[time-nuts] Fury - Rubidium - PIS

J. Forster jfor at quik.com
Tue Jul 27 22:02:56 EDT 2010

> That part I understand (your drawing), its a basic phase lock loop.
> What I am having trouble with is the Fury's commands relationship.

OK. Sorry for the BW.

Basically you tune a loop by starting with the P, I, and D set to zero.
You slowly crank up the P until it starts to become unstable. (Put in a
small step perturbation and look at the response for ringing) Then crank
up the D until it stabilizes, then crank up the P again. When you have got
a stable fairly well performing loop, you introduce some I. You may have
to tweek P and D to keep stability.

It looks like your system has an overall gain (DACG) and a P and D
controller gain. This is not uncommon to avoid switching a bunch of caps.



> The Fury controller has the following SERVO commands to set up the loop:
> SERVo:DACG which is the DAC gain, a control voltage range ?
> range is 0.1 to 10,000 -- the DAC is 0 to +5V
> SERVo:EFCS which is the EFC Scale, proportional gain of the PID loop
> range is 0.0 to 500.0 -- 0.7 example for a good double oven and 6.0 for
> a simple single oven
> SERVo:EFCD which is IIR filter time constant
> range is 0.0 to 4000.0 -- example between 10 and 50
> Thanks - Brian KD4FM
> On 7/27/2010 8:36 PM, J. Forster wrote:
>>> I read the article on PID on Wikipedia last night.  I do not fully
>>> understand it, but I see/learning some of the relationship.
>> Here's a very quick primer:
>> Consider a very simple control position servo loop:
>> Pos. Input --- + (SUM)--- PID --- AMP>  --- MOTOR ===== Output Pos
>>                     |-                               ||
>>                     |                               POS Sensor
>>                     |                                 |
>>                     -----------------------------------
>> If you put an upwards step into the Pos Input the output of the SUM goes
>> up. This is applied to the AMP via the PID network and the MOTOR stasrts
>> up, turning the output shaft. As the Output shaft turns, the position
>> sensor output rises. That subtracts from the commanded position in the
>> SUM, reducing the AMP input.
>> Thats how the P = Proportional signal drives the loop to null.
>> However, in order for the motor to turn some non-zero voltage needs to
>> be
>> applied. As the SUM output approaches zero the motor drive ceases and
>> the
>> loop never reaches null. So the I = Integral term is added. If the loop
>> stops just shy of null, the SUM output will not be zero. The I
>> Integrator
>> takes the near-null voltage and integrates it (Vsum dT) which will
>> eventually rise sufficiently to drive the motor to null.
>> However, the motor does not stop instantly when the SUM reaches zero
>> because of inertia, so it overshoots. So the D = Derivative term
>> (dVsum/dT)is added in to cut the motor drive as the loop approaches
>> null.
>> Note, in general the I term is destabilizing and the D term is
>> stabilizing, as long as you are considering frequencies below where the
>> othy components have significant phase shift.
>> FWIW,
>> -John
>> =================
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