Optimal Designs of the Structure Configuration of the Two-Axis Steering Mechanism of the FAST

The feed support system of the Five-hundred-meter Aperture Spherical Telescope(FAST)suspends and drives the receiver to track astronomical objects in high precision for observation. A two-axis steering mechanism has an auxiliary function in the process of real-time adjustment of the feed position and altitude angle.We design such a mechanism of truss configuration to satisfy requirements on mass and rigidity. In numerical optimization of the designs, we use an objective function reflecting constraints on compliance and weights and adopt cross sections of various rigid rod components and geometrical parameters of the structure as variables. The optimization is subsequently achieved by the steepest descent method of 1-dimensional search,which is based on the sensitivity analysis of the variables(reduced to be dimensionless)in each iteration. Several sets of initial conditions for optimization have been randomly generated. Corresponding optimum results have small mutual deviations. With the similar optimization method but using nine controlling points of heights as variables, we find a design of configuration with varying heights along the ring of the mechanism. Numerical experiments favor the varying-height design as it has higher stiffness-to-mass ratios.