my office. If I think about it I can take a look.
 
Not worth you spending you time on. I will find the answer. You have much larger project to deal with, than some trivial question like this.
I will post back later once I have found the answer.
 
For my tension devices on my machine I use periodic. Main reason for this is I set the interval the same as the RPI for my analogue input card (100ms in my instance).
 
RPI for the modules is set under "CompactBus Local" in the IO tree.

Thank you very much I got pulled off to fight fires. I still have the project open and ready to start looking for it. I guess I don't have to look any more.

I will drink a cold adult beverage for you tonight.
 

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