slew). If the earth’s trajectory then forced the dr ive to “flop,” the drive would slew 180 degrees in  the opposite direction (3rd slew). Finally, overcoming the st arting torque again and moving in 0.2  deg increments is required to resume  the regular operation duty cycle. 

Since the viscosity of the Braycote grease increas es non-linearly with a reduction in temperature,  meeting this requirement was a challenge. De velopment testing suggested that the no load  running torque for the input bearings, harmonic dr ive, and output bearings increased significantly  when the temperature drops from +25 °C to -70 °C (at 50 RPM at the input, the no load running  torque increases 4 times with grease plate and 8 times with grease plate + 10% grease fill for  these components.) Through measuring or estima ting the starting and running torques internal  and external to the drive, the r equired motor torque was determined. 

After calculating the expected torque on the motor shaft as a function of environmental  temperature, a thermodynamic model was used to  estimate the peak motor rotor temperature  during the operating conditions described above. To  pass the test, two conditions must be met:  • The motor must not overheat  • The peak armature resistance must be low enough to allow rover system to deliver the  current limit for that temperature at the mini mum operating voltage (22.5V). Note: As motor’s  resistance increases, the peak current the system  can deliver decreases, for a given voltage.   Given the results of the devel opment testing, the following torque margins were predicted: 

Table 2.  Predicted Driv e Torque Margins Under Maximum Load at Temperature  -70 C -60 C +25 C Running T orque Mar gin 1.4 2.0 2.0 Starting T orque Mar gin 1.5 2.0 2.0Temperature

Since a 2.0 torque margin at the minimu m protoflight operating temperature (-70 °C) could not be  predicted, the HGAG drives were all tested and verified at the minimum flight acceptance  operating temperature (-60 °C). 

Pointing Accuracy A nalysis  The maximum uncertainty in each drive’s point ing angle was conservatively estimated by  summing the following sources of error at each level of the gear train.    • Detent forcing the motor shaft into the s ubsequent detent well: 90 deg at motor shaft (or  0.017 deg at the drive’s output)  • Backlash in the gearbox: 0.37 deg at gear moto r output (or 0.006 deg at  the drive’s output)  • Backlash in the spur gear stage: 0.14 deg at spur gear output (or 0.003 deg at the drive’s  output)  • Hysteresis and repeatability error within the har monic drive: 0.02 deg at  the drive’s output  • Gear train windup on Mars (Under starting to rque condition): 0.07 deg for Azimuth drive and  0.06 deg for Elevation drive  • Expected tolerance stack of housing misalignm ents (including thermal distortions): 0.186 deg  The total HGAG maximum pointing uncertainty was then found by an RSS combination of each  drive’s error. The maximum estimated uncertain ty in the HGAG’s pointing angle was 0.43 deg,  which met the 0.5 deg requirement. 

Backdriving A nalysis  When the Rover drives over uneven terrain, the induced g-loading may apply a torque on the  azimuth and elevation drives. Since the elevat ion drive was nearly balanced, the induced loading  would have to be very large to cause any conc ern. The azimuth drive, however, was more  susceptible to back driving since its center of mass was offset from its rotation axis by  242