TRIM_AIRSPD_CM and TRIM_THROTTLE Parameters

Posted by Jacques Eloff on January 21, 2016 at 2:29am

Hi

Can someone please explain the interaction (if any) between TRIM_AIRSPD_CM and TRIM_THROTTLE? I know these parameters are unrelated, but is there a situation where they can both be trying to influence airspeed? 

Presumably TRIM_AIRPSD_CM is more important insofar as speed control is concerned in Auto flight mode? What flight modes is TRIM_THROTTLE used?

Thanks,

Jacques

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Replies

  • Developer
    Grant Morphett February 1, 2016 at 12:05am

    James is correct.  The pasted in definitions by James are correct.

    No airspeed sensor only TRIM_THROTTLE is used and its a % of your throttle RC PWM min and max.  If min is 1000 and max is 2000 and TRIM_THROTTLE is 60 then PWM output on throttle is targeted at 1600.

    Note TRIM_ARSPD_CM is not used in FBWB and CRUISE modes.

    Thanks, Grant.

    • Jacques Eloff > Grant Morphett February 6, 2016 at 10:14pm

      Thanks Grant.

      I remember reading that if no airspeed sensor was fitted, then an approximation of airspeed was made using GPS, accelerometers etc. Presumably that's only used for stall prevention if it is not being used to manage speed in Auto mode?

      I am using airspeed sensors on all 3 of my craft, so not too concerned either way - just for my background/understanding.

      Cheers,

      Jacques

       

  • Abolo January 21, 2016 at 9:41am

    Hi Jacques Eloff

    I am still having some problem with airspeed and auto modes , :(

    I am not sure , but I think TRIM_THROTTLE used when you don't have Airspeed connected , which with give static throttle in Auto mode ,

    maybe others can help with sure information,

    regards ,

  • James Downing January 21, 2016 at 5:18am

    I believe TRIM_ARSPD_CM is used when you have an airspeed sensor because it's unit is cm/s.  TRIM_THROTTLE is in %, and I believe is used when you have no airspeed sensor enabled.

    • Jacques Eloff > James Downing January 21, 2016 at 6:51am

      Hmmm, not so sure about that James. Bear in mind that if you are not using a pitot tube then an approximation of airspeed is made using GPS, accelerometers, etc. It's just not as accurate, but TRIM_ARSPD_CM will still be used.

    • iskess > Jacques Eloff January 21, 2016 at 9:01am
      My understanding and experience is that TRIM-ARSPD is not used unless there is an airspeed sensor. I share the question about to what degree TRIM_THROTTLE is used when an airspeed exists.
    • James Downing > iskess January 21, 2016 at 10:35am

      It may use it as a starting point when in auto mode, since there is no preset "neutral" to a ESC, then the PIDs take over after that to attempt to match TRIM_ARSPD_CM.  That's the only way I can imagine a PID controller utilizing both inputs.

    • James Downing > Jacques Eloff January 21, 2016 at 6:55am

      I was largely going off of these definitions:

      http://plane.ardupilot.com/wiki/arduplane-parameters/

      Throttle cruise percentage (ArduPlane:TRIM_THROTTLE)

      The target percentage of throttle to apply for normal flight

      Throttle nudge enable (ArduPlane:THROTTLE_NUDGE)

      When enabled, this uses the throttle input in auto-throttle modes to ‘nudge’ the throttle or airspeed to higher or lower values. When you have an airspeed sensor the nudge affects the target airspeed, so that throttle inputs above 50% will increase the target airspeed from TRIM_ARSPD_CM up to a maximum of ARSPD_FBW_MAX. When no airspeed sensor is enabled the throttle nudge will push up the target throttle for throttle inputs above 50%.

      Target airspeed (ArduPlane:TRIM_ARSPD_CM)

      Airspeed in cm/s to aim for when airspeed is enabled in auto mode. This is a calibrated (apparent) airspeed.

    • Jacques Eloff > James Downing January 22, 2016 at 1:55am

      Hi James - looks like you may have been correct.

      Either way, would be good to get this issue cleared up by one of the gurus!

      Jacques

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