Case Studies
Go home on the 5th! Application of motion capture system in spacecraft rendezvous and docking research
College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics
Motion capture, air flotation platform, spacecraft ground verification, rendezvous and docking, semi-physical simulation
Three-degree-of-freedom air floatation table

On December 17, 2020, Beijing time, the Chang 'e-5 returner of the lunar exploration project landed successfully in the scheduled area of Siziwangqi, Inner Mongolia, which marked a successful end to the three-step development plan of "circling, falling and returning" of the lunar exploration project in China. A series of key technologies realized in the Chang 'e-5 mission are of milestone significance for China to upgrade the space technology level, improve the lunar exploration engineering system, carry out lunar scientific research, and organize follow-up lunar and interstellar exploration missions.

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During the whole journey of Chang 'e-5, one of the major difficulties is the rendezvous and docking of the lunar orbit of the riser, orbiter and returner combination. Spacecraft rendezvous and docking, as an important space activity, provides basic technical support for space missions such as on-orbit service and manned spaceflight.

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Because of the difficulty and high cost of space rendezvous and docking, it is necessary to conduct equivalent ground simulation test to verify the effectiveness of the control system before the spacecraft carries out the space mission. The difficulty of the ground test is to simulate the microgravity environment in space. The ground semi-physical simulation platform based on the air flotation platform can simulate the actual state of the satellite's on-orbit motion and control to a great extent, and realize the ground simulation with high confidence.

Spacecraft ground semi-physical simulation platform is usually composed of marble platform, air flotation platform (air flotation simulator), communication system and measurement system subsystem. Professor Kang Guohua's research team from the School of Aerospace, Nanjing University of Aeronautics and Astronautics designed the ground semi-physical simulation platform of spacecraft based on 3-DOF air flotation platform, and integrated NOKOV optical motion capture system as a measurement system in the semi-physical simulation platform. Eight NOKOV optical motion capture cameras are erected around the marble air-floating platform in the site, and markers are placed on the top of the air-floating platform according to the designed rules, and the markers move synchronously with the air-floating platform. The motion capture camera, as a state observer, collects and calculates the real-time position and velocity (angular velocity) and other motion state information of the landmark points. The control module combines the above information with the navigation reference signal (relative position and attitude between two air-bearing platforms), and outputs the control force after processing. After thrust distribution, it outputs the required spray force to each nozzle of the actuator. The actuator modulates the signal into a solenoid valve switching pulse signal with PWM to realize air injection, control the air-bearing platform to move with the reference signal, and quickly verify the robustness and convergence performance of the spacecraft rendezvous control algorithm.

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In addition, the team also proposed a centroid calibration method based on position observation, aiming at the change of centroid position (movement of robotic arm, consumption of fuel and combination and deformation of satellites) of planar air-bearing platform in the semi-physical simulation system of micro-satellite, and collected data on the semi-physical simulation platform, and calibrated the centroid of air-bearing platform in real time through the real-time motion trajectory of any three fixed points on the three-axis air-bearing platform body obtained by the motion capture system, which verified the effectiveness of the method.


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