On Sun, 6 Dec 2009 19:31:56 -0600, "Karl Townsend"
wrote:

My first winter project is on my new high tunnel (greenhouse) High
temperature control by ventilation is critical for good growing. I will be
ventilating by using a rollup side curtain. I plan to automate the process
by using a small 12 volt DC gear motor and wench. I've settled on my
mechanical design and plan to order parts tomorrow.

My query is on control. KISS is the order of the day here. I plan on two
thermostats - Hi and Lo. Also two limit switches - full open and full
closed. The control is simple - if you make Hi; roll up for a few seconds
and go to delay timer. If you make Lo unroll a few seconds and go to delay
timer. Stop on limit switches. Repeat 24X7.

I'm thinking this is PLC territory. I know nothing about them. Is there an
inexpensive simple to program unit that runs on 12 volt DC? Alternatively, I
can see a way to do it with a timer that continuously does a few seconds on
then several minutes off continuously.

Karl

If outdoor temp is higher than your minimum, if the temp ever got high
enough to open the curtain it'd stay open until outdoor temp drops.

If outdoor temp is lower than your min but there's enough sun to heat
the space to above max with curtain closed, then the system would
cycle, alternately opening and closing.

If outdoor temp is lower than min and there's not enough sun to reach
max temp, then the curtain would stay closed.

If you only partially move the curtain and then go to a delay timer
for a while to see if that move drops temp below max, might it take
unacceptably long to fully open on a high temp condition? The time
constant will be quite different on calm days vs windy days.

A PID controller with a temperature sensor would provide better
temperature stability, but at the expense of battery life because the
curtain would be making small moves very often. You'd need curtain
position feedback too.