Last winter we tried heating our green house without success and large electric bills. This year we tried Warm Dirt. I made a wood box about coffin size. Melanie and I lined the bottom with Electric Heat Cable, poured in 4 inches of sand, then placed her clay pots on the sand. Keep the roots warm (above 40F) and you're good to grow.
Here I'm putting the plants to bed. Yes yes, I do get dressed up to tend the garden. Don't you?
I first started with an simple AC timer that switched the heater tape on and off, but we didn't really know any temperatures. So I whipped up some probes and started looking at the data. Here's the four temperatures I was collecting: heated dirt, potted dirt, air inside the box, air outside the box. The purple line dropped at 10AM because that's when we removed the box lid that morning.
The timer worked fine for a while, but then obsession took over, I needed more data, more control, more power. I must be the master of my domain!
I started the WarmDirt project.
Here's the WarmDirt README file
Here's the WarmDirt System Architecture. Not to complicated.
Here's the WarmDirt schematic. Click on the image for hi-res version
Sure there's little problems, like via hole placement. No show stoppers though.
The top silkscreen isn't too high resolution, but I can live with that.
It soldered up like a dream!
I mounted WarmDirt in a NEMA-4 outdoor electrical enclosure. Its got to be waterproof, this is a greenhouse application. I also used IP68 cable glands.
WarmDirt uses a triac-based solid state relay, SSR, for AC power switching. With an SSR, I can pulse-width modulate the AC powering the heater element for a much finer control of temperature. I fussed with a PID control algorithm for a week or so and tweaked the PID constants into shape. This plot looks like a mess, but dark blue line tells the story, the temperaturepotteddirt never goes below setpoint of 47F. Now we're talkin'
You PID jockeys will see that my constants aren't quite tuned here, the green line has some ringing. The system time constant is about 3 hours and
around 3/3 I was still playing around with P and I. Its much better now.
WarmDirt supports either 1 stepper motor or 2 servo motors for
lifting the box lid. Here's the original motor, a stepper with a
3D printed spool that's kinda cool.
While cool, it failed. The holding torque wasn't enough to hold the wood box cover open. OK, plan B, use a linear actuator.
The completed WarmDirt box. The control algorithm opens the lid
when the greenhouse temperature is above 43F and closes below 41.5F.
I don't need to put on my tuxedo everyday now, yay!
I received the linear actuator this week, then spent Saturday
afternoon rigging it to the lid. This linkage isn't
stiff enough (see the twisting) and the coupler is crappy, I'm
an EE not a ME. When I learn to weld, I'll replace this.
Click to watch Warm Dirt in Action
This is a time lapse movie. There's also a power supply issue with WarmDirt. The current design only supplies the linear actuator with 9 volts at 1 amp of power. It works but is really slow, 5 minutes to open or close. I'll also replace components for more power in the future. WarmDirt is a work in progress, just like all gardening projects.