crice said:Thanks for your reply.
The output should preferably be a DC voltage. I'm interfacing this anemometer with another device, and the most ideal situation for me would be to "poll" the output voltage at any given time to obtain the correct windspeed (after doing calibrations).
I forgot to add that I also thought about what you had mentioned, philba. I played with the idea of using a reed switch + written software to count the number of pulses from a spinning set of cups or fan. The main problem here is that I think I would have to keep my device on at all times to obtain an accurate reading of the speed. I believe for the DC voltage case, I can wake my device up, get the voltage output reading, then make the device go back to sleep so that I can save precious battery life.
EDIT: What I'm most interested in is a frequency-to-voltage chip that requires a very low voltage supply.
crice said:The output should preferably be a DC voltage.
philba said:Does the output have to be a DC voltage?
dknguyen said:DC motors put out a DC voltage when back driven, while AC motors put out an AC voltage when back-driven. The internal commutation does that. So using a DC motor as a tachometer would give you a DC output.
If you need a to count pulses for frequency but need the signal to be unipolar, maybe use an AC motor with a diode overvoltage clamp or something to produce a square wave.
crice said:Hello all, I am in need of some design advice.
I am designing a low power anemometer. The main hurdle here is that I can work with only a 2.5V supply (with 3V as the absolute maximum because I am running on 2 AA batteries). Also, I need to have a voltage output. So, if I were to look at the output voltage of the anemometer, I can correlate the voltage to a wind speed. I only need to detect wind speeds of roughly 150 fpm to 1200 fpm (sorry I am from the US). I have toyed with many ideas of how to approach this problem.
First, I thought of using a DC brushless fan to generate voltage. When a wind forces a dc fan to spin, it generates a small but measurable voltage. I thought of somehow correlating wind speed to the output voltage in this way, but I could see no discernable pattern or correlation. Also, the fan was not very sensitive to low wind speeds because of the internal metal coils. I tested this with a 40x40x6mm Sunon DC fan. Anyone have ideas? This would be the most ideal solution for my design, because it requires no extra circuitry to function. All it needs is a DC fan.
My next idea was to build the homemade easter egg anemometer as documented here: https://www.otherpower.com/anemometer.html
However, the problem with this anemometer is that LM2917 frequency-voltage chip they use requires a very high supply voltage. Is there a low power version of this chip? I cannot seem to find a frequency-voltage chip that needs less than a 9V supply.
This is where I am stuck. I have been researching some analog tachometers, but I am not sure how they apply to my design. So my dilemma is this, I need to design an anemometer that outputs a voltage and only uses a 2.5V supply.
Any ideas would be greatly appreciated!
philba said:That's an interesting idea but has some issues.
- weatherproofing the pot. not impossible but needs to be able to withstand any weather, including moisture/humidity/...
- you will only use a small part the pot's swing - say 45 degrees.
- above a certain speed you will have value compression rendering it useless. for example, the difference between 30 and 31 KPH may well be less than the noise level.
- the assembly needs to pivot to face the wind. more complexity.
- what effect will rain have on this? it will make the vane heavier than thus not deflect at the same level.
- I don't see long term reliability in that design. pot wear, hinge pin wear, corrosion, dirt, ...
I disagree that timing pulses is complex. No moving parts other than the actual cup assembly. The electronics can be sealed to the weather, analog noise is not an issue, no need to pivot into the wind direction and the upper limit on speed is basically the cup assembly's structural integrity.
Klaus said:Well, if you think about it, you would also have to:
weatherproof the shaft of a rotating cup anemometer (I built one and its NOT an easy job if the tiny ball bearings should last. Its been going on my roof for over 20 years now and it does need maintenance every 6-8 years or so).
You can use a simple plastic gear set to increase the pot travel 2 or 3 times.
You lost me with your value compression statement but I would challenge you
to build a rotating cup anemometer that can reliably differentiate between 30 & 31 Kph. You are doing very well if you get reliable increments of 5 units over the whole range. Keep in mind that most mortals do NOT have access to a wind tunnel to calibrate the thing.
Keeping a vane facing to the wind is no big deal, its done all the time on horizontal axis wind generators.
Rain does affect the rotating cups too, the effect on a vane can be minimised by a water repellant surface.
Prevention of wear & dirt ingress, corrosion, etc. is common to ALL electromechanical anemometers.
All I did is suggest a simple system that can cope with the limited supply voltage. You would be battling to drive the timing pulse electronics from just a little 3V battery as the original poster specified.
A vane is certainly MUCH simpler to build than a reliable rotating cup anemometer. it should be quite sensitive if the vane is partially balanced. The original poster DID specify a range of windspeed and it could be tailored to that.
Now, if you could figure out a way to get an electronic readout from these ball in a tube venturi anemometers then you would have cracked the simplicity challenge and I' tip my hat to you
Klaus
philba said:the compression issue is that the reading are non-linear. as the deflection increases, the force pushing back against the wind increases and thus increasingly more force is needed to deflect the vane. as the deflection increases, the "sail area" decreases as well. above a certain wind velocity there will be no deflection. the vane deflection approaches this point asymptotically. as the vane approaches this asymptote the differential readings will decrease. Since this is an analog system, noise will come into play. when the difference between 2 readings (my example between 30 and 31) is less than the system noise level you can't distinguish.
with a pulse measurement technique, it is trivially easy to very accurately measure the rotational frequency. not only can it discern between 30 and 31, it could discern between 30.9 and 31, if that matters much. Yes, you will need to do filtering as the wind velocity is constantly changing. And, yes, we do agree that calibration is an issue for both designs.
the problem with the rain issue on a vane is that it also changes the force on the vane. I'm not even sure how to model it though.
more mechanical stuff = complexity and unreliability. I agree that both approaches have corrosion issues but fewer moving parts is better, imo. that's why I think a rotating cup approach is better.
by the way. roller blade bearings are pretty good. they are very cheap, take side force well, are sealed and last a long time.
crice said:Thank you for all your input.
However I'm sorry, I forgot to update my original post. This anemometer that I am designing will be used inside air ducts within buildings or houses, hence the very small range of wind speeds. I won't have to worry about natural, outdoor corrosion or related problems.
I've been looking into magnetic reed switch sensors to provide the necessary pulses for the frequency-voltage IC. Anyone have experience with these?
Nigel Goodwin said:They work fine, but you have the added weight of the magnet to move with your 'fan'.
Have you considered ultrasonic methods?, no moving parts!.
crice said:Can you elaborate on the ultrasonic methods?
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