DIY Air Speed Sensor [Archive]

Gary Warner

Jun 02, 2006, 07:15 AM

There are many examples out there. I've been kicking around the idea of a DIY airspeed sensor to feedback to elevator control. What I've found is that the sensors (pressure) are hard to use at RC speeds, especially sailplanes. They can also be expensive. So I've put some thought into making my own sensor.

The sensor that seems to be best would be an impeller who's RPM's were read. I'm not big on an impeller out in the open, so I'm thinking a pitot tube running into a 'box' (inside the plane) that would enclose an impeller. The box would also have an exit tube that would connect into the backside of the air stream (push-pull system).

Another idea was the use of a strain bar with an end of it exposed to the air stream. Another is the same idea, but with a spring loaded fin and a position sensor on the fin. Less of a 'good' option, but it shows I'm thinking about this.

Gary
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vintage1

Jun 02, 2006, 07:40 AM

Gary Warner

Jun 02, 2006, 08:11 AM

Prpbably the simplest is a hot wire anemometer.

Until it rains ;) ....you may need to calibrate for pressure too..but it respinds nicely to air density anway so changes in altitude and hence stall speed would be compensated...
That's a good idea.

I've seen the Radio Shack wind speed/direction unit that uses four transistors in the same way. Any idea where I could read up on the circuit theory?

Gary
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kak8

Jun 02, 2006, 08:49 AM

GPS can read speed - but that is not for amateurs :-)

How about a MAP sensor from a "modern" car?
MAP - Manifold Air Pressure - could be used to messure pressure and the pressure is higher if you go faster - just an idea. Its not much bigger than 1"x 1"

lazy-b

Jun 02, 2006, 09:13 AM

Guys, how about an Ultrasonic transducer, here we have Windspeed and Wind direction Monitoring using 2 pair of Ultrasonic transducer arrange in Perpendicular to each other.

a pair of Ultrasonic transducer is facing each other 3 feet apart, the speed of the wind can affect the time travel of echo sound.......by measuring the time travel of echo sound you can estimate the wind speed........with 2 pair of this, you can detect the speed and direction of the wind.

We have a Mobile Weather Monitoring Station using this technique.

I guess, if you have big plane, you can put a Ultrasonic Transmitter near the front and Ultrasonic Receiver near the Tail, as the plane increase its speed, the travel time of the sound will be shorter.

Gary Warner

Jun 02, 2006, 09:42 AM

Guys, how about an Ultrasonic transducer...
At 1130 feet per second that works out to .0008849 (say .000885) seconds per foot.

A PIC running at 20mhz can count at Fosc/4 or 5 MIPS or a resolution of .0000002 seconds. What's the finest resolution of speed that can be detected by this PIC? Is it 3.9159... MPH? I'm sure I don't know the math for this.

Edit: .000885/.0000002 = 4425 times the PIC can count in one foot-second of sound.

Edit: What does this say? 770 MPH / 5000000 = .000154
Does this say that the counting resolution is .000154 MPH?

Gary
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vintage1

Jun 02, 2006, 03:41 PM

That's a good idea.

I've seen the Radio Shack wind speed/direction unit that uses four transistors in the same way. Any idea where I could read up on the circuit theory?

Gary
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Don't think there is much..you shove a constant current through a bit of wire, and measure the volts across it. The more the wind blows, the less the volts..

They used it in fuel injection systems a bit - it works well to accomdate pressure changes and even sligghtly heavier damp air, but it went haywire when it rained...

Gary Warner

Jun 02, 2006, 04:10 PM

Don't think there is much..you shove a constant current through a bit of wire, and measure the volts across it. The more the wind blows, the less the volts..

They used it in fuel injection systems a bit - it works well to accomdate pressure changes and even sligghtly heavier damp air, but it went haywire when it rained...
That's what I think the Radio Shack meters are doing. They are supposed to be using a small constant current bias collector to emitter and the base to emitter voltage drop needed to hold this constant current is a measurement of the heat dissipated (so I've been told). By using 4 in each cardinal position it can figure out both wind speed and direction by differential cooling. Also, the sensor only needs to be held vertical - it doesn't need to be pointed into the wind.

I'd like to see some additional documents on this. It's a cool device.

Gary
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Hell-e-Guy

Jun 02, 2006, 09:47 PM

vintage1

Jun 03, 2006, 03:26 AM

That's what I think the Radio Shack meters are doing. They are supposed to be using a small constant current bias collector to emitter and the base to emitter voltage drop needed to hold this constant current is a measurement of the heat dissipated (so I've been told). By using 4 in each cardinal position it can figure out both wind speed and direction by differential cooling. Also, the sensor only needs to be held vertical - it doesn't need to be pointed into the wind.

I'd like to see some additional documents on this. It's a cool device.

Gary
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Ah...yes that might work...the mass of a transistor is a bit huge though..a nice tungten filament out of a bulb or something would be far more responsive..and, in general, we KNOW whch direction a plane is flying in..;)

ElectroLawndart

Jun 03, 2006, 04:23 AM

That's what I think the Radio Shack meters are doing. They are supposed to be using a small constant current bias collector to emitter and the base to emitter voltage drop needed to hold this constant current is a measurement of the heat dissipated (so I've been told). By using 4 in each cardinal position it can figure out both wind speed and direction by differential cooling. Also, the sensor only needs to be held vertical - it doesn't need to be pointed into the wind.

I'd like to see some additional documents on this. It's a cool device.

Gary
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Nuts and Volts Magazine had something like that called a Micro Gust Thermal Anemometer in their Feb '06 issue.
http://www.nutsvolts.com/toc_Pages/feb06toc.htm

Dart

Quacker

Jun 03, 2006, 06:29 AM

I've made several types of very sensitive instruments using small, glass bead thermistors. They are very good at low airspeeds in the self-heating mode. They function like a hot wire sensor except that they are much more sensitive. The hotter you run them, the less temperature compensation is an issue. If you mount 2 next to each other in a tube, and put them in a bridge, you can easily tell the direction and volume of air moving in the tube. This makes a good vario. By porting the tube in selected locations on the airframe, you can make an AOA or yaw sensor ;>)

ElectroLawndart

Jun 03, 2006, 10:48 AM

I've made several types of very sensitive instruments using small, glass bead thermistors.

I think thats how the Nuts and Volts article did it. They pried the ceramic oven off of it. I didn't read it, I guess I'll have to put it back on my reading pile now. :D

@Quacker
How sensitive is your design to changes in ambient air temperature? Does moving from shadow to direct sunlight have an effect?

Dart

Gary Warner

Jun 03, 2006, 11:29 AM

Guys Cool stuff !!!
I liked the GPS idea as it has no mechanical setup at all...
Seems to be the most expensive but also the most elegant.
Does GPS give a good speed resolution ?
what is the altitude resolution?
HEG,

GPS will measure ground speed very well - down to about one tenth of a MPH. Problem is that that's ground speed, not airspeed. The GPS ground speed comes in real handy for time-to-distance navigation since airspeed is poor for the same. Each has it's place in real planes.

Is it a navigation or an airspeed issue you want want to address?

Gary
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Gary Warner

Jun 03, 2006, 11:34 AM

I think thats how the Nuts and Volts article did it.
I have that issue here on my desk. It's good for seeing changes in airspeed in very small amounts. I might have a place in an airplane but I wonder if the speed ranges we use would work with this example.

They do heat the sensor to very high temps so it should be fairly stable at higher airspeeds. The article warns you not to touch the exposed sensor when heated as it will cause a nasty burn.

I wonder about the current needed to heat up so much. Seems like a waste of battery resources.

Gary
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Quacker

Jun 03, 2006, 12:27 PM

I have that issue here on my desk. It's good for seeing changes in airspeed in very small amounts. I might have a place in an airplane but I wonder if the speed ranges we use would work with this example.

They do heat the sensor to very high temps so it should be fairly stable at higher airspeeds. The article warns you not to touch the exposed sensor when heated as it will cause a nasty burn.

I wonder about the current needed to heat up so much. Seems like a waste of battery resources.

Gary
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Gary:

The thermistors I used were so small that a few MA is all it needed to heat substantially

Quacker

Jun 03, 2006, 12:36 PM

I think thats how the Nuts and Volts article did it. They pried the ceramic oven off of it. I didn't read it, I guess I'll have to put it back on my reading pile now. :D

@Quacker
How sensitive is your design to changes in ambient air temperature? Does moving from shadow to direct sunlight have an effect?

Dart

Electro:

By using thermistors that could get very hot(~150C), ambient temp effects were drastically reduced. Sun to shade would not influence anything compared to just small air movements.

When set up in a bridge, temp effects are common mode and are reduced even more.

Those itty-bitty thermistors are very hard to find these days. Panasonic makes some that are a little bit larger (size 0201), but are surface mount. The mount will suck all the heat out and make them less effective, so some other means of mounting on very fine wire would be needed. None the less, it might be worth it.

Hell-e-Guy

Jun 03, 2006, 10:42 PM

Gary,
I am interested in knowing the ground speed of my pylons.
A lot of guys at our flying club would also like to know who's hot-liner is fastest.
Do you have in mind a small GPS receiver that I won't need to morgage the house to get.

bratcher

Jun 04, 2006, 12:29 AM

Do you have in mind a small GPS receiver that I won't need to morgage the house to get.

I've always wondered just how hackable this (http://www.geeks.com/details.asp?invtid=UT-41&cat=GPS) is. The price is certainly right...

Hell-e-Guy

Jun 04, 2006, 02:34 AM

If it's a two chip solution (one GPS receiver and one USB Controller) it should be fairly easy.
The USB Chip should probably have an SPI/UART/I2C interface with the receiver.

Gary Warner

Jun 04, 2006, 02:19 PM

That's what I was thinking. Too bad the native com is USB. If it was RS232 it would be plug and play with a PIC.

Gary
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MatC

Jun 04, 2006, 08:56 PM

ultrasonic looks pretty good, I might do this one when I get round to building my auto pilot.
Sound is about 330 m/s, so that's 3ms per metre, 300us for 10cm.
1us timing would then give accuracy of around 1m/s or about 2.25mph, but I'd think some form of crazy PLL/frequency based/mixing/something system would give much more accurate results than PIC timing.

It's very nicely constrained too, as mentioned a pair of them would give very good x,y speeds.

Quacker

Jun 04, 2006, 09:47 PM

The ultrasonic technique has it's apeal but I couldnt find any transducers that were small enough.

kak8

Jun 05, 2006, 03:40 PM

Take a look at this if you don`t know it allready.
With optional GPS http://home.germany.net/100-173822/d_logg2_engl.htm

sguty

Oct 07, 2006, 01:46 PM

I see that the link directly above uses a MPX2010 differential pressure sensor and a classic pitot tube (you have to follow the link to the older datalogger page to see the details auf Deutsch.)
Does anyone else have any practical experience with the MPX2010 or 5010DP devices?
I'm thinking of using one of these (the 2010s seems easier to find in non-OEM quantites) to measure airspeed inside the ducting of an EDF and wondering how nuch inaccuracy I'd incur if I took the readings from a simple probe and the duct wall versus a full pitot tube.

Steve G

JimDrew

Oct 08, 2006, 02:03 AM

Acetronics

Oct 08, 2006, 04:04 AM

And pressure sensors are FREE as samples from various producers ...

Thanks to them ...

Alain

tungym

Oct 08, 2006, 08:17 AM

How about this?

http://www.kitsrus.com/pdf/k168.pdf

But need to modify to have logging function.

Andrew McGregor

Oct 09, 2006, 05:04 AM

Why not use what we are going to use with the XtremeLink... two pressure transducers. A differential model input connects via tubing to a pitot tube and the other input is internal (current air pressure). The output voltage is proportional to the difference in pressure. The other transducer is a gauge type with actual air pressure generating a voltage. By knowing the actual pressure, and differential pressure, you can calculate the air speed and the altitude with the same setup. These sensors are about $2.00 each in 100 unit quantities, about $3.50 each in single quantity.

Are you going to set them up so we can install a proper static port too? Lots of planes are pressurised by propwash, so an internally ported sensor won't be very accurate... take a look at Hyperion's SU and Yak designs, there's no firewall, so the fuse is open at the front.

Just a case design issue, but one I hope you've thought of.

peterangus

Oct 09, 2006, 06:59 AM

Eagletree dataloggerwww.eagletreesystems.com/plane/plane.html

Deiter Meissner's datalogger home.arcor.de/D_MEISSNER/d_logg2_engl.htm

JimDrew

Oct 09, 2006, 11:26 AM

If both the differential and absolute sensors are located in the same airstream, prop wash really won't matter too much. However, our sensor will be are shielded and can be mounted in any configuration, including dual pitot tubes.

birdofplay

Oct 09, 2006, 11:59 PM

how about this ...
http://www.bnbproducts.com/pages/features.htm
There is provision for a Tachometer input already.
Maybe some sort of tiny fan and magnet arraingment
could be fashioned to provide airspeed as well as all
the rest of the electronic data.
It's fairly cheap and ready to go.
and a bit more easily calibrated for you car.