Sure. The absolute amplitude of a peak is just the number you read off the graph; in OP’s picture, the Z vibration has an amplitude of ~1100. But this number is really not useful because it depends on flight time, so you can’t really use it to compare to other flights.
When looking at relative amplitude, you’re just comparing the height of the different peaks to each other, like “my Z vibrations are much strongest at 140Hz” or “after changing my FC mount, the 70Hz peak is much larger in comparison to the 140Hz peak than last flight.” This is about as precise as our analyses can be with these tools, but generally it’s good enough.
As for doing this yourself, the steps are:
- Enable batch sampling (Copter 3.6 or later) with
INS_LOG_BAT_OPT = 1
. Also, setINS_LOG_BAT_MASK
to choose which IMUs to log. Set it =1 for first IMU, =3 for IMU1 and 2, or =7 for three IMUs. - Fly the drone
- Download the dataflash log.
- In mission planner, press Ctrl+F and click the FFT button. In the new window, click the “New DF Log” button and select the flight log.
If logging multiple IMUs, you might need to make sure all the relevant IMUs are enabled with INS_ENABLE_MASK
. Also, later versions of ArduCopter (including current master) will allow us to collect post-filtered IMU data by setting INS_LOG_BAT_OPT = 3
, which will be especially useful for configuring our filters.
There’s a long forum thread which has some more information regarding this here:
Just note that the “fast IMU logging” they refer to has been superceded by the batch logging I included in my instructions.