ATT dataflash messages: "desired" attitude vs attitude

Can someone explain "Desired Roll" vs. "Roll"?

I have an unexplained crash that occurred at the end of a long autonomous flight.  Suggestion in the manual on log file analysis indicates differences between desired and actual values might point to a mechanical failure. However, that doesn't appear to be the case in an s900 hex crash with a pixhawk. 

I notice the desired roll becomes quite large (positive) and large oscillations occur around the roll axis. 

Is this a failure of the algorithm controlling attitude or am I misinterpreting the telemetry?



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  • Desired Roll is what the Flight controller wants the copter to do. 

    Roll is what the copter is actually doing.

    If there is a discrepancy between the two (amusing the PID are correct and the copter power to weight ratio is OK), this indicated there may well be a hardware problem. 

    Your log is indicating the above is good. (PID are correct and the copter power to weight ratio ok) as the roll (ACTUAL ROLL) has been closely following the the desired roll up until around 11 1/2 mins into the flight.

    There are some odd blips earlier in the flight also, just after 4, 7 and 9 mins in. This could be way-points?

    I note the voltage drops down to 20.38V which is LOW for a 6 cell. This may cause the ESC to shut power down to the ESCs. This would cause a graph like yours.

    • Thanks, Anthony, for the response.

      The copter is well below it's max weight, has been through the auto-tune process, and flies with very good stability and precision.  And yes, the small perturbations in roll you identified are waypoints. The lines were flown perfectly.  When the helicopter reached the end of the last line and began RTL is when the problem started.

      I see the 20.38V, but that's a noisy plot. I think the real average is ~20.7, which is the failsafe voltage -- that's 3.45V/cell -- should be OK for the 1 minute flight to home, using the 21,000mAhr battery. I think the ESC's aren't setup for low-voltage cutoff--but not 100% sure.

      This is a hex-X configuration. Motor 2 is left; motor 1 right. The log would lead me to believe motor 2 stopped providing lift suddenly--hence the quick roll to the left, and perhaps the autopilot shut down motor 1 to try to correct (see RCOuts).  I notice the current drops and maybe by 1/3, which seems to fit my hypothesis that motor 2 isn't using current, nor motor 1, because it's been shut down.  Not sure WHY motor 2 would stop.  Testing after crash, all motors still spin fine...  lots of other broken part though!!!  However, not a single prop broken, including ones on motor 2.

      My real question is why doesn't the desired roll stay near zero? Is it really the "desired correction", as opposed to the actual desired roll attitude? Even if so, why would it remain at such large positive values as the heli is pitching around the axis?

  • Bump.

    I am curious for some sort of explanation to this as well....

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