Go-‘n’-Get-MSL

Been a busy week in general.  Neglected posting, and in particular I neglected our electric-off-roader, off-planet.

Rocker-bogie wheels- click for large size

The MSL rover used its arm-mounted rock lens to take self portraits last week.  For perspective, each wheel is big enough to fit your head easily, and probably your shoulders too.

Click for large size

But, how does it get places?  I mentioned the nuclear generator, which then recharges two lithium batteries.  Each wheel then has its own brushless electric motor; the corner wheels have steering motors and pivots, seen here.  The aluminum “rims” and treads mount to titanium spokes.  The thin spokes act as fine suspension springs; coarse suspension is via the “rocker-bogie” arms between wheel and body.  The layout as a whole gives both offroad flexibility and mission redundancy.

The spacecraft electrical bus runs at 28 volts, a bit high for computers nowadays.  I’d guess the motors also run at this, to prevent the efficiency losses of step-up electronics.  Anyone know for sure?  28V would then be slow for an EV.  The motors then turn reduction gears, increasing torque. Overall, the drivetrain and computer algorithms can move the rover at up to 90 meters per hour.  Not miles- meters.

Isn’t that slow?  Given the uncertain terrain, and the essentially irreplaceable national asset on Mars, mission managers aren’t taking too many chances.  First, the rover cannot be actively driven- there isn’t a guy on Earth treating MSL like a video game.  Instead, controllers review the previous landscape photos.  After a target destination is chosen, waypoints are selected, leading to the target along a presumed safe path.  The driving plan (target and waypoints) are then transmitted.  Between each waypoint, the rover computer is usually smart enough to pick its own path; if it truly can’t decide on a safe route, or finds itself in a bind, it will simply sit there, report its status, and await new plans.

Secondly, the drivetrain is built for robustness, not speed.  The design must show torque margin, in difficult terrain, even after wear, contamination, etc., before managers will approve it for construction and launch.  Both of the previous MER rovers had a wheel show issues, with the Spirit rover losing one wheel’s motor outright.  Since rovers can also take pictures and run limited experiments in the middle of drives, speed is a luxury.

The offroad capabilities of the suspension are more impressive.  The rocker-bogie design can climb rocks twice the height of a wheel.  In development since the late ’80s, the basic drivetrain layout has been demonstrated on ground mules, prototypes, and then, the 1996 Mars Pathfinder rover (“Sojourner“).

Rocky II test rover

Rocky IV test rover

Nicknamed “Rocky” for short, these test units are well-characterized, so the software programmers can write comprehensive, debugged code.  Even then, the MSL mule is still taken to the California deserts, so driving parameters can be tweaked and bugs found, for later upload to Mars.  In other words, the rover will get better, even on another planet with no shops or mechanics.

Even rodders and racers know: electronic engine management and drivetrains are an opportunity for hacking, not a restriction or even liability.  There are people trying hacks on my Zero right now.

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