Accelerometer digital low pass filter

Yet more indoor testing, indirectly GERMS related, more aimed at investigating drift against the accelerometer dlpf setting.

Speculatively, the net is that

  • alpf of 3 or less (>= 41Hz) means there is no vertical drift, but there is horizontal drift because real X- and Y-axis noise is not being filtered out and the motion processing thinks there’s X- / Y-axis acceleration when there isn’t.*
  • alpf of 4 or higher (<= 20Hz) gives no horizontal drift, but there is vertical drift because real Z-axis acceleration is being filtered out as noise meaning the motion processing thinks there’s less acceleration than there is.*

Again speculative, I think X- and Y- axis noise was due to unbalanced props leading to asymmetric noise which when integrated leads to non-zero drift velocity: if one of the props has slight damage, this will generate the assymeteric noise very well.

My props do have nicks and muck on them, so I cleaned them up, and tried alpf 3 again, but the drift still remained – to some extent expected as the cleaned props still had nicks in them.  So out came a set of unflown props.  Balanced as best I can, I tried again at alpf 3. Some vertical drift had appeared unexpectedly, so I tried alpf 2, and it was perfect.  So that’s what I’ll be sticking with.

The only down side of this is that the noise getting through at alpf 2 means my germs idea in its basic form is a no go.  It’s shelved for the moment as it’s no longer necessary for my short flights, and instead, I’ll see how far I can extend the complementary filter to see how long a zero drift flight can be achieved.

*To be more precise, it the integration of the acceleration (i.e. the velocites) that are wrong. The Z-axis velocity is greater than zero in reality but zero according to the motion processing due to the acceleration that had been filtered out. In contrast, the X- and Y- velocities are zero according to motion processing, but drifting in reality as the dlpf is letting through asymmetric noise.


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