When updating attitude during arm motion, gyroscope propagation error soon becomes greater than the tilt being monitored. The solution is to add an accelerometer-only mode that can update attitude without using gyroscopes, and to add the ability to monitor pitch and roll separately. Accelerometer-only mode is used only when it is safe to assume that any vehicle shifting in yaw would also be accompanied by shifting in pitch and/or roll. For surface contact operations, it is important to monitor change in tilt of the vehicle resulting from arm motion.

The approach has been to update attitude using gyroscope and accelerometer data, and monitor for changes in attitude and tilt. Attitude is updated in three axes based on gyroscope data, and then corrected in pitch and roll based on accelerometer data. This pair of corrections is done at the attitude update rate, which is 8 Hz for surface operations in Mars Science Laboratory (MSL). This results in a continuous growth in yaw error over time (while the amount of pitch and roll error does not grow).

The consequence was that changes in attitude detected were due to yaw error, rather than actual changes in rover tilt. The operational workaround was to stop monitoring for tilt altogether, and assume the rover was not moving. This is reasonable when on the flat, but becomes risky on a slope where the rover is more likely to shift. Instead of totally ignoring rover motion, the option of ignoring yaw motion (which only the gyroscopes can sense) while continuing to monitor tilt (which the accelerometers can sense) was added. Monitoring for tilt was also updated to be able to separately monitor for roll and pitch.

With this update and use of this new mode, attitude is now updated and tilt monitored during long periods of arm activity with no growth in attitude error. Error in remote sensing pointing and arm placement on surface features designated in images is also reduced because changes in tilt are no longer ignored.

This work was done by Stephen F. Peters, Charles A. Vanelli, and Matthew L. Robinson of Caltech for NASA’s Jet Propulsion Laboratory. This software is available for license through the Jet Propulsion Laboratory, and you may request a license at: https://download.jpl.nasa.gov/ops/request/request_introduction.cfm . NPO-49660

NASA Tech Briefs Magazine

This article first appeared in the September, 2016 issue of NASA Tech Briefs Magazine.

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