Re: Shameless Plug

From: YD (yd.techHAT_at_techie.com)
Date: 11/30/04 

Date: Tue, 30 Nov 2004 20:56:35 -0300

On Tue, 30 Nov 2004 10:59:17 -0800, Tim Wescott
<tim@wescottnospamdesign.com> wrote:

>Guy Macon wrote:
>
>> Tim Wescott wrote:
>>
>>
>>>Guy Macon wrote:
>>>
>>>
>>>> "Why do some controller boards have an option to reverse the
>>>> phase of the D? What is that good for?"
>>>
>>>OK, I'll bite -- what _is_ it good for? I've never done closed-loop
>>>control with prepackaged controllers and I've never seen that done
>>>elsewhere. I can certainly see reversing the phase of the whole thing,
>>>or reversing the phase of the D term if it's coming from some other
>>>feedback source (which would imply a second input) but I _can't_ see the
>>>point in intentionally establishing an unstable zero in your control system.
>>
>>
>> My experience is more hands-on than theory, but here is the answer
>> I gave my classes:
>>
>> In N years of setting up servos, I have never once found a use for
>> it, nor have I found any literature that explains when it might be
>> of some use. I think that somewhere back in the early days someone
>> was told to put in a jumper that reverses the phase of the entire
>> servo (quite handy when someone miswired a section that is really
>> hard to get to), got it wrong, and some other manufacturers have
>> been copying the "feature" ever since.
>>
>> If one of the theory boys has a better answer, I am all ears.
>>
>Being able to reverse the sign of the whole thing is good -- one of the
>old curmudgeonly engineers from whom I learned practical control liked
>to say that when designing one of these things you should count up all
>the sign changes in the loop -- then throw in one extra for the one you
>missed. His circuits always had at least one spot where you could
>rearrange the inputs to an op amp and reverse the sense of a signal. I
>follow that now: there's always at least one place in my software where
>one can insert a '-' and change the sign of the whole thing.

All controllers I've seen, be they in software or hardware, have a
flag or switch to reverse the controller action. Some even let you
reverse the output action. Eases up a lot in initial implementation.

I've seen the ones that let you reverse the terms individually but
haven't found a need for trying them out. One accidentally had the
integral term reversed and I went bonkers trying to tune it before
finding the error.

The derivative term is useful when you have a slow process with
inertia. Once the controlled variable starts to respond it puts a lid
on further controller action which might cause unacceptable overshoot.
On the few occasions I do apply it it's usually in homeopathic doses.

- YD. 

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