No more flippin’ flipping

[With thanks to Dave for getting me rethinking an area I thought I’d already solved.]

I’d had problems in the past when the velocity PID target is changed (e.g. Hover @ 0.0ms-1 to Ascent @ 0.3ms-1) – I was trying to reduce noise at the time, and I speculated that one cause could be that the target changed immediately and the fix was to change the target incrementally over a short period of time.  Normally this “soft” transition between targets for the outer velocity PIDs would be provided by the human being operating their sticks, but Phoebe has to do this herself because she doesn’t have the human feedback.

Revisiting that yesterday, I recognised that for significant transitions, there would be a significant power surge, and this could be what’s causing Phoebe’s flipping.  The transition from hover to ascending could require such a current surge that the voltage of the LiPo drop to under 10v temporarily, causing the Raspberry Pi to reboot – 10v is the lower limit of the DC-DC converter I use to get the 5v to power the Raspberry Pi.

So I changed the transition time from the fixed 0.5s I was using to a dynamic period depending on how large the change in vertical velocity PID target was.  Horizontal velocity target changes don’t need the power sapping current surges (their PID gains are a factor of 600 smaller as they don’t have gravity to fight), so they stay at the default 0.5s transition time.

A quick test flight just now shows it’s working – no more flipping – so I can go back to the equally frustrating drift control again.

3 thoughts on “No more flippin’ flipping

  1. Oh good, one problem solved 🙂
    Now, you know the paper “Quadrotor Dynamics and Control” by Randal W. Beard, I got stuck on page 2 🙁
    What does that = sign with a small triangle on top of it mean? Apart from that symbol, page 2 looks legible but the symbol crops up regularly in the paper and I’ve never seen it before.

    • I skipped the first few pages as they were too obscure, but if you head to page 6 and read section 1.2 a few times, it starts to make enough sense that I felt I could use it, although as it turns out in the next post, I don’t think I got it quite right…

      I just assumed the = + triangle just meant equals for a matrix and ploughed on regardless of my ignorance!

      • It was surprisingly easy to find an explanation of the ≜ symbol with the right search terms 🙂
        (I went with ‘mathematical symbols dictionary’ in Google).
        According to http://en.wikipedia.org/wiki/List_of_mathematical_symbols the ≜ symbol is used to denote a definition, which seems to make sense. So the first instance in the paper says that p1 is defined as a column matrix. Don’t know how to reproduce the math here, so anyone reading will just have to take a look at the paper to see what I’m talking about.

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