Mars 2020 Mobility Actuator Thermal Testing and Model Correlation

This paper describes the thermal testing and model correlation of a mobility actuator
planned for use on the Mars 2020 Rover. The mobility actuator is identical to those which
have been successfully flown and operated on the Mars Science Laboratory (MSL) Curiosity
rover since its successful landing on Mars in August of 2012. The actuator consists of a
motor, brake, and encoder paired with a four stage planetary gear box. In this thermal test,
the actuator was instrumented with a number of thermocouples on both the interior and
exterior of the gearbox. Heaters and a cold plate were used to generate thermal gradients
across the actuator in vacuum, low pressure GN2, and low pressure CO2 environments in an
effort to correlate a thermal model and develop a better understanding of how heat flows
through the mechanism. This testing resulted in the successful model correlation of a
simplified thermal model, and yielded important insights regarding the conductance of ball
bearings and gear-to-gear contact. Ball bearing thermal conductance in a low pressure
environment can be estimated by using correlations for vacuum thermal conductance along
with a multiplier to account for increased gas conduction, and gear-to-gear conductance can
be estimated by accounting for gas and grease conduction between two gears.