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bitcraze / crazyflie-firmware

The main firmware for the Crazyflie Nano Quadcopter, Crazyflie Bolt Quadcopter and Roadrunner Positioning Tag.
GNU General Public License v3.0
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EKF crashing when flying with VL53 sensor only #182

Closed ataffanel closed 7 years ago

ataffanel commented 7 years ago

When flying with the VL53 deck and the kalman filter, the EKF is crashing at random interval on a NaN assert:

SYS: The system resumed after watchdog timeout [WARNING]
SYS: Assert failed at src/modules/src/estimator_kalman.c:789

It corresponds to this line: https://github.com/bitcraze/crazyflie-firmware/blob/72146a6a46e319f3af38eab6c64dcdd504466ba9/src/modules/src/estimator_kalman.c#L789

Further investigation seems to indicate that in at least one case PHTd[STATE_X] was NaN.

Tested with the version 72146a6a46 and the config.mk:

ESTIMATOR=kalman
CFLAGS+=-DUPDATE_KALMAN_WITH_RANGING
ataffanel commented 7 years ago

Found that H of the tof update could contain a NaN: https://github.com/bitcraze/crazyflie-firmware/blob/72146a6a46e319f3af38eab6c64dcdd504466ba9/src/modules/src/estimator_kalman.c#L958

h[STATE_Z] = 1 / cosf(alpha);

This is a good reason for the ekf to crash.

ataffanel commented 7 years ago

The problem comes from R[2][2] > 1.0. Since R is a rotation matrix it is not supposed to be possible.

ataffanel commented 7 years ago

I can confirm that there is rounding errors when calculating the rotation matrix R, I printed R[2][2] when it is > 1 and I get R[2][2] = 1.0. Which means that the value is really close to 1 but just over when compared to 1.

This should be fixable by making sure all element of R are within [-1, 1].

ataffanel commented 7 years ago

Fixed by removing the VL53 aperture angle compensation: the effect was very small (I cannot see any difference) and it removes the acos() that was causing the NaN.