[time-nuts] Poor man's oven
Chris Albertson
albertson.chris at gmail.com
Tue Jun 6 14:00:57 EDT 2017
On Tue, Jun 6, 2017 at 7:34 AM, Attila Kinali <attila at kinali.ch> wrote:
>
> 1) You want the control loop as stable as possible
> 2) Stability is directly related to controllability
> 3) The larger the heat flow, the better the controllability
> 4) therefore the outside temperature should be as low as possible
I think you are correct but within reason of course. It is easy to see
that the extremes can't work. If the internal set point is very close to
ambient the oven is uncontrollable. because you only use the first bit of
the DAC to control the heater and after a few seconds you have overshoot.
Moving the set point up lets us use the full range of current on the heater
can gives us 8 or 10 bits of control and the rate of change is slow enough
that we have time take thousands of samples and see a rate of change in
temperature. The PID algorithm needs something that is slow to change
compared to the control loop cycle. So you want a good size thermal mass
compared to the amount of heat.
At the other extreme, where the set point to far above ambient we would
need to run the heater full time and also loose control. So I disagree
with #4 above. The heater would have to run full-on at 100% duty cycle.
(In other words avoid using liquid nitrogen baths)
There is an optimum were it peaks but I don't know how to find it. Look
at the specific heat of the thermal mass (likely you are using aluminum)
and multiply that by the mass and I think you want that to be large
compared to the heat from a full-on heater so that the rate of change looks
slow compared to your control loop cycle.
--
Chris Albertson
Redondo Beach, California
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