APTMRS: Autonomous Prism Target Maintenance Robotic System for FAST

The Five Hundred Meter Spherical Radio Telescope (FAST) is the largest spherical radio telescope in the world, and there are more than 2,000 prism targets distributed on its reflector that require regular maintenance. These prism targets are screwed into the corresponding threaded holes by target bolts. At present, manual maintenance is mainly used, which is inefficient and unsafe. To address this issue, we develop an autonomous prism target maintenance robotic system, called APTMRS. This system integrates a mobile robot with an assembly robot and a collaborative robot, enabling it to move and perform the prism target replacement task on the reflector. Its technical development is threefold. First, a robotic end-effector that can be used for screwing the non-standard target bolt from the side is designed. Second, a novel pose measurement framework, which incorporates a feature point extraction and matching method, is utilized to measure the pose of the prism target. Third, the manipulation policies for the challenging steps in the workflow are presented, including picking and placing, initial thread mating, and final tightening. Independent experiments indicate that the performances of these methods are satisfactory. Moreover, tests in both simulated and real FAST scenarios confirm the effectiveness of APTMRS in achieving autonomous prism target maintenance.
Note to Practitioners
—Due to its excellent versatility, threaded connections have been widely adopted in a multitude of scenarios. Threads are not only present on standard bolts and nuts but also on a myriad of non-standard components. The robotic system proposed in this paper offers a solution for assembling and disassembling target bolts for FAST. Specifically, the scenario applied in this paper covers several extremely challenging problems in autonomous threaded assembly. Firstly, the object of assembly is a non-standard bolt, with obstacles above it, rendering traditional tools inapplicable. Secondly, the surface of the object is weakly textured and highly reflective. These pose significant challenges for the vision system to accurately acquire its pose. Lastly, the assembly environment is harsh. The threaded holes reside on an incline that is hard to access, with a maximum slope of
$56^\circ$
. To address these challenges, this paper presents a robotic system supplemented by three techniques including a novel robotic end-effector, a new pose measurement framework, and manipulation policies. The experimental results indicate that this robotic system is capable of autonomously disassembling and assembling non-standard target bolts. We explore the potential for generalizing these designs and methods to other scenarios, with the hope of contributing new insights to the development of threaded assembly systems.