Receiver failsafe issue with APM2

I have Hitec R/C equipment in my APM2-equipped Telemaster.  The rx is the Optima 7.  This rx has a failsafe capability where you can select the throttle setting and the control surface position when signal is lost.  Loss of signal results in the failsafe selections within one second.   My configuration setting is zero throttle and neutral control surfaces.

I have tested this setup (no APM connected) with partial throttle and control surface deflection.  Turning off the tx results in advertised failsafe behavior.

My understanding is that in APM2 MANUAL mode, receiver outputs are supposed to be directly passed through without modification to the servos and ESC.  Thus I expect failsafe settings to be passed through unchanged when I test it with APM2 connected.

The results have left me concerned!  It appears that the control surfaces behave properly, ie, they go to neutral.  The throttle, however, does not go to zero.  There is a modest reduction in the motor speed but it is nowhere near zero.

Do I have reason to be concerned?


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  • Everyone also needs to remember that if anything causes the APM to lock up you're still SOL - it will hold last known parameters until reboot. (throttle, steering, etc) All the discussion about failsafe modes only applies so long as the APM is functioning.
  • I would update the PPM encoder firmware as JB mentioned.  I think there's every reason to be very concerned with the current default firmware.

    A big thank you to JB for addressing this important issue!

  • Moderator

    @Vernon, I think Fred meant "zero throttle" to mean "lowest stick postion."

    I've got the same setup on my FrSky... you bind by holding the bind button on the Rx, position your sticks and switches, and then click the bind button again.

    I position my APM mode switch to the setting I've mapped to RTH.

    @John, thanks for the patch!

  • My configuration setting is zero throttle and neutral control surfaces

    You just did the same thing the 9x variants do which is BAD. You should never go to 0 on any channel, at least output a VALID value to close a throttle servo ~900

    The 9x variants do the same thing out of the box (drop the throttle signal altogether) and have caused some issues. Null is bad because then a servo just sits at it's last position as if it's not powered on. The assumption is that an ESC would shut off with no signal, but that doesn't work in every situation, which is why setting a failsafe at the lowest value is better than no value. Then you know it shuts it down.

    It's known the PPM encoder current shipping firmware will ignore a null value on the throttle channel if the other channels are valid. There is an update posted for PPM that corrects this loss of throttle signal while other values are correct.

    Again, you just need to output a low value not a null value, and it's not a bad idea to update the PPM in case the unthinkable happens when the throttle wire only gets loose.


    DeveloperPermalink Reply by John Arne Birkeland on Sunday

    I have updated the ArduPPM firmware for the APM2 (v2.2.67) so that it will detect a missing throttle signal and set the the throttle value to 900us. APM1 version will come later.



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