Altitude and Airspeed

[AndrewM]

I decided that I would like to include a method of measuring altitude and airspeed for one of the projects I am working on, so after a quick search on google I came across this article:

http://www.geocities.com/SiliconValley/Orchard/6633/altimeter.html

Seems like a pretty simple method of measuring altitude that can be adapted to any microcontroller, although the MPX4115 mgiht not be accurate enough for low altitude flying.  Has anyone done anything similar to the article, using air pressure to determine altitude, and if so what did you use for a pressure sensor and how did it work out?

The second part of wanting to know a relative airspeed.  I have no intention of compensating for outside air forces (wind, a fan, etc), just a very base guage.  So my thought was this, place the altitude pressure sensor (MPX4115, etc) someplace on the robot where it would not get hit with direct airflow from propulsion (someplace in the back of the fuselage.  At the front of the craft, facing forward, place another pressure sensor.  Using the altitude sensor as reference, compare the pressure difference with the front to get a best guess at airspeed.

Think that will work?  Anyone tried something similar before?  Any other possible ideas that might work?  I thought initial about just a propeller attached to an encoder, but it would require too much room and likely slow the craft down too much.

[dunk]

what about sonar to measure airspeed?
have the transmitter and receiver in line with the direction of travel and measure the time the sound takes to travel the distance.

dunk.

[hgordon]

I believe Sparkfun has an SPI-bus barometer that is popular.  Barometers are commonly used for airspeed and altitude.  You can also use the z-axis of an accelerometer to measure change in altitude. 

For landing sensors, ultrasonic rangers are fairly popular, though I've heard of mixed results with helicopters.  Do a web search on open source UAV's, and you will probably come up with some examples.

[Admin]

You can also use the z-axis of an accelerometer to measure change in altitude. 

Not really . . . it only works if your air vehicle is perfectly horizontal - you'd have to use a 3-axis accelerometer and break the data into components. But then its also affected by typical robot motion and gravity, and it won't know which is which. So then you'd need a 3 axis gryo as well. It also won't detect small changes in acceleration. You will find it quickly drifting . . . basically it won't work.

I think a barometer would be best.

[hgordon]

Not really . . . it only works if your air vehicle is perfectly horizontal - you'd have to use a 3-axis accelerometer and break the data into components. But then its also affected by typical robot motion and gravity, and it won't know which is which. So then you'd need a 3 axis gryo as well. It also won't detect small changes in acceleration. You will find it quickly drifting . . . basically it won't work.

I think a barometer would be best.

Depends on the airframe - I looked at using this for a hovering device, but you're right about vibration overwhelming other measurements by an accelerometer.  For a blimp or balloon, the accelerometer would be reasonable, but generally I agree that a barometer is a more general solution.

[AndrewM]

In looking at the MPX4115 (and similiar chips) data sheets ( http://www.freescale.com/files/sensors/doc/data_sheet/MPX4115A.pdf ) , which would make up the barometer circuit for determining altitude and airspeed it does not seem like it is sensitive enough.   If I am reading the datasheet correctly and remember the goofy altitude pressure stuff right, it looks as if I will get a 45.9mV different per kPA difference, which is around 500 feet of altitude (might be off there).   If my robot hits 500 feet above sea level I can basically kiss it goodbye because it is in the great outdoors at the point spreading murder and mahem.

Anyone know of a more sensitive barometer that could get down into the 10feet or lower difference range?

[AndrewM]

Scratch that, found the SCP1000 pressure sensor with 1.5PA of resolution

[Admin]

If its not sensitive enough, just amplify the output signal:
http://www.societyofrobots.com/schematics_voltamp.shtml

[555 timer chip guy]

GPS works well for that to.