Without a D term, PI loops can be unstable when the gain (P) is increased. If you will, with a large error, the correction will itself be large and as the system corrects itself, it may overshoot the target value, going into a low (or high if you really blew it) level oscillation around the target value. The D term slows it down just enough and minimizes that overshoot while maintaining a high gain (low steady state error) and a fast response.
Didier KO4BB On January 24, 2015 8:05:38 PM CST, Bob Camp <kb...@n1k.org> wrote: >Hi > >A classic control loop in it’s simplest form has only one term. That is >often referred to as a proportional term. When the control signal (or >error) changes by A the output changes by A times that term. Often in >shorthand notation this term is refereed to as a P term. > >The next thing that some people add to a control loop is an integrator. >It looks at the control signal (or error) has a constant offset of A, >the integrator sums up the A’s. The output of an integrator would >eventually go to infinity with a constant control input (or error) into >it. This term is often referred to as an I term. > >Lastly people add a term to the control loop that responds to the rate >of change in the control signal (or error). The faster the change, the >bigger this signal gets. This is commonly refereed to as a Derivative >term. In shorthand it’s talked about as the D term. > >The net result is a three element control loop running what’s called a >PID algorithm . > >The P and I can also be described by a time constant and a damping. >That’s what the Trimble software lets you do. The implication is that >it’s just a PI loop. In fact it appears to be a PID loop and you can’t >get at the D term. > >For a much more clearly worded explanation of all this, there’s > >http://en.wikipedia.org/wiki/PID_controller > >Bob > >> On Jan 24, 2015, at 6:35 PM, Cash Olsen <radio.kd5...@gmail.com> >wrote: >> >> Bob, >> >> I am relatively new to the list and still learning the jargon and >> concepts. You wrote: "There does appear to be a D in the TBolt loop. >> For what ever reason, that’s not a changeable value. The D does scale >> with the time constant." >> >> Could you or one of the other members elaborate on the what is meant >> by "D" above. Does it have anything to do with a flat spot in the >> loop? >> >> -- >> S. Cash Olsen KD5SSJ >> ARRL Technical Specialist >> >> Message: 10 >> Date: Sat, 24 Jan 2015 09:18:15 -0500 >> From: Bob Camp <kb...@n1k.org> >> To: Discussion of precise time and frequency measurement >> <time-nuts@febo.com> >> Subject: Re: [time-nuts] Questions regarding tuning Thunderbolt with >> Lady Heather --> GPSDO's >> Message-ID: <6581eb02-9792-432f-b143-25b41fb29...@n1k.org> >> Content-Type: text/plain; charset=utf-8 >> >> >> There does appear to be a D in the TBolt loop. For what ever reason, >> that’s not a changeable value. The D does scale with the time >> constant. >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > >_______________________________________________ >time-nuts mailing list -- time-nuts@febo.com >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. -- Sent from my Motorola Droid Razr HD 4G LTE wireless tracker while I do other things. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.