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Sep 24, 2013, 02:06 AM
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Fusing barometer & sonar









The jesus barometer goes on.



& gets earplug foam insulation. Earplugs are decent sources of breathable foam.


The temperature compensated MS5611 output




is compared with the raw MB1240 output




in a 1m flight over carpet. Despite the temperature compensation, the MS5611 has a huge amount of drift, leading to 1m of unknown absolute error. Its short term relative readings are quite good for something that measures air pressure.


For the 1st time, you can see just how many sonar readings are invalid. It generates 7650 when it knows the reading is invalid, but sometimes it errors low. It never seems to error high.


The object of the game is to fuse the air pressure & sonar readings to eliminate the errors. Sonar still has to be the master.




In this flight, sonar was used for guidance, while a fused reading was calculated.


So the last 10 pressure readings are averaged & normalized to the average of the last 10 sonar readings to compensate for temperature drift. The most recent pressure reading is subtracted from the average pressure & scaled to mm to get the most recent altitude based on pressure.


The algorithm doesn't produce any glitches, but is very noisy compared to sonar. You want to use sonar if the sonar change is under 100mm. If it's over 100mm, use air pressure.



A flight using fused sensors for guidance was very subtly smoother, but overshot the altitude changes a lot more. It switched to pressure, which lagged sonar. The sudden altitude changes due to glitches were gone, but there were audible glitches in motor speed.


Why not just freeze the sonar value when it changes by more than 100mm? Because some of those changes are legitimate. The air pressure picked up the legitimate altitude changes.
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