[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


Hi

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

Bob


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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