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Topic: Altimeter? (Read 16797 times) previous topic - next topic

Nachtwind

Hi..
Has anyone an idea to build a somewhat precise altimeter?
I thought of using a pressure sensor which i would calibrate every day at a place i know the exact elevation a.s.l.
Does maybe someone know exact systems that maybe work already with absolute and not relative data?

westfw

GPS will give you altitude data.  There's at least two GPS shields for Arduino.
One from Adafruit: http://www.adafruit.com/index.php?main_page=product_info&cPath=17_21&products_id=98
One from Sparkfun: http://www.sparkfun.com/commerce/product_info.php?products_id=9487
(note that neither shield actually includes a GPS module!)
Most GPS receivers communicate via 4800bps async serial (compatible with the serial port on the arduino), so it is "relatively easy" to interface many gps receivers to arduino...  I read somewhere that most of the cheap ($25-40) USB GPS modules you can buy on eBay include a serial port internally and use a USB/Serial converter (like the one on arduino), so connecting to an arduino only requires bypassing the GPS USB converter...


mem

#2
Dec 24, 2009, 10:09 am Last Edit: Dec 24, 2009, 10:10 am by mem Reason: 1
I posted some altimeter code in this thread: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1223819446/0

it uses this pressure module: http://www.futurlec.com/HP03D.shtml

Nachtwind

Gps is not a technology to use in my case. When i was testing this i had with three different modules about 10(!)m difference between the values - and the expected altitude. The pressure sensor altimeter of you, mem, looks interestering..

Thanks for the ideas

kg4wsv

GPS altitude precision sucks, especially if you're moving.  I suspect averaging would help.

A pressure sensor is a lot cheaper, but as you point out pressure altitude varies with weather.

Rocket altimeters seem to work on AGL vs. MSL - i.e., relative altitude instead of absolute.  This neatly sidesteps the problem.

I've got working code for the Freescale MPXAZ6115/MPXHZ6115 sensors.  The Perfectflite avionics use this one.

-j

sylvie369

I'd be interested in learning a lot more about the inner workings of the Perfectflite altimeters, as my main project involves sending telemetry from them.

Rocket altimeters generally spend some time on the pad reading pressure, averaging them, and then deciding that the pressure they're reading on the pad will be an altitude of zero (AGL, of course). I use Perfectflites and Missileworks, and they both are very good at what they do (though of course I have no idea how accurate the altitude readings I get are).

retrolefty

#6
Dec 24, 2009, 08:38 pm Last Edit: Dec 24, 2009, 08:39 pm by retrolefty Reason: 1
One factor on accuracy when using an absolute pressure sensor as an altimeter, is to be sure the sensor has internal temperature compensated or you are using external temperature compensation if not.
I found one easy method for a calibration reference was to get a portable radio that has AM aircraft band reception (116-136mhz). Any close airport, even many small ones have continuous broadcast of their local barometric pressure and if you know the elevation difference between that airport and your location you have all the information needed to set your starting pressure/elevation reference.

Lefty

kg4wsv

I've considered using a combination GPS and pressure sensor.  My theory is that the payload (rocket or balloon) will spend some time on the ground, and the GPS can lock and obtain a relatively accurate altitude measurement which would be averaged.  This would be used to adjust the pressure altitude from the pressure sensor.  I'd need a GPS anyway, for lat/lan position reporting.

-j

Joshua47

After poking around for a good 15mins looking for information I can't find anything about how long it takes to get a measurement..
I am thinking it would wait for a stable measurement anyways (useless for us.. we will never get one whilst flying), so it's more so the actual laser diode and reciever setup that I am interested in.
If I could get it to give me the raw data I could filter it pretty easily and it would make a very nice altimeter. :)
Yet to find a DIY project for a time of flight laser rangefinder yet..
If anyone has anymore info regarding this please share, I am very interested in developing a low cost laser altimeter for UAV landing purposes.

GrooveFlotilla

Someone posted a cheap DIY parallax laser rangefinder recently - wouldn't this be simpler than time-of-flight?
Some people are like Slinkies.

Not really good for anything, but they bring a smile to your face when pushed down the stairs.

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