strength margin analysis where the maximum performance motor was used). The assumed motor  had the lowest torque constant, the minimum speed c onstant, and the maximum armature resistance.   

The gear motor assemblies were supplied by Star sys Corporation and used a three-stage, 81.37:1,  planetary gearbox to minimize the effects of viscous losses caused by the bearings and harmonic  drive on the motor. Once again, the gear motor analysis assumed worst case starting torque and no  load running torque for the gear moto r, as reported by Starsys. 

Spur Gear Stage   Since the RF system is routed coax ial with the drive axis of rotation, a spur gear stage is used to  offset the gear motor. The spur gear stage also provides a 1.33:1 gear reduction. Both the gear  motor’s pinion gear and the spur gear were made from 15-5 PH corrosion resistant steel. The spur  stage was lubricated with MoS 2 filled Braycote 601 to meet t he mechanism’s life requirements. 

Harm onic Drive Stage   The HGAG used a custom-built, hollow shaft har monic drive made by HD Systems, Inc. The  harmonic drive was based on HD Systems’ standard m odel SHF 14-50 drive (with a 50:1 gear ratio)  with changes made to accommodate the mounting requi rements for the HGAG.  This harmonic drive  was chosen for the following reasons:  • Hollow design accommodated RF routing  • High gear reduction in a compact package  • Drive stiffness was high enough to meet accu racy requirements but low enough to prevent  damaging hardware inside and outside the gear train when the mechanism rotates into its hard  stops  • The drive was sufficiently accurate (low ba cklash, small hysteresis, and high repeatability)  For the harmonic drive, JPL in itially explored using special aerospace grade materials recommended  by HD Systems, Inc. that would be less susceptible to cold embrittlement. However, based on the  limited flight history for the special grade mate rials and the limited development time for the MER  program, JPL chose to use the standard commercial materials instead. Because of this decision,  additional cold temperature / high load testing was required on the EM unit to validate the robustness  of the harmonic drive.  Gear train Analyses   The principle analyses performed on the gear train were:  • Ensuring the motor would not overheat during worst-case loading and duty cycles. (As the motor  runs, current flowing through the motor’s arma ture warms the windings. Exceeding the maximum  allowable winding temperature of 110 °C would damage the motor).  • Ensuring the gimbal would meet its pointing accuracy requirement s (backlash, hysteresis, windup)  • Assessing whether the mechanism would backd rive under MER mobility loads (induced by  situations like driv ing ov er or of f of large rocks).   • Ensuring that the mechanism would not damage it self or surrounding hardware if the mechanism  drove into a hard stop at peak voltage. 

Motor Overheat Analysis  Each drive was required to perform the fo llowing operations in rapid succession at both  protoflight operating temperature extremes  while at the minimum operating voltage:  1. Slew from hard stop to hard stop at 6 deg/sec  2. Stall for 2 seconds against the har d stop at the motor current limit  3. Slew back to the other stop at 6 deg/sec  4. Slew back 180 deg at 6 deg/sec  5. Rotate one 0.2 deg step (i.e. overcome starting torque) 

This test was designed to ensure that the drive would be able to perform operational  requirements, even after the Rover suffered a power reset condition. Thus, if the drive was at the  end-of-travel hard stop, ground cont rollers could home the drive (1st slew plus 2 second stall),  and then send the gimbal to the end-of-travel hard stop as if to resume communications (2nd  241