Solid State Wind Sensor

CounterRotatingProps

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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Franklin

I don't think the ping would be the best way to go because there is additional hardware/programming in the ping to do easy distance measurements and that is not what the wind sensor is doing at all. You would be better off getting the transducers and starting from scratch.

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- Stephen

Phil Pilgrim (PhiPi)

I think it could be done with four Ping)))s. Two Ping)))s on opposite sides would be triggered and send out pulses simultaneously. You would have to measure the difference in their return echo times (actually the pulses from their mates) to determine windspeed and direction along the corresponding axis.

-Phil

W9GFO

The Mars Viking landers used an array of hot wire thermocouples to measure wind speed and direction. I couldn't find any description of that but I did find this.

Rich H

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The Servo Boss, a 12 channel servo tester kit from Gadget Gangster.

Phil Pilgrim (PhiPi)

I've often wondered if self-heated thermistors would perform well enough to measure windspeed.

-Phil

CounterRotatingProps

RE 4 pings

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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Phil Pilgrim (PhiPi)

I think the effects of temperature and humidity would cancel each other out. The speed going in one direction would be:

····V_sound + V_wind

and in the other,

····V_sound - V_wind

Subtracting one from the other, you get,

····2 * V_wind

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

Tracy Allen

I have experience with the Vaisala WMT50, which is a low end ultrasonic anemometer, but still pricey (~$800) compared to low end cups and vane.
http://www.vaisala.com/weather/products/wmt52.html

Here is Vaisala's description of the operating principle:

The wind sensor measures the transit time (in both directions) along the 
three paths established by the array of transducers. This transit time 
depends on the wind speed along the ultrasonic path. For zero wind 
speed, both the forward and reverse transit times are the same. With 
wind along the sound path, the up-wind direction transit time increases 
and the down-wind transit time decreases.

The wind speed is calculated from the measured transit times using the 
following formula: 

W = 0.5 * L * (1/Tf - 1/Tr)

where

W = Wind speed
L  = Distance between the two transducers
Tf = Transit time in forward direction
Tr = Transit time in reverse direction

Measuring the six transit times allows Vw to be computed for each of 
the three ultrasonic paths. The computed wind speeds are independent 
of altitude, temperature and humidity, which are cancelled out when the 
transit times are measured in both directions, although the individual 
transit times depend on these parameters. 
Using Vw values of two array paths is enough to compute wind speed 
and wind direction. A signal processing technique is used so that wind 
speed and wind direction are calculated from the two array paths of best 
quality.

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Tracy Allen
www.emesystems.com

CounterRotatingProps

thanks guys, for the detailed replies ... mulling this over as I'm on another more pressing project at the moment.
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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dev/null

I am trying to understand the principle behind these detectors.

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

Searider

Dev/Null,

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

---

Franklin

Google doppler effect for info on what the process is.

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- Stephen

dev/null

I know the doppler effect from relativity. I just haven't dealt with it in this scenario before (moving medium).
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

CounterRotatingProps

Mike, take a closer look at Phil's and PJ's math earlier - they have it.

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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dev/null

Ye maybe unrealistic example. The question is how sensitive the PING's are to the frequency. I'd suspect they are fairly sensitive, so you'd get problems even at low speeds.

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Tracy Allen

Like the PING, these ultrasonic wind sensors emit very short bursts of sound and the µprocessor measures the time it takes for the burst to travel a fixed distance. The arms on the Vaisala WMT50 pictured above are 10 cm apart, and in still air it takes only about 0.3 millisecond for sound to travel that distance. The WMT50 emits at 300 kHz. I'm not sure how many cycles it emits in each burst, but it might not be more than 10, a 33 microsecond burst, which would create a packet of sound about 1 cm long.

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

Franklin

So, staying with the conveyor belt analogy the wind speed is the same as the conveyor belt speeding up or slowing down. That would change the time at which the package arrived and you would calculate the speed of the belt?

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- Stephen

Duffer

An analogy that helped me to understand the principal is that of a coast to coast airline flight. If the plane maintains a constand air speed, its ground speed will be slower (and thus·the flight·will take longer) if it's flying up-wind rather than down-wind. That's the reason that nearly all·passenger flights in the US·have a longer duration·going east-to-west. The jet stream·flows west-to-east and has a significant effect on an aircraft's ground speed.

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

dev/null

Tracy you are right, but your argument is cumbersome.

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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CounterRotatingProps

I thought of it (loosely) as a doppler effect, but instead of fixed speed of sound and moving object compressing/decompressing the wave front, the *object* is fixed and the air itself is the thing that slows or speeds up. Pretty much the same example Duffer uses: an airplane moving against differing air-flow.

- H

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Tracy Allen

dev/null,

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

stephenwagner

C R P
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

stephenwagner

CRP

Here is a link to the heathkit schmatic.

http://www.nostalgickitscentral.com/heath/schematics/heathkit_schema_ci1080.gif

SJW

CounterRotatingProps

SJW - thank you --- that is very interesting...

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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dev/null

Tracy

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.

Tracy said...
but I hold to the conclusion that there is no Doppler effect in a steady laminar air flow

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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Phil Pilgrim (PhiPi)

dev/null said...
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.

Tracy is right. In your example both emitter and detector are moving at the same velocity. For the Doppler effect to come into play they must be moving at different speeds (i.e. separating or moving closer together). Think of it this way: if the wind is moving away from the emitter, the wavelength will be dilated relative to the moving air mass. But since it's moving toward the detector at the same velocity, the detector will see it recompressed by the same amount as the dilation; hence, no apparent change in frequency.

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 9/21/2009 11:50:19 PM GMT

dev/null

Umm... This is "almost" exactly what I said in my previous post Phil. Read the part above the quote.

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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Phil Pilgrim (PhiPi)

No, there is no Doppler effect, no matter which way the wind blows, as long as the emitter and detector do not move relative to each other. You can separate the wind into two orthogonal components — one parallel to the axis between emitter and detector, and one perpendicular. The perpendicular component contributes nothing to either the measured windspeed or to the observed emitter frequency. And the parallel component, as we both agree, contributes nothing to the obseved emitter frequency.

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

dev/null

I'm sorry Phil but you are wrong: Read section 3 - relative doppler effect:
www.glafreniere.com/sa_Doppler.htm

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Tracy Allen

Hi dev/null, That site has beautiful animations, and thank you for pointing it out. I'll have to take a deep breath before diving into it seriouslly though. The author presents an eccentric theory of the universe that starts with the aether and builds up within that a wave theory of energy, matter and evolution. He may be onto something, but my BS detection antennae are activated! Have you worked carefully through the math?

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Tracy Allen
www.emesystems.com

Tracy Allen

With regard to sound propagation, I'm supplying microphone mounts to biologists, who are monitoring bats (the flying kind). A lot of this has to do with wind turbines, where bats are killed by direct encounters with the blades or by being caught in the slipstream. There are also basic ecological studies, too. There is a volume in space wiyhin which the microphone is able to pick up bats that may be present. Sound at higher frequencies is attenuated strongly with distance, and the bats are in motion in any direction, and temperature, humidity, wind and turbulence introduce a whole new dimension of complications. We are thinking about setting up experiments in a wind tunnel, but it is kind of hard to decide what would be meaningful.

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Tracy Allen
www.emesystems.com