Accurate altitude initiating

[dindibo4]

Hey, I'm planning to build an Arduino based drone but before I will, I'm trying to figure out what kind of sensors I would need and especially the one that tells the arduino the altitude of the drone so it will be able to maintain the altitude.
At first, I thought about an accelerometer but I'm not sure it will work because it might be that the sum of the forces of the vertical axis on the drone would be 0 and thus there will be no acceleration but it might still freefall downwards at a constant speed.

So what is your opinion which sensor should I use?

[groundFungus]

BMP280 is an absolute barometric pressure sensor that might fit the bill.  The BMP280 is available from many sources on a breakout board.  https://www.adafruit.com/product/2651  Adafruit has a tutorial and Arduino library available, too.

[dindibo4]

BMP280 is an absolute barometric pressure sensor that might fit the bill.  The BMP280 is available from many sources on a breakout board.  https://www.adafruit.com/product/2651  Adafruit has a tutorial and Arduino library available, too.

According to the documentation of the BMP280, the relative accuracy of this sensor is 12 Pascal and the absolute is 100 pascal this means that the inaccuracy of this sensor is between 0.9-8.65 meters.
it's not an optimal solution because if the drone would go up and down in a range of several meters it won't
stay exactly where it should be and it will be very hard to control.

[jremington]

The LPS25H is much better than the BMP280.

[dindibo4]

The LPS25H is much better than the BMP280.

Well, this sensor is better but it still has bad inaccuracy of 85 cm
it still not gonna work.

[jremington]

bad inaccuracy of 85 cm

Nope. In high resolution mode, +/- 1 Pa, or about +/- 8 cm at low elevations.

You are dreaming if you think you can do better with consumer grade sensors. Good luck!

[dindibo4]

Nope. In high resolution mode, +/- 1 Pa, or about +/- 8 cm at low elevations.

You are dreaming if you think you can do better with consumer grade sensors. Good luck!

If so that's a good sensor that can solve the problem,
but I've read the documentation of this sensor at this site:http://www.st.com/content/ccc/resource/technical/document/datasheet/58/d2/33/a4/42/89/42/0b/DM00066332.pdf/files/DM00066332.pdf/jcr:content/translations/en.DM00066332.pdf
and this is what is written there:

Paccrel Relative accuracy over
pressure(2)
2. Characterization data. Parameter not tested at final test
P = 800 to 1100 hPa
T = 25°C ± 0.1 hPa

as you can see it's 10 pascals of inaccuracy and not 1 Pascal, maybe this document that isn't reliable
but if you found something that suggests only 1 unit of Pascal inaccuracy like you said, please let me know

[jremington]

if you found something that suggests only 1 unit of Pascal inaccuracy like you said, please let me know

"inaccuracy" is not the correct term to use.

The data sheet for the part states RMS noise of 1Pa in the high resolution filtered mode. You cannot detect pressure changes smaller than that. As suggested by this figure, I have verified that 8 cm differences in altitude are easily detected over the short term.

You should not confuse this spec with the spec relative accuracy over pressure (range 800-1100 HPa) of 10 Pa, as that is irrelevant for an application like short term "hold altitude". The error in barometrically determined altitude is limited entirely by atmospheric pressure variations, which are equivalent to +/- 50 meters or so.

So, if you want your drone to fly at 10 meters above ground,  you have to zero the measurement, that is, determine the reference pressure at ground level, before every single flight.

[dindibo4]

"inaccuracy" is not the correct term to use.

The data sheet for the part states RMS noise of 1Pa in the high resolution filtered mode. You cannot detect pressure changes smaller than that. As suggested by this figure, I have verified that 8 cm differences in altitude are easily detected over the short term.

You should not confuse this spec with the spec relative accuracy over pressure (range 800-1100 HPa) of 10 Pa, as that is irrelevant for an application like short term "hold altitude". The error in barometrically determined altitude is limited entirely by atmospheric pressure variations, which are equivalent to +/- 50 meters or so.

So, if you want your drone to fly at 10 meters above ground,  you have to zero the measurement, that is, determine the reference pressure at ground level, before every single flight.

Ok I got it now, so it's really a good sensor after all and I guess I'll use it for the drone.
Thanks for the help you helped me a lot.