Design, Development and Ground Testing of the Hexapod System

Abstract

An accurate positioning system is required to precisely adjust the mirrors of a space telescope, and efforts have been made for developing such a system. This telescope is intended to be accommodated within launch vehicles and incorporates segmented mirrors to effectively accommodate large mirrors. Following the unfolding process, the segmented primary mirror becomes pivotal in focusing light from distant celestial bodies. The configuration of these segmented mirrors significantly influences the resulting image quality. To address this, six actuators have been designated for the six degrees of freedom of these segmented mirrors. They are strategically positioned behind each segment, allowing for both fine-tuning and coarse adjustment. As the full mirror is divided into smaller segments, meticulous design is essential to ensure seamless alignment after unfolding and positioning. These actuators are constructed using commonly available mechanical components, such as flexures, gears, bearings, camshafts, and ball screws. The coarse-drive Tumbler coupling facilitates independent fine motion control. The engagement of these shafts produces combined motion, while disengagement results in only fine motion. A crucial component of the design is the motion reduction structure, a sophisticated passive system that smoothly converts coarse input (in micrometers) into precise output (in nanometers) without any backlash. The actuators have been assembled and tested which deliver both coarse and fine motion adjustments, achieving a remarkable resolution of less than 10 nanometers across a 5-millimeter range. A comprehensive control mechanism, inclusive of feedback control, is implemented to precisely manage both coarse and fine motions, achieving the desired positions. In the initial phase, two actuators are integrated into bipod structures for 2 degrees of freedom control. Subsequently, the study progresses to the hexapod assembly and control and optimization techniques involved in developing the actuators and the entire 6 degrees of freedom hexapod assembly.