Figure 9.  Azimuth driv e “flipper” hard stop 

Structural Analysis Overview   Two of the significant driving requi rements for the HGAG system were  the launch locked first fundamental  frequency of 50 Hz and the deployed first fundamental frequency of 14 Hz. A detailed finite element  model of the HGAG assembly was developed to predict the overall stiffness of the system and assist in  finalizing the design. The model was statically and dynamically calibrated to provide accurate mass  distribution. 

Stiffness predictions for the el evation and azimuth bearing sets were determined using BEAR3D [2], a  nonlinear program that allows the analysis of thr ee-dimensional assemblies of linear elastic beams and  bearings. The computer program is based on the sti ffness method of structur al analysis and the bearing  technology presented in the New Departure Engineering Handbook of 1946 [3]. The New Departure  developments were established for two-dimensional bearing analysis. The techniques were extended to  full three-dimensional analysis with five degrees  of freedom to represent each bearing. BEAR3D  generates a bearing stiffness matrix that is easily incorporated into a MSC.NASTRAN stru ctural system  model of the mechanism. In addition to the bearing stiffness predictions, the minimum manufacturer  published values were used for the harmonic drive spring rate. 

Nonstructural lumped masses include the gear motors , the HGA, the pyrotechnic pin puller, and the  potentiometers. All lumped masses were attached to the structure in such a way as to not stiffen, nor  strengthen it. The pin puller mount was modeled as four  titanium bars of square cross-section. The pin  puller was solidly attached to the apex of this four-s trut mount. The modal predictions and test results are  shown in Table 3. The deployed first mode is largel y dependant on the spring rate of the harmonic drive.  Because the lowest published value for the spring ra te was used in the FEA model, the prediction was  lower than the actual test results achieved by the flight hardware. 

Table 3.  First mode frequencies of the HGAG w ith the HGA attached  Launch Locked Deployed Requirement 50 Hz 14 Hz Predicted 54 Hz 19.5 Hz Actual 51 Hz 26 Hz

Analysis was also performed on the structural stresses in the assemb ly induced by launch, landing, and  mobility loads as well as thermal gradients exper ienced while operating on Mars. For the launch and  landing loads, the 45 g quasi-static load was applied to the model in each of the three primary orthogonal  axis with the launch lock restraint engaged. For t he mobility loads, the expected maximum g load was  applied in each of the 3 axis with the launch lock restra int eliminated. In all case s, the structural elements  of the gimbal showed acceptable positive margin. 

The HGAG is exposed to the environmental temperatur e extremes during Martian operation.  The thermal  stress analysis for the structure was done using t he same FEA model. Overall system temperature  changes (-120 °C to +110 °C) induce no appreciable stress in the system because all the structural  elements are either identical (tit anium) or closely matched in CTE (440C steel bearings). However,  thermal gradients in the gimbal sy stem can develop from some component s seeing direct sunlight while  others remain in the shade. It was important to analyze whether or not these gradients could induce  enough additional load in the pyrotechnic pin puller system  to cause it to bind prior to being fired. The  worst-case gradients come from the hot transient case . During this transient case, it was predicted that  the assembly could develop a 17.9 MPa peak stress and a 9.8 N side l oad on the launch lock pin. This  result was found to be well within the perform ance capabilities of the pyrotechnic device.  248