A recent study published in Space: Science & Technology by a research team led by Dr Honghua Zhang of the Beijing Institute of Control Engineering, breaks down the design of the Chang’e-5 GNC system. “We hope that the GNC methods presented in this article find application in future lunar exploration, crewed space exploration and other missions,” said Dr Zhang. The Chang’e-5 spacecraft consisted of an Orbiter and Return Module (ORM), which remained in lunar orbit in space, and a Lander and Ascender module ( LAM), which landed on the lunar surface.
The ascending part of the LAM then returned to the ORM with lunar samples. The ascender then separated from the ORM and performed a controlled desorbit to land on the lunar surface. While previous Chinese lunar missions, Chang’e-3 and Chang’e-4, had successful soft landings, Chang’e-5 was the first to plan a return to ORM, offering a new challenge for the overhaul of its GNC system. The former helped control the angle and angular velocity of the LAM, allowing for rapid and stable attitude reorientation. Sloshing was addressed by the filter, which limited the pitch, yaw and roll of the LAM, improving the robustness of the control system and helping to avoid instability. Finally, the double observer obtained better control performance by estimating and compensating for disturbances such as sloshing, misalignment of the center of mass and disturbances induced by the thruster.
The scientists used a reconfigurable three-part attitude control that included a “quaternion” partition control, a phase and gain stabilization filter, and a dual observer. The LAM used various sensors to aid in its descent and landing. The inclusion of multiple sensors for the same purpose has improved the redundancy of the system, which is crucial for space missions where there is no possibility of repairing a faulty sensor.
The LAM should be released from the ORM in a ‘horizontal’ orientation but land in a ‘vertical’ orientation. Designing the correct method of guidance that allows for this righting process, called attitude reorientation, while avoiding sloshing of the liquid propellant, which would create an imbalance of internal forces in the LAM, derailing the descent, was crucial. Chang’e-5’s LAM contained new components, including a “light surface tension tank” for the increased amount of propellant needed to revive the LAM from the lunar surface.
“The data fusion methods described in our work for on-board sensors, such as inertial measurement units (like gyroscopes and accelerometers) and velocimeters, have the potential to improve future air vehicles, both on land. and cosmic. ” But how did the Chang’e-5 detect the faulty sensors and choose others? This was achieved through a process called intelligent heterogeneous sensor data fusion, where the outputs of many sensors with similar use were compared. If the output of a sensor was found to be significantly different, the sensor was deemed to be faulty and one of the other sensors would be chosen for navigation. According to Dr. Zhang,
The Chang’e-5 spacecraft returned to Earth on December 16, taking with it the youngest lunar samples (only 1.2 billion years old!) Ever obtained by mankind. The success of the Chang’e-5 mission, due in large part to its innovative GNC system, paves the way for future space exploration missions such as the landing of an asteroid, as well as the most daring dream of humanity: traveling in space with a crew. Originally posted on Eurek alert
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