Alt Hold to Stabilize Mode Transition Question

I am more than a little confused by the transition from, say Alt Hold mode, to Stabilize mode. Let's say I launch and determine that a stable hover is at 1/4 throttle (no matter the altitude). Now I ascend to 20', return to 1/4 throttle, and enter Altitude Hold mode. 1/4 throttle now represents an altitude of 20'. I then move the throttle stick down to, say 1/8 throttle, and descend to 10' and stably hover there. Finally, at this 10', I return to Stabilize mode - won't the multicopter quickly drop and probably crash since the throttle stick is sitting at 1/8 which is way under stable hover (determined to be at 1/4 throttle when initially in Stabilize mode)? Perhaps I need to mark the throttle position when entering Alt Hold mode and only enter Stabilize mode after returning to the altitude at which I entered Alt Hold mode. Or am I more completely confused than I know...?

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  • From the wiki on Alt Hold mode;

    • While in Alt Hold mode altitude can still be manually changed by raising or lowering the throttle control beyond a large central dead band.
      • After using manual override to change altitude it is important to re-position the throttle stick back to as near your normal hover position as possible.
      • When you switch back to Stabilize mode from any Altitude Holding mode, the copter will immediately go in the direction of any throttle stick displacement from the neutral hover position. It can drop or rise at an alarming rate

    So, you don't lower the throttle stick to a position and leave it there, you lower to descend and then put the stick back into a dead zone area.

    Note the last point!  You can climb or descend sharply when you switch out the mode if the throttle stick isn't close to the hover position.  I tend to set alt hold and not move the throttle stick...when I want to climb or descend, I'll switch to Acro or Stablize mode, go to new alt and then switch back to alt hold mode.

  • One solution might be to automatically hold the same altitude when exiting alt-hold mode.  Then slowly ramp to the real throttle input over a few seconds.  I think it would be easy enough to implement.

    But in general there always seem to be issues when you override user input.  In this case you might need to have full manual control due to a wind gust or power lines close above.  Then you are stuffed because you cannot use throttle to control it.

    I would like to know of a solution because I would implement it myself just for safety.  I think this issue has affected a lot of people because it's a silent trap (going from alt hold to normal without checking your throttle first).

  • Simply put, the transistion should be monitored, especially if you have an airframe that hovers at 25% throttle.

     

    If you are switching from alt-hold to stab you should be very ready with the throttle, then it's easy, just do what is required with throttle input, the same way you do in stab mode anyway.

     

    It's really easy, I don't worry about it at all, as I know in stab i have manual throttle control, so when i return to stab, if i want to hover my stick need to be roughly at hovering point, no matter which quad/octa/hex I'm flying

     

    If i was decending fast in alt-hold, then switching to stab and stopping the decent I'd give it a blip more beans then back to hover point, same as in stab mode.

     

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