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IDAC carried out linear static structural analysis of two satellite antennas for HITEC. Both designs, namely VINA and GTTC, were built with the purpose of ground tracking and control of satellites for commercial use, respectively, for the European version of the GPS constellation. As the current design stands, the antenna consists of a reflector dish that is shaped and held together by a skeletal back-up structure, which then is attached to the centre hub. The entire assembly sits on a Yoke that can rotate in the azimuth and elevation planes. When analysing the antennas, five million degrees of freedom had to be evaluated and analysed.
The primary objectives of the analysis were twofold. The first was to ensure the stiffness of the assembly under its own weight and the second was to validate the design under wind and ice loading conditions.
The analysis for different loading conditions was conducted using ANSYS Classic and Workbench environments in conjunction with each other.
Structural integrity under gravity
Ensuring the entire structure did not undergo excessive displacements or bending moments whilst standing under its own gravitational weight was a key design parameter. Aside from obvious safety concerns, the functional reasons for this, lay in achieving a high pointing accuracy for the antenna. Whilst a stiffer structure may not always yield better Pointing Accuracy, designing for minimal differential displacements does enhance this parameter; the lesser the displacements, the smaller is the resultant signal noise and distortion.
Given the size of the FE model, it was decided to break the analysis into components that could be analysed separately. In order to make sure that the right forces were applied on each component as a boundary condition, a preliminary analysis of the entire assembly was first carried out. The relevant loads from the complete model were then applied to the corresponding component, which were then analysed separately. In ensuring that each component provided minimal displacements, several design iterations were carried out to meet this requirement. Once the mandated structural stiffness was reached, the entire assembly was once again put together and analysed, using the new designs. Working closely with the client, IDAC was able to reduce the weight of the overall structure as part of this process.
Wind and ice loading
During regular operations in the year, wind gusts and ice formation are repeating phenomena that add external loadings to the antenna assembly. With the basic structure validated under its own gravitational weight, a separate analysis was conducted for each loading condition.
Under icing conditions and wind loading, each subcomponent and the overall structure proved to be structurally stable. Having analysed each separate loading scenario, a combination load of gravitational weight + wind loading + ice load was then applied to the antenna structure. Again simulation data suggested that the resultant stresses and deformations fell within the design limits established by HITEC.
Stress due to varying orientations
With the antenna rotating in the azimuth and elevation directions, the loads due to wind gusts specifically changes with orientation. The wetted surface area increases or decreases depending on the direction of the wind and the relative orientation of the antenna. Consequently, it is important to analyse the stresses in various orientations before allowing the structure to be put into full operational use. The specific orientations that were analysed were the results of prior wind tunnel testing and analytical modelling carried out by HITEC. A macro was created in ANSYS Classic that calculated the resultant stresses, regardless of the orientation.
Using a combination of ANSYS Classic and Workbench interfaces, IDAC was able to assist in a design process that maximised strength, whilst minimising the overall weight under different loading conditions. At the end of the project, IDAC had the ability to provide a complete analysis run from model generation to post processing, within a time frame of just one week. By exploiting ANSYS’ ability to interact with external geometry, while also taking advantage of DesignModeler’s dedicated FEA geometry tools, run times for the entire model were kept to just over an hour. As a consequence of small run times and the extensive use of macros, several load cases could be investigated in one day.
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