I am developing my own autopilot system with arduino Mega 2560 i added most features that the arducopter has like gps position hold, waypoint, point of interest but the thing that i cant be success is altitude hold. I am usning Bosch BMP085 barometric pressure sensor. When I examine the data results of BMP085 about 3 min. its not accurate enough to use inside the PID, like it is roaming about -1m between 1m. when i use Low Pass Filter the results become stationary (Sensor was stationary when i took the readings) .But this time its become so slow and my quadrotor crash the ground or fly-up. I tried changing the coefficients of PID, Low Pass ratio but no luck! I Know INS is a good solution for this but when i try to use my accelerometer data for the speed estimate the data is too noisy because of motors. I also use anti-vibration mounting gels. So Do you have any idea for this problem?
Sorry for the bad english :(
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In my case I got as much as 500m of difference, I must say it was completely exposed to ligth and propwash, it ended up in a spectacular crash of course:
http://purposefulscience.blogspot.com.es/2014/04/crius-aiop-v2-foam...
That particular barometer is the one we use to use with the APM1. It's very noisy which is why we moved to the MS5611.
You'll just need to filter it a lot and make the PIDs very low. Best of luck.