Oh what now?

I managed to squeeze in another flight today, and got diagnostics. The flight was only 3 seconds before I had to kill it – she then flipped over and landed upside down on the roof before sliding down (phew) and landing on the grass.  No damage done to the house or Phoebe.

During the first second or so, the flight plan was zero movement in all three axes while the props got up to speed. For the next two seconds, Phoebe rose, but she rose to over 3m in those two seconds instead of the target of one meter.  She also drifted gently to the right and possibly backwards (but that’s harder to see from behind).

This graph shows accelerometer readings over the course of the flight, with gravity (a fixed reading taken during the flight warm-up period and rotated to the same orientation) subtracted:

Acceleration Drift

Acceleration Drift

I’ve added linear trend lines to the graph which, at least over these 3 seconds, do fit with a linear drift in the accelerometer readings.  Note that the drift in readings seems independent of whether she’s stationary on the ground for the first second, or climbing for the remaining flight time.

So what could be the cause?  For once, it isn’t my code – at this point no motion processing has happened – this is raw data read from the sensors.  It also can’t be calibration as the trend lines would be horizontal with a fixed error.

My first suspect for the drift was temperature, but this graph of sensor temperature over the course of the flight shows that’s unlikely:

Flight temperature

Flight temperature

Could this be dlpf filtering out important stuff or including unnecessary stuff?  It was set to 3 – 44Hz and 4.9ms lag.

I have 2 plans of attack here:

  1. try upping the dlpf to 2 – 94Hz and 3ms lag and down to 4 – 21Hz and 8.5ms lag and see what happens.
  2. try physical filtering by isolating the sensors from the prop noise.

Option 2 is Zoë – the genetically engineered clone of Phoebe’s software and electronics with Chloë’s hardware, with a special dash of silicone dampers to isolate the Raspberry Pi and Beret lower platform from the noise from the props / motors transmitted along the arms to the top platform.  The remaining parts are arriving tomorrow.

I do hope this works because otherwise this project has just hit the biggest metaphoric brick-wall possible – think of the great wall of China, but built of granite.

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