As already stated, there are many different ways to design and or tune a
temperature controller so it does not overshoot.
The trade off being that it may then take a little longer to get to the
finial temperature.
BUT SO WHAT??? If one is concerned about a little oven overshoot then they
are missing the much BIGGER picture.
Why was the oven off in the first place? That will cause such a bigger
problem, No one is going notice a little overshoot.
The Osc needs to be kept powered up with the oven on.
If the primary temperature sensing thermistor is on top of the heater,
you have already compromised the oven design because the xtral is not going
to be at that exact same temperature.
If you need to go that far to stabilize a hard to control loop, then better
to add a second thermistor.
(Not a dual oven, Just One heater, with two temperature sensors, where the
outer thermistor ia outside the error loop but acts as a feed forward to the
PID loop)
Concerning the 11 steps outlined on how to make a HP10811 better
Several of them are unnecessary and way overkill, or there are simpler ways
to get even better results, and some I find even undesirable.
Just one example, don't always need some super low noise voltage reference
to drive the EFC.
Just need to reduce the range to no more than necessary because the best
voltage reference you can use is ground.
Example:
If you're going to use a GOOD pot to set the EFC voltage them best to limit
the pot's tuning range to about 1000 times the desired resolution.
1e-12 resolution x 1e3 = 1e-9 total Pot tuning range, which means the Pot
voltage should be divided down by about 1000 before it is connected to the
EFC.
ws
***********
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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