On Wed, 09 Dec 2009 11:16:46 -0600, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Don Foreman fired this volley in
:

Yes, because the servo has internal position sensing feedback.

That still doesn't make the entire system a cybernetic loop. The error
signal generating equipment derives no feedback from the effector. It's a
"dead reckoning" system; the controller tells the curtain to "move to x",
and only assumes it has happened.

LLoyd

Agreed, the entire system isn't a cybernetic loop --unless you include
the operator as part of the cybernetic loop. Either way, there is a
position feedback pot in the servo. The servo is a subsystem with
feedback.

You are also correct that the process varible of interest here is
temperature, so the primary loop sensor for PID control would be a
temperature sensor -- not a curtain position sensor.

That said, curtain position is the time integral of output (motor
drive) so that creates a big fat pole in the system response. If a
secondary loop made curtain position a fn of the PID's output the
system might be both more responsive and easier to stabilize. Curtain
position determines rate of heat loss which the PID control then
matches to rate of (solar) heat input to minimize error from setpoint.
But it seems to me that Karl isn't trying to control to setpoint, he
just wants to try to prevent excursions above a max or below a min. So
a more "fuzzy" approach might be a lot simpler, more responsive and
stable with a simple algorithm something like:

start
if temp_too_high then open_curtain_a_smidge
else if temp_too-low then close_curtain_a_smidge
waitabit
goto start