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Looking for a reasonable looptime for my PID controler!!

Good morning!

I am trying to do a PID control of a combustionchamber.  The setpoint and process value are the output  temperature, the value to  operate  is  the massstream of  the fuel.  My  in- and output datas are refreshed every 50 ms.  My problem is...

What is a reasonable looptime for my PID control? 


Do you have any ideas???

thanks RT
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Message 1 of 9
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Systems that change quickly or have complex behavior require faster control loop rates

It all depends on your process and the settling time you expect

Message 2 of 9
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Thanks for the quick answer....
but how does it depend on the settling time?
let`s say the settling time is about 20 s, which looptime would you suggest?
Is there a kind of mathematical description for this dependency??

And here komes an additional question:
Do my Ti, Td values depend on the looptime?

thanks RT
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Message 3 of 9
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Hey RT,

Do read this simple tutorial on PID

It should answer your questions to an appreciable extent

I am not sure about Ti, but Td does depend on loop time.( if loop time is too slow, your Td can make your system unstable)

Hope this Helps

regards

Dev

Message 4 of 9
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My group has used the Sun Electronics PC-1000 Process Controller for some time, with good results. There are some instructions in the beginning of this Engineering Note (pdf link) that discribe how to iteratively determine the PID coefficients for a linear system. I'm not a mathematician, so I can't vouch for whole document, but their somewhat antiquated instruments DO workSmiley Happy

Now is the right time to use %^<%Y-%m-%dT%H:%M:%S%3uZ>T
If you don't hate time zones, you're not a real programmer.

"You are what you don't automate"
Inplaceness is synonymous with insidiousness

Message 5 of 9
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Sorry, but these answers were only little help...

look´s like there are no real helpful rules to get the right, only signs for bad cicle-times... Smiley Sad

...
CU
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Message 6 of 9
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Hi RT,

Concerning to the loop time to take, as dev said it depends on your application, .....but you getting the values from the sensors every 50ms, so i would suggest to take this also for the loop timing!

Concerning Ti and Td, they will be handled as you set them, sure there is a relation between them and the dt, because of the digitalization (you are working with discrete values). But this all will be handled in the background of the PID control.

Regards, Christian

Message 7 of 9
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... I figured out that for some different configurations it is better to take a higher looptime (so the PID will react slower), because it is the only possibilty to keep the oscillation in a tolerable limit, even if this ist to high for my construction....

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Message 8 of 9
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Hi RT,
 
Sure, the looptime is one value which takes influence of your control system, but i would suggest
to set it due to your systems benefits, and then change the values (Ti, Td, V) of the PID controller
to avoid the oscillations.
 
Christian
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Message 9 of 9
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