Temperature Controlled rasPi 3B

[Achim Gratz]

I've had another go at the temperature control loop.  I'm not using an
integral term for now since it leads to large oscillations.  Instead I
opted to use delta control using weighted data from three different time
scales.  It still oscillates a bit since I do want it to react very
quickly to transients caused by CPU spikes from cron jobs and the like.
The upshot of all this is that the short-term temperature fluctuations
are a bit larger now, but the average is much closer to target (the
remaining bias seems to be caused by the fact that the upwards and
downwards time constants are not identical).

I've also added the ability to change the setpoint by writing it to a
file and then notifying the control loop via signal that it should be
read again.  Besides just changing the setpoint, it's also possible to
change to the new temperature with a controlled ramp rate.  When that
was in place, I've added another parameter to do linear sweeps around
the setpoint, too.  That was easy since I only needed to change the
endpoint handling; either stopping at the endpoint or swapping it around
for the next leg of the sweep.

So here are the last few days of my rasPi 3B seeking it's zero-TC point
(it's placement & configuration got changed in the meantime, so it's a
bit different from the old one).

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Regards,
Achim.
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