dual baro sensors for improved accuracy???

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csurf
Posts: 65
Joined: Mon Dec 23, 2013 5:59 am

dual baro sensors for improved accuracy???

Post by csurf »

MWC noob & non-expert here...
I own a HK multiwii pro mega board.
I also have an extra BMP085 i2c break-out board.

I'm curious if there would be any possible benefit to setting up a separate, external BMP085 baro sensor via i2c, and then somehow averaging the readings from the two baro sensors (onboard & external sensor)? I'm not sure what kind of math would need to be done to best combine these readings for maximum accuracy, but I figured that it might possibly be a good way to improve stability of baro readings for alt-hold.

If this wouldn't work, then please disregard this idea.
However, if so, can someone please help me out on how to best set this up for testing? (I'd be willing to test it out as well)

timecop
Posts: 1880
Joined: Fri Sep 02, 2011 4:48 pm

Re: dual baro sensors for improved accuracy???

Post by timecop »

Money might be better spent on DJI NAZA V2 if you want to do something that requires "altitude hold"
You know that saying about a dead horse?

csurf
Posts: 65
Joined: Mon Dec 23, 2013 5:59 am

Re: dual baro sensors for improved accuracy???

Post by csurf »

I understand your point of view, but I wouldn't personally be spending anything
(I already own both the MWC and the extra baro sensor breakout board).

It would take me less than 5 minutes to solder a pin header to the i2c pads and then connect the baro sensor. After that, it would be a question of reading the sensor and some how mixing the readings from the two baro's together. I don't think it would be such a huge effort to get this going, at least for testing...

I'm just trying to think outside the box and see if there might be any good that could come of trying to set this up.
Can anyone else please provide some helpful feedback? thanks...

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Crashpilot1000
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Joined: Tue Apr 03, 2012 7:38 pm

Re: dual baro sensors for improved accuracy???

Post by Crashpilot1000 »

I did some sensorcombining (not baro but anyways..). The result will depend on the partners (exactly the same sensors etc) but generally you will see a benefit from combining sensors even of the same type. At least you can expect an increased polling rate and that alone is not a too shabby effect in baros. The baros must have different I2C addresses or different I2C busses or different bus systems (I2C/SPI) - you will have to shoot through the datasheets to figure out what the best/most practical way is. It's more a question of practical effect/purpose (different locations of baros to avoid groundeffect etc). So if you want to play around with that subject, you will be most probably rewarded with better function and/or reliability. It may be reasonable to combine one baro with a different sensor that can measure a distance between objects (sonar/radar/laser/future gps with better hight resolution etc) - that may be more reasonable than putting your effort on using 2 baros. Multiwii and other popular flightcontrols I know of only support one baro at a time.
Cheers Rob

EDIT: I think this: http://www.zerouav.com/en/Product/chanp ... i/540.html will have 2 baros :) .

csurf
Posts: 65
Joined: Mon Dec 23, 2013 5:59 am

Re: dual baro sensors for improved accuracy???

Post by csurf »

Crashpilot1000 wrote:I did some sensorcombining (not baro but anyways..). The result will depend on the partners (exactly the same sensors etc) but generally you will see a benefit from combining sensors even of the same type. At least you can expect an increased polling rate and that alone is not a too shabby effect in baros. The baros must have different I2C addresses or different I2C busses or different bus systems (I2C/SPI) - you will have to shoot through the datasheets to figure out what the best/most practical way is. It's more a question of practical effect/purpose (different locations of baros to avoid groundeffect etc). So if you want to play around with that subject, you will be most probably rewarded with better function and/or reliability. It may be reasonable to combine one baro with a different sensor that can measure a distance between objects (sonar/radar/laser/future gps with better hight resolution etc) - that may be more reasonable than putting your effort on using 2 baros. Multiwii and other popular flightcontrols I know of only support one baro at a time.
Cheers Rob

EDIT: I think this: http://www.zerouav.com/en/Product/chanp ... i/540.html will have 2 baros :) .


Thanks rob...
I looked into it, and it turns out that it's not that simple to run two BMP085 sensors on the same i2c bus. You have to switch each one into an 'off/suspend' state before reading the other, and apparently there's a delay before you can read from a sensor that's been switched back on (read somewhere to wait 10ms before reading it otherwise you get bad data or something).

Anyway, it looks like it's going to be too complicated, so I'll have to ditch the idea. Perhaps it might work with a different model of sensor, but I only have the BMP085. oh well, seemed like a good idea. I get a variation of about 0.5-1.0 meter when the onboard BMP085 is at rest, covered with foam, no wind/prop wash or direct sources of light or heat... seems like a very unstable sensor :(

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Crashpilot1000
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Re: dual baro sensors for improved accuracy???

Post by Crashpilot1000 »

Yes, BMP085 has a lot of noise and produces the observed +-0,5m accuracy. On top of that it also has a noticeable awful lag.. so the necessary filtering will make it worse. I did a little spikefilter, that has no to minimal lag and it will get rid of BMPs spikes quiet a great deal but it can not overcome the +-0.5m of course...
After all you can get a decent althold from BMP but no match for MS and flying 2m aboveground relying on BMP is also not a good idea. So if you are flying higher and don't care for hight gains/losses too much you can go for BMP otherwise use MS5611 for now. I've seen some ad of an cheap Baro that *could* rival the MS baro on the first sight.. but digging deeper the datasheet had too many contradictions and no info on acutal samplerate etc - so I guess it's just china crap.

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