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May 31, 2006, 04:21 PM
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Thread OP
Discussion

DIY Air Speed Sensor


Hi Boys and Gals,
Has anyone built a speed sensor for R/C use.
What are the options for building one?
is a pito tube the only solution?

Thanks.
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Jun 02, 2006, 06:15 AM
York Electronics
Gary Warner's Avatar
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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Jun 02, 2006, 06:40 AM
Registered User
vintage1's Avatar
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...
Jun 02, 2006, 07:11 AM
York Electronics
Gary Warner's Avatar
Quote:
Originally Posted by vintage1
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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Jun 02, 2006, 07:49 AM
kak8
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"
Last edited by kak8; Jun 02, 2006 at 07:56 AM.
Jun 02, 2006, 08:13 AM
Registered User
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.
Last edited by lazy-b; Jun 02, 2006 at 08:19 AM.
Jun 02, 2006, 08:42 AM
York Electronics
Gary Warner's Avatar
Quote:
Originally Posted by lazy-b
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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Last edited by Gary Warner; Jun 02, 2006 at 09:10 AM.
Jun 02, 2006, 02:41 PM
Registered User
vintage1's Avatar
Quote:
Originally Posted by Gary Warner
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
--
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...
Jun 02, 2006, 03:10 PM
York Electronics
Gary Warner's Avatar
Quote:
Originally Posted by vintage1
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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Jun 02, 2006, 08:47 PM
Registered User
Thread OP
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?
Jun 03, 2006, 02:26 AM
Registered User
vintage1's Avatar
Quote:
Originally Posted by Gary Warner
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
--
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..
Jun 03, 2006, 03:23 AM
Plane & Computer crasher
ElectroLawndart's Avatar
Quote:
Originally Posted by Gary Warner
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
--
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
Jun 03, 2006, 05:29 AM
ClearView Rocks!
Quacker's Avatar
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 ;>)
Jun 03, 2006, 09:48 AM
Plane & Computer crasher
ElectroLawndart's Avatar
Quote:
Originally Posted by Quacker
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.

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

Dart
Jun 03, 2006, 10:29 AM
York Electronics
Gary Warner's Avatar
Quote:
Originally Posted by Hell-e-Guy
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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