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ArduPilot / MissionPlanner

Mission Planner Ground Control Station for ArduPilot (c# .net)
http://ardupilot.org/planner/
GNU General Public License v3.0
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Incorrect FFT frequencies #2069

Open tridge opened 5 years ago

tridge commented 5 years ago

The FFT option in MissionPlanner is producing incorrect frequencies from the ISBD and ISBH data. I've put a log here that has exact frequency noise in it: http://uav.tridgell.net/FFT/fft-x100-y150-z200.bin in that log the X axis (both accel and gyro) has 100Hz vibration, the Y axis has 150Hz vibration and the Z axis has 200Hz vibration. With MAVExplorer I get this: image In MissionPlanner I get quite different frequencies. See this PR for the way to generate the exact vibration freq logs: https://github.com/ArduPilot/ardupilot/pull/10189 Note that SITL quite deliberately sets the sample rates of the two IMUs differently, to better emulate a Pixhawk1. The rates are:

These rates are reflected in the smp_rate numbers in ISBH messages. I've seen the same affect of bad frequencies in FFT in MP on real aircraft logs as well.

rmackay9 commented 5 years ago

Great to find this issue!

tridge commented 5 years ago

I've added another log with fast sampling enabled, so we get 8x sampling gyros and 4x on accel. http://uav.tridgell.net/FFT/fft-x100-y150-z200-fast-sample.bin

meee1 commented 5 years ago

seems to work well after the first change, and pushed a new beta

tridge commented 5 years ago

thanks Michael, It is much improved, but the scaling on this log is odd: http://uav.tridgell.net/FFT/fft-x100-y150-z200-fast-sample.bin we also need to work out some units for the Y axis, preferably using a common method for MAVExplorer and MissionPlanner, and check that the Y value is independent of the number of samples. Right now MAVExplorer and MP give very different values on the Y axis. Leonard has offered to help with this (ping @lthall )

tridge commented 5 years ago

I've done a test with this log: http://uav.tridgell.net/FFT/fft-x100-y150-z200-fast-sample-last-50.bin that is just the 2nd half of this log: http://uav.tridgell.net/FFT/fft-x100-y150-z200-fast-sample.bin with MAVExplorer I get the same Y values with both logs. With MP I get half the value on the Y axis when I use the half-sized log. That implies that MP isn't scaling for number of samples.