Hybrid Planning System for In-Space Robotic Assembly of Telescopes using Segmented Mirror Tiles

The use of large telescopes is increasingly important in the field of astronomy. However, for larger telescopes there is a technological limit for producing primary mirrors made of a single rigid piece. A similar problem arises for space-based telescopes, where the cargo areas of launch vehicles provide an upper limit for the maximum aperture of the telescope, even if it is deployable as the James Webb Space Telescope. As larger telescopes are required, in-space assembly emerges as a feasible alternative, making use of a highly autonomous robotic system that directly constructs the structure on orbit. Although this might reduce the overall costs of construction and maintenance of the structures, such robotization brings additional challenges for an automatic planning system. This paper presents the study and implementation of an autonomous assembly planner for in-space robotic assembly of telescopes using segmented mirror tiles. The proposed planner takes into consideration different constraints. On the one hand, robot physical constraints such as joint position and torque limits, and moments of inertia of the sub-assemblies must be considered. On the other hand, semantic constraints representing the possible types of connections between tiles must be specified to ensure the mechanical stability and electric and data connectivity within the structure. In order to generate a plan that considers these constraints, we propose a hybrid assembly planner with a global and a local layer. The global layer uses graph search strategies to explore a graph representation of possible assembly sequences, ensuring that the potential solutions fulfill all the semantic constraints describing the allowed connections between tiles. The local layer uses a constrained path planning algorithm combined with a dynamic simulation to ensure that the transitions between states (representing sub-assemblies) fulfill the required physical constraints. The final assembly sequence to be executed is selected by minimizing a suitable cost function. The planner is integrated into a ground-based demonstrator for autonomous robotic assembly of mirror tiles, developed within the PULSAR EU Project. Simulations for larger telescope structures demonstrate the generality of the approach. In order to illustrate the flexibility of the planner, different physical constraints and optimization goals were implemented and verified. The planner successfully adapts the assembly plan accordingly.