# [time-nuts] (no subject)

Didier Juges shalimr9 at gmail.com
Sun Jan 25 07:42:07 EST 2015

```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 <kb8tq at 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.kd5ssj at 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 <kb8tq at n1k.org>
>> To: Discussion of precise time and frequency measurement
>>        <time-nuts at febo.com>
>> Subject: Re: [time-nuts] Questions regarding tuning Thunderbolt with
>> Message-ID: <6581EB02-9792-432F-B143-25B41FB290DB at 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.
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--
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