On Tue, 08 Dec 2009 21:51:02 -0600, Don Foreman wrote:

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.

No problem.

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.

I'm not so sure -- Karl's control rule as stated could possibly find a
sweet spot.

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

Which is the best you could expect, unless you want an alarm to ring so
that Karl knows he's got to go install heaters.

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.

This is what Karl is describing, and you share your concern with me. I
think it could be alleviated considerably with tapered vents -- when it's
cold and/or windy the vents would stay mostly closed, so make sure that
the vents are narrow there, such that an incremental opening of the
curtain doesn't open the vent up much.

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.

Possibly. A controller with a bit of built-in hysteresis wouldn't. Or
Karl may find that really small motions don't drain the battery badly
enough for it to be a concern.

You'd need curtain position feedback too.

I don't think you do, necessarily, beyond limit switches. The motor/
curtain system will just act like an integrating plant; this means that
Karl really only needs a PD controller, probably, or a P-double D (ick).

Position feedback on the curtain would help if you were really going for
low power and/or precise temperature control, however. So would a
monitor on the outside temperature, for that matter.

--
www.wescottdesign.com