Don Foreman fired this volley in
:

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


In which case, no position sensing would be necesary, at all.

If the curtain hits a limit, it just stops at a mechanical limit switch.
If the controller senses that the temperature stays a certain amount past
a warning limit either way, and isn't making an excursion toward the
desired temperature, it signals an alarm saying it can no longer cope.

That's a good idea, Don. Dead-simple, too. No PLC is necessary if the
out-of-control alarms aren't needed: Just a timer and temperature
comparator with a "safe range of temperatures" dead-band. Wait a period
of time, and decide to move the curtain, or not, then do it again, over
and over.

LLoyd