These Wheels Are Made for Arc-ing: Two New Mobility Commands to Improve Wheel Wear Outcomes

Abstract

The Mars Science Laboratory (MSL) Curiosity rover has driven over 29 kilometers since landing on Mars in August 2012, as of sol 3600 (September 2022). MSL Rover Planners use a variety of different driving commands and sequencing strategies to reach science targets of interest in diverse terrain. However, 490 Martian solar days (sols) and 4600 meters into the mission, the wheels began to exhibit significant wear, manifesting as cracks and breaks in their aluminum-alloy construction. Hardware mitigation was not possible due to the location of the vehicle; instead, engineers applied software techniques to extend the lifetime of the wheels. This initially led to the development of new Terrain-adaptive Wheel Speed Control software for the rover, which has proven helpful and now has been in use contin-uously since sol 1646 (April 2017). But some inefficiencies remained. When performing a precision approach to a goal location, the standard driving strategy alternates arcs (specified using a small number of fixed, preselected curvatures, running in open-loop motion) with closed-loop turns in place. But turn-in-place commands were found to exacerbate wheel wear rates due to their slower drive speeds and additional steering. So when a new flight software (FSW) release called R13 became possible, the project decided to implement two new commands that will help reduce unnecessary rover motion. These new mobility commands are now part of MSL's next major FSW release and offer new commanding strategies that should reduce unnecessary wheel wear and allow rover planners to generate more flexible drive paths. While similar in name to existing arcing commands, these two commands have completely new behaviors that allow the rover to maneuver in ways never possible before: dynamically choosing the precise arc curvature appropriate for the precision goal location, and using the gyro-measured heading change to terminate motion. This update removes the prior restrictions on which specific curvatures can be used, letting the rover choose whatever is appropriate given its current pose relative to the current goal. These two commands can potentially replace the turn-in-place maneuvers currently used to reach a specific goal, a known significant contributor to wheel damage in terrains with embedded rocks. The new commands were exercised during algorithm development both in simulation and also on MSL's Vehicle Systems Testbed (VSTB), an engineering model of Curiosity typically deployed in JPL's Mars Yard, an outdoor test area with a variety of terrains and slopes. Once development was complete, the VSTB was also used to conduct a formal Verification and Validation (V&V) test campaign over 5 days in the Mars Yard, testing the commands to their limits to ensure readiness for Mars. During this test campaign, the expected behavior of the commands was confirmed which led to approval for inclusion in the R13 FSW update after a review of the V&V results by a board of Mission Subject Matter Experts. We anticipate these commands will be deployed on Curiosity as part of the R13 update expected in early 2023.