[time-nuts] Strange oscillations

Alberto di Bene dibene at usa.net
Sat Sep 4 12:01:13 EDT 2004

   Bill Hawkins wrote:

Proportional control does not have any hysteresis. The thermistor
and the oven control should also be free from hysteresis. You are
correct that you need hysteresis to have a limit cycle. What you
have is a limit cycle because the amplitude is relatively stable.


     thanks for your observations. Could it be that the PI(D) thermal
   controller of the oven
   has a residual oscillation ? Just a guess. Certainly it is not a
   bang-bang, as 0.18 Celsius
   seem way too low for a bang-bang controller.

Your reference oscillator is cycling. What are you using to measure
to 10E-11 accuracy?

   I measure it indirectly. I measure the EFC correction voltage as
   computed by the AVR
   microcontroller. That OCXO has a slope of about 1.5 Hz/V, so when I
   see that the EFC has
   changed by 70 uV, I reckon that the change in frequency is about 100
   uHz, which at 10MHz
   is 1E-11

Your line voltage (heater supply) is cycling. Proportional control
needs some error to change the heat to the oven, so it is sensitive
to changes in all ambient conditions. Can you measure the heater
supply voltage to 10E-11? Probably not, but if the proportional gain
is 100 you only need 10E-9, still an expensive measurement. Try using
a battery for the oven supply.

   That's a thing I can do, albeit I don't think that is the cause. The
   12V to the oven heater
   come from a 7812 regulator, a cheap device, agreed, but it is not
   known for oscillating
   with a period of 5 - 6 minutes.

It is possible that the designers were counting on the enclosure to
stabilize the oven. If the controller is actually proportional plus
integral (and maybe plus derivative) then the controller tuning may
be too sensitive for an oven exposed to ambient air. Try insulating
the OXCO with packaging foam or fiberglass so that it is completely
enclosed except the wires. That is easier than adding thermal mass,
but if the OXCO is supposed to mount on a plate, mount it on a plate
about twice the area of the mounting surface. Copper would be nice
but aluminum is cheaper.

   I will stuff the box with packaging foam when closing it, and then I
   will check again the behavior.

Remember that you are making a very sensitive measurement. Your
presence in the room could change the heat required because you
radiate about 500 watts in the infrared.

   Yes, as I said a 24 hour log showed clearly the disturbances produced
   bt my presence in the room.
   What I didn't know is that I am a 500 W furnace...:-)
   If now the list server is more benign (thanks John), I try to embed a
   capture just done this afternoon
   Thanks for your help
   73  Alberto  I2PHD
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