Am 14.02.21 um 03:40 schrieb Mark Rivers:
>> That means KI must *always* be non-zero.
>
> No, that's not true. Consider the case of DC motor where the EPID
> output is a DAC that controls the motor velocity. If the VAL field
> is the motor position, then when the motor is at the target position
> the velocity is 0, so the I term must be 0.
Right. I should have added "... except in case the output variable
should be zero when the target value is reached". I didn't mention it
because it seemed like a very exceptional situation, though your example
suggests it may not be that exceptional after all.
Cheers
Ben
> Am 13.02.21 um 13:32 schrieb Mark Rivers:
>>> Suppose we have been at the target position for some time, i.e.
>>> VAL=OVAL, so that E(n)=0 for all n. Then all terms become 0 and the
>>> output is OVAL=0 as well. But shouldn't it be OVAL=VAL instead?
>> Your statement that "we have been at the target position for some
>> time, i.e. VAL=OVAL" is also incorrect. When it is at the target
>> position VAL=CVAL, not OVAL.
> Yes, sorry, I have confused OVAL and CVAL here.
>
>> I think your statement that "all terms become 0" is incorrect. The
>> integral term (I) does not become zero even when sitting at the
>> desired setpoint for infinite time, because it is the integral from
>> time=T0, where T0 is the time that the feedback was turned on.
> So that means if we start the feedback when E=VAL-CVAL=0 then we will
> first move the system *away* from the desired VAL (unless it happens to
> be equal to zero). Then we accumulate the errors, increasing I at each
> step, until after a while I is large enough to compensate our initial error.
>
> That means KI must *always* be non-zero. I find it a bit strange that it
> initially has to move away from the correct output value only to arrive
> at that value some steps later. Is it possible to preload the I with a
> non-zero value if one has a good guess what its initial value should be?
>
>> The integral term is what prevents "droop", e.g. it is what supplies
>> the constant power required to keep a furnace at 500 C even when the
>> error is 0.
> Ok, I think I understand this now. Thanks.
>
>> I use the EPID record for lots of tasks and it seems to work fine.
>> Have you observed an issue which led you to post this message?
> Not me personally. Some physicists wanted to understand the behavior of
> one of our control loops better, looked at the formula and came up with
> the above objection. I agree that with a non-zero KI it should work
> (after enough initial cycles).
>
> Cheers
> Ben
> --
> I would rather have questions that cannot be answered, than answers that
> cannot be questioned. -- Richard Feynman
>
>
--
I would rather have questions that cannot be answered, than answers that
cannot be questioned. -- Richard Feynman
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