Throttle Failsafe not working correctly (clashing PWM)

Hi All,

I'm pretty new to UAVs and DIY drones, and I've been tasked with building a Y6 as an environmental research platform (seemingly a common thing...). I've got pretty far with my build and just waiting on propellers so I can complete transferring to the Y6B configuration, but I've found a problem with my throttle fail-safe that I'd like to resolve before starting test flights.

I think that minimum throttle PWM should be greater than the Tx off PWM, so the failsafe can be set to trigger at an in-between threshold; however, at the moment both minimum throttle and Tx off result in a PWM of 990 (received by the APM).

Can anyone offer any guidance on resolving this? (I'm using a Taranis X9D transmitter, FrSky X8R receiver, and APM 2.6. I think I might need to increase the PWM output from the Tx when the throttle's in the minimum position, maybe by reprogramming that channel's minimum from -100% to -90% or -85% and re-calibrating the radios, but I'd really appreciate someone letting me know if this is a daft idea!)

Thanks for any input,


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  • Did you redo the radio calibration using the -90% to +100% TX range?  If not, then Arducopter is waiting for your old calibrated minimum throttle, rather than your new higher value.  Basically it is never seeing minimum throttle, and therefore will not arm.

    • Ah that's correct, thanks. I was looking in in the wrong place, thinking the problem was where I'd set the FS threshold; renewing radio calibration allowed arming.

        In the meantime, I also calibrated the voltage readings from a 3DR power module. Whereas originally all six motors started fine, now only two motors (Y6B; M1 & M2) will start, I haven't done anything major in the 'power deck', and the connections from the APM-PDB-ESCs all look okay (I've tried with both the USB and MAVlink connections to GCS). The fact that all four don't start makes me think the problem is something I've done in Mission Planner start up but I can't work out what - any suggestions?

    • You will now also need to redo the ESC Calibration since your TX is only outputting -90% to +100%.  This could be why it is acting funny.

      This is all why I suggested using the extended limits on your transmitter to set the failsafe at -120%.  That way nothing regarding normal operation has to change.

    • Excellent. Glad you got it working.

    • Spot on yet again, thanks for the help @P2P. I hadn't initially implemented your suggestion as I wasn't sure how to extend the limits on my Tx; doing so has resolved my issues with misbehaving motors (although I've also repeated the ESC calibration just in case I'd altered something).

    • By starting up, do you mean spinning automatically when you arm them, or spinning as a result of increased throttle from your Tx?

      I always have one or two motors that won't spin when the "VERY SLOW" prop-rotation-on-arm is applied by the APM. If I go into Stabilize and raise the throttle, they eventually all start spinning.

    • Yes I'd encountered that earlier so increased the MOT_SPIN_ARMED to 90 to give all motors enough push to start. They (M1 & M2) spin automatically when armed, and are also the only ones responding to increased throttle; above ~60% throttle the APM starts beeping furiously (which I guess might be a warning as the motors are unloaded without props).

  • Thanks both, I'd missed that page on the Wiki sorry.

    I've set up the throttle failsafe: Tx failsafe mode to sent -100% on channel 3 (990 pwm), normal throttle range -90% to 100% (min 1040 pwm) and MP failsafe threshold at 1010 - this works in that if I turn off the Tx, the APM/Mission Planner switches the the selected behaviour (RTL); however, I now cannot Arm, due to a failure of the PreArm Check:FS_THR_Value. Checking in the advanced parameters page confirms that this parameter is set to 1010 (within the 925-1100 constraints), and I've restarted MP and reconnected to the APM.

    Has anyone got any suggestions what I'm missing?

  • Here's how you do it:

    1. On your transmitter, you should have it the throttle limits set for -100% to +100% (normal).
    2. Perform the Arducopter radio calibration.
    3. Note the lowest PWM value on the throttle during calibration.  It will probably be 980-1000.
    4. In the failsafe settings, set the FS PWM for something less than the normal minimum value, such as 950.
    5. Set the transmitter throttle limits for -120% to +100%.  This will allow you tell the receiver to go less than normal throttle on failsafe.
    6. Put the throttle stick all the way down, so the reciver is seeing below normal PWM,  It will probably be seeing 900-930ish.
    7. Follow your receiver's instructions for locking in those values for failsafe response output.
    8. When you're done, set the transmitter's throttle limits back to -100% to +100%.
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