[time-nuts] Making a HP 10811 and similar types better in the real world Part I

Bob Camp lists at rtty.us
Wed Sep 21 23:08:02 UTC 2011


If the thermistor is on top of the heater, the heat will cut back before anything inside the oven overshoots on warmup.


On Sep 21, 2011, at 7:02 PM, J. Forster wrote:

> That is not always as easy as it sounds.
> The thermal equivalent of a "rigid body" does not exist. If you apply heat
> to a block of metal at one end, it takes a while for it to propagate to
> the other end. In fact, a long thin rod looks a lot more like a
> transmission line than an isothermal block.
> This matters because if you try and increase the loop gain, the wrap-up of
> the phase shift soon reaches 180 degrees, and the thing becomes unstable
> as negative FB at low frequencies becomes positive FB at higher
> frequencies.
> -John
> ===============
>> Hi
>> Actually, overshoot is pretty easy to eliminate on a conventional OCXO by
>> picking a good location for the thermistor. The heater will always run
>> "hot", but the rest of the stuff does not have to.
>> Bob
>> On Sep 21, 2011, at 6:37 PM, Magnus Danielson wrote:
>>> On 09/22/2011 12:30 AM, Chuck Harris wrote:
>>>> Perry Sandeen wrote:
>>>>> GM List,
>>>>> ..............................................................................The
>>>>> ovens are proportionally controlled. On start-up all ovens,
>>>>> proportional or not,
>>>>> will have over-shoot. Some more, some less. An inescapable fact of
>>>>> life.
>>>> Imagine that the set point is variable, and can be set below the
>>>> desired
>>>> temperature. Then imagine that the set point can approach the desired
>>>> temperature more closely as it gets closer to the desired temperature.
>>>> ... And you will have discovered (100 years late) the PID controller.
>>>> PID controllers do not have to overshoot the desired temperature. It is
>>>> not an inescapable fact of life. 30 years ago I was designing PID
>>>> controllers,
>>>> with a little microprocessor magic, that could quickly arrive at the
>>>> set
>>>> point
>>>> temperature and never, I repeat, never, exceed that temperature.
>>>> Someone's
>>>> internal organs would have become toast if it did.
>>> Overshot is fairly easy to avoid for a well controlled PID loop simply
>>> by setting the damping factor properly.
>>> PIDs is nice in that you can control loop bandwidth and damping factor
>>> fairly well. Overshot properties vs. damping factor is a well researched
>>> field and already tabulated before I was born.
>>> Cheers,
>>> Magnus
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