approximately 150 cm. In order to estimate whether the azimuth drive could withstand the  maximum expected 6.6 g loading without back driving, the maximum expected drive train  efficiencies were conservatively estimated:  • Harmonic drive back drive torque: 0 Nm  • Harmonic drive starting torque: 0 Nm  • Planetary and spur gear efficiency: 1.0  • Harmonic drive efficiency: 0.85  • Minimum detent holding torque: 2.7 mNm  Using these assumptions, the azimuth drive c ould only withstand a 2.2 g load before it would  start to back drive. Because the drive could not meet the 6.6 g r equirement without any  movement it was important to understand how much  movement would result from the event. A  finite element analysis indicated that the 6.6 g loading resembled a 20Hz sine wave that would  decay over 1 second. Under this condition, it was estimated that the maximum angular excursion  for the drive would be 0.07 deg. This 0.07degree excursion was deemed a cceptable by the MER  system.  In addition, it was determined that t he minimum motor detent size required to stop  motion would be 1.6 mNm which is well below t he minimum detent holding torque of 2.7 mNm for  the HGAG motors . 

Drive Im pact Analysis  To ensure that the drive would not damage itself  when driving into a hard stop, the maximum  torque that would develop as the hard stop hal ted motion was determined. This was found by  estimating the total inertia of the moving parts , the maximum angular velocity, the torsional  stiffness of the gear train, and the maximum contribution of the detent in adding to the detent  torque. Since the reflected inertia of the gear trai n increases with the square of the gear ratio, the  majority of the total inertia was the gear train’ s early stages. In fact, the inertia of components  attached to the motor shaft was 10 times larger t han the inertia of the entire mass attached to the  drive’s output. Fortunately, the harmonic drive’s torsional compliance mitigated the peak torque  developed and helped to ensure that no damage woul d occur when the drive impacts the hard  stops. 

RF Sy stem   Once the drives have correctly  positioned the High Gain Antenna to send and receive communication  signals, the HGAG must provide an RF  system to transfer that inform ation from the antenna to the Rover  electronics. The HGA and coax routing within the gimbal comprise the RF system.  The HGA used was  almost identical to the HGA used for the Mars Path finder Lander, with structural modifications made to  accommodate the cantilevered mounting on the HGAG. Given the 0.5 dB maximum loss requirement, it  was important to minimize RF cable length and the number  of RF fitting interfaces used, in order to limit  transmission and insertion losses.  

The RF signal must be carried across both the rotating el evation and azimuth drives of the gimbal.  It was  not desirable to accommodate the large rotation angles  required in the drives through bending or flexing  of the RF cables. To avoid this, the design employs tw o RF rotary joints to transmit the signal over the  rotating interfaces. The rotary joints are positioned inte rnal to the gimbal and are aligned with the axis of  the azimuth and elevation drives.  The hollow center design of the drive train allows the RF system to be  located right on the center axis of  each drive which simplifies the ov erall RF system. Routing the RF  system predominantly internal to the HGAG also hel ps to protect the cables and rotary joints from  damage as well as large temperature gradients. The ro tary joints were supplied by Kevlan Corporation  under contract from JPL. In order to limit the amount  of torque required to rotate the RF joints, small  heaters are placed near the bearings in the joints in order to maintain a minimum operating temperature.  Between the two rotary joints, the RF signal is carri ed through semi-rigid coax cables supplied by W.M.  Gore under contract from JPL. 

The final RF design areas that needed to be addressed we re how to provide the torque required to turn  the rotary joints without carrying the torsion load th rough the semi-rigid RF cables, and how to protect the  internal drive components from contamination enteri ng through the RF system. In order to prevent the  rotary joint torsion load from being carried by the cabl es, a “torque tube” is attached to each drive output.   The torque tube reaches back down the center hole in each drive and engages two flats on the input to  243