Pic based Wind Speed meter

[Charles Linquis]

I'm advocating that you NOT use the traditional hot wire approach. Most of the hot wire sensors use either a (special) purchased wire, or have you break a small incandescent bulb to get the wire. Both are relatively fragile. When I did this project I found some small (approx 3/8" diameter X 1/8" thick) PTCs in the DigiKey catalog. I don't see them there anymore. You could perhaps use one from

http://www.specsensors.com/custom/co...er.asp?thm=srh

The idea is that they have a low resistance so that when you apply 5V you can get enough power into them to easily heat them up to their "knee" temperature. The ones that I chose had a 5 ohm resistance and a knee temperature of 110C. When you apply 5V across them, they initially draw 1A (5Watts), which quickly heats them to their knee temperature, at which point their resistance rapidly increases and the current falls. If you try to cool them (by moving air across them), the current draw increases. Because of their self-regulating nature, they draw as much current as needed to keep their temperature at 110C. If you put a small (.1ohm or so) resistor in series with the PTC thermistor, the voltage drop across that resistor is proportional to the cooling effect of the air - which is proportional to the velocity. You do have to amplify that voltage in order to get it p to a good range for the PICs A/D converter.

The airspeed to voltage drop across this resistor is not a linear function. Also, be aware that you must compensate for ambient temperature, since a 20 MPH wind at 0C will cool a hot thermistor more effectively than a 20 MPH wind at 30C, so you need to add a second temperature sensor to measure ambient and use that in a calibration routine. I can't really give you the calibration code that I used because I'm certain your thermistor will be different than the one I used.

Still, with a little work and some calibration, the method outlined performs very well. I have used it to measure air velocities in electronic cabinets.

[Bobbo_ZA]

Dave, that would work, although I would expect it to need quite a stiff wind to get over the mass of the bells? what did you use for a pickup? are there any more details of your project posted anywhere?

I was thinking about it again the other night while playing on the breadboard, would it be possible to have the magnet cause an interupt, and then meassure the time ellapsed when the magnet triggers it the second time and then reset and wait for the magnet again?

I know this will likely result in a fairly "slow" refresh but im assuming we are talking milliseconds anyway, :-)

Any hints on where i can start looking to find info on timing the triggers?

[Dave]

Well Bobbo_ZA, I used 4 rare earth magnets about 1/4 inch square epoxyed to the shaft. The bottom of the shaft sat on a single ball bearing as the thrust bearing. the upper shaft outer bearing was made from a piece of teflon rod, drilled out to slightly biger than the shaft diameter. I calibrated it in the car, on some old country roads when there was no wind early in the monring. The main shaft was made out of stainless and the cross arms to hokd the cups were made of brass rod. I used a hall efect switch for the pickup and it was housed in an outside vertical junction box.

Dave Purola,
N8NTA

[Bobbo_ZA]

mmmm, this is all sounding way past my current / very limited PicBasicskills, sounded quite "easy" at first but seems to be very deceptive.

Had another thought that may make this a bit easier to handle, :-)

If i were to use a differential pressure sensor with the pitot tube mounted on a rotating platform that would point into the wind regardless of direction (weather vane of sorts), I should then be able to measure the airspeed / wind speed from that?

I think the mechanics of rotating the pitot tube will be a challenge but that part I’m quite comfortable with.

Your thoughts?? (Something like: http://www.picbasic.co.uk/forum/show...ressure+sensor - in this topic Melanie references an earlier topic of mine using a 3.9kpa sensor which should give me a reasonable speed range)

[Charles Linquis]

The differential pressure at normal wind velocities would be very, very low.
If I were you, I would opt for the "spinning cup" version. When you look at all the little monitoring stations the government has put around in the national parks, all have spinning cups. And as I mentioned before, I have built such a device that worked - for awhile. My only issue was the fragility of the cups. I like Dave's idea of using old telephone bells. They are a bit heavy, but - being brass,
they should be easy to drill and solder.

And again, I used a 5.25 floppy disk drive motor. It had two good bearings, and most importantly, it had a tachometer output. I decided to use that as my starting point, since I used to work extensively with those disk drives (shows my age). You should be able to find some of those drives in a junk shop.

A few brands (Siemens for one) had a pure, amplified Hall-Effect tachometer output. You could the turn the motor shaft very slowly, and it would still put out a 5V p-p signal. Most of the others just used a separate winding.
You have to use a comparator to get a 5V p-p swing from those. And most of those motors will produce a tachometer output WITHOUT powering the motor. Just try a few different brands. The brand I used was Tandem.

My setup got thrown away when I moved from Iowa to California 19 years ago, otherwise I'd send it to you.





At any rate, the motor had great bearings

[Bobbo_ZA]

Going to go dig in my old PC drawers tonight and see what i can dig up, was planning on a DC brushless fan motor but could not get it to give me anything usefull, perhaps a DVD/CDrom motor may also work.

[Charles Linquis]

DC brushless fan motors often have tachometers. The tachometer is open-collector (you MUST have a pull up), and is powered by the + input.

If you want to use a DC brushless fan only for its bearings and tachometer:

Take the label off the motor (on the hub)
Remove the snap-ring off the shaft to allow the rotor to be removed
Pull off the rotor (blades)
CUT one wire on each of the windings (there will be 4, 6 or 8). This will prevent the fan from running.
Re-assemble

GND the BLACK wire
Put a 2.2K resistor from the TACHOMETER wire (usually white, yellow or blue) to 5V
Connect the RED wire to 5V
Use the signal on the TACHOMETER wire. It will be 5V p-p. and will give 2 cycles/revolution. The duty cycle may not be
exactly 50%. If you use PULSIN you should measure a LOW half-cycle and a HIGH half cycle and add them together.

I suppose if you had a big enough fan, you could use it to measure wind directly. It won't start
spinning in a gentle breeze, however.