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Unread 18-02-2015, 17:18
jwhite jwhite is offline
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Re: PID Tuning Help

Quote:
Originally Posted by SousVide View Post
3. slowly bump I up until the desired time to target is good for you even if it begins to become unstable.
Again, for more completeness, where should you start? We mostly gave up on trying for PI controllers, because we found that even apparently small values of I led to instability. (The 1114 guide to PID tuning suggests PD; a bunch of smart people I know swear by PI. I have no real experience other than sitting with students trying this for a few nights in a row).

We successfully tuned one with I, but only when our I was several orders of magnitude less than the P term.

Is there a rule of thumb? Start with an I that is X of P, and then increase slowly?

As a side note, if you're tuning I, one of the challenges you may face is that the smart dashboard allows a tiny number of significant digits. (Afair, the lowest we could go was 0.001). We ended up making a scaled pid output just to get around that problem.


Quote:
Originally Posted by SousVide View Post
5. if necessary, D can be adjusted up slightly to eliminate any further overshoot. Too much D and the system will become unstable again.
And, again, any rule of thumb? As we were tuning PD, we tended to start D at the P value, or perhaps below, and then gradually increased from there.
 


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