Solid State Wind Sensor
![CounterRotatingProps](https://forums.parallax.com/uploads/userpics/360/nWQG6ZTFJSJXM.png)
Never having thought of using ultrasonics to measure wind speed, I was left scratching my head after seeing this $2,550 sensor:
http://www.stevenswater.com/catalog/stevensProduct.aspx?SKU=%2793560%27
With the water-resistant ping))) on the near horizon, I was wondering if two of them - or·two hacked versions could be configured to do the same thing - for a heck of a lot less. Of course the accuracy would likely be far less, but still seems like a cool idea?
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http://www.stevenswater.com/catalog/stevensProduct.aspx?SKU=%2793560%27
With the water-resistant ping))) on the near horizon, I was wondering if two of them - or·two hacked versions could be configured to do the same thing - for a heck of a lot less. Of course the accuracy would likely be far less, but still seems like a cool idea?
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Comments
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- Stephen
-Phil
Rich H
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The Servo Boss, a 12 channel servo tester kit from Gadget Gangster.
-Phil
would the ultrasonic freqs be correct for this?
how much would humidity affect the true speed?
RE The Beagle on Mars
Nice find W9GFO - the circuit described does both temp and windspeed - neat.
- I'll try to dig up some of the refs from that biblio.
"Range 0.5 to 30 m/s " So the top end on Mars is ~= 67 mph ... not even a Category 1 on Earth [noparse]:)[/noparse])
@Phil:
are those "heated film strips" in that circuit what you call 'self-heated thermistors' - is that the same as a "hot wire" thermo?
" The hot film form one half of a voltage divider, with an amplified output. "
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····Vsound + Vwind
and in the other,
····Vsound - Vwind
Subtracting one from the other, you get,
····2 * Vwind
I was wrong about subtracting times, BTW. You need to compute the velocities first from the complementary times and subtract those.
I have no idea about the heated film strips. I was referring to garden variety thermistors, like you might get from Radio Shack.
-Phil
http://www.vaisala.com/weather/products/wmt52.html
Here is Vaisala's description of the operating principle:
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Tracy Allen
www.emesystems.com
Tracy, you have some interesting weather related stuff at emesystems!
I think the ultrasonic, non-moving sensor is just too cool ... but I'll probably end up using the cups be cause they're just too inexpensive [noparse]:)[/noparse]
- H
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If air was moving at the speed of sound, and you sent a sound signal along the same path, would the speed of the sound wave with regards to the air be zero? This would cancel out the sound wave? Like it would have an infinite wavelength? What about doppler effect? Just curious.
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Don't worry. Be happy
I suspect that these type of wind speed sensors would start to fail near the speed of sound. For more nomal ranges of terestial wind speeds they seem to work pretty good and no moving parts can be a good thing.
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Searider
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- Stephen
A simple calculation. Assume two PING sensors are paralell to the direction of the wind:
Windspeed: 50 m/s
SpeeOfSound = 340 m/s
Frequency: 40kHz
f = (340+50 / 340 - 50) x 40 kHz = 53kHz.
I guess that would pose a problem for the PING sensor?
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Don't worry. Be happy
RE doppler - the airspeed thing is the same as the moving-fire-truck siren phenomenon, but instead of sound, its air.
> Windspeed: 50 m/s
Yikes!!
at 111.8 mph, that's the low end of a Category 3 hurricane, man!
At that point, I'd be less worried about the function of my PINGs))) than I would be about saving my *ss))) :-P
- H
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Don't worry. Be happy
It would be a nice application for a Prop as the µprocessor. The emitters on these things are very specialized though. Not off-the-shelf.
It has nothing to do with the Doppler effect. Maybe look at it this way. Someone sends you a package via UPS ground. It arrives in one week. If they sent it via UPS red it would arrive the next day. But if they send you one package a day, either service will subsequently bring you one per day. After the first one arrives. The identifiable package is analogous to the "group velocity" and the passage of packages tick tick tick is analogous to the "phase velocity". Maybe it is better to think of a conveyor belt. If the distance between you and the person putting widgets on the conveyor belt is constant, and the speed of the belt is constant, then your rate of widget arrival is also constant. But Doppler effects happen if the person putting things on the belt is moving toward you or away from you, or if the belt itself is accelerating..
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Tracy Allen
www.emesystems.com
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- Stephen
If you were to measure the flight time of a plane flying a "race course" pattern from say, Denver to Atlanta at a constant air apeed, you could tell both the direction and speed of the wind by measuring the difference in time between the two halves of one "lap".
Duffer
If you propagate a wave in a moving medium, the wave will experience a doppler effect. This is a fact.
The point here is that the effect is cancelled on the receiver side, because the receiver is moving with equal speed in the opposite direction of the medium.
If you were moving inside the air, along the path of the wave, you will see the doppler effect, but then it gets cancelled when you stop.
But if the emitter and receiver are placed perpendicular to the moving air (or any angle), you'd have a problem.
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Don't worry. Be happy
- H
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I agree that the UPS truck and conveyor analogies are lame, but I hold to the conclusion that there is no Doppler effect in a steady laminar air flow. I'm not sure if you agree with that or not? That is, the tone you hear from a fixed whistle will be the same in a 20 kmph steady wind as it is in still air. No frequency shift for the observer, no Doppler effect. Granted, the wavelength in the wind moving along with the sound wave will be longer than it is in still air, but it is a also cumbersome to argue that the Doppler effect is canceled because the receiver is moving with equal speed in the opposite direction of the medium. In these wind sensors, the arms are solidly fixed in space.
The Doppler shift occurs when the air flow itself gusts and lulls, but the effect will not be huge and will be within the detection limits of the device. These wind sensors depend on the group velocity, how long it takes a packet of sound to travel the distance between the arms, not on the phase velocity or exact frequency. Wind gusts might affect the frequency, but how much is wind speed going to change during the 33 microseconds of each sound burst?
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Tracy Allen
www.emesystems.com
This might work.
Two low impedance thermisters and two fixed resistors form a wheatstone bridge configuration.
The two thermisters share a common current node. The two fixed resistors share the other common current node.
Both thermisters are self heated by a current source and are exposed to the ambient air, however, only one is exposed to the wind.
With no wind, the bridge is balanced and the resulting differential voltage is zero regardless of temp. and humidity.
With wind, heat is carried away disturbing the balance and differential voltage.
The differential voltage is now proportional to the wind speed.
And yes it is very sensitive.
Heathkit used the same idea for their old engine exhaust gas analyzer. One thermister is exposed to the ambient air. The other is exposed to the exhaust gas after the exhaust gases passed through a heat exchanger to bring them down to ambient air temperature. There is a direct mathematical relationship between the thermal conductivity characteristics of the exhaust gas and the air fuel ratio.
I hope this helps.
SJW
Here is a link to the heathkit schmatic.
http://www.nostalgickitscentral.com/heath/schematics/heathkit_schema_ci1080.gif
SJW
I loved heath kits back in the day!
@PJ Allen - OK, let me repeat back to see if I get it:
So the pacing of the ultra-sonic packets, the "tick tick tick" is directly affected by the windspeed? (Duffer's Airplane)
e.g., packets going against the wind arrive later that packets in still air? And, complimentary:
packets going with the wind arrive sooner than in still air?
Hmmm. ... the wind is whistling between my ears on this Doppler thing ~[noparse]:)[/noparse])
- Howard
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Try to look at it this way. Instead of having the air move, make the sensors move while the air is still. This is equivalent. You'll see that there for sure is a doppler effect.
My point was that since both emitter and receiver are moving with equal velocities (they are mounted on something), the effect gets cancelled. This only goes if the air is moving paralell to the sensors. The "stretching" in one end becomes "compressing" in the other.
The doppler effect holds for air as well as any other medium. That is, if the emitter, receiver, or both is moving relative to the medium. If you had a bunch of air moving at the speed of sound, and you sent a sound wave out in the same direction, the wave would'nt propagat at all, and would be completely cancelled by the doppler effect. (what an interesting experiment that would be).
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Don't worry. Be happy
-Phil
Post Edited (Phil Pilgrim (PhiPi)) : 9/21/2009 11:50:19 PM GMT
The doppler effect occurs if the emitter and observer are moving relatively to each other *OR* the medium between them moves. Your statement is not correct. In this case we are just lucky that the wind is blowing parallel to emitter and observer, and the effect gets cancelled. If the wind blows in any other direction, the doppler effect orccurs, and you need some serious vector integrals to calculate the magnitudes.
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Don't worry. Be happy
The only time a "Doppler" effect might be observed with stationary transducer pairs is in the case of non-laminar (i.e. turbulent) flow, as Tracy alludes to. In this case the windspeed oscillates due to eddies as it traverses the gap between emitter and detector and will arrive at the detector with a frequency that appears to modulate about the centroid frequency of the emitter. Turbulent flow and the short-term effect it has on measured wind velocity is one reason the transducers in the illustrated wind sensor are so skinny: to minimize turbulence.
-Phil
Post Edited (Phil Pilgrim (PhiPi)) : 9/22/2009 7:57:29 AM GMT
www.glafreniere.com/sa_Doppler.htm
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Don't worry. Be happy
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Tracy Allen
www.emesystems.com
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Tracy Allen
www.emesystems.com