SUI Yi, SUN Haining, HUANG Wei, DONG Qiang, LI Guangyu, ZHANG Jianyong, ZHANG Yajing
[Objective] The Tianwen lander has to follow the same sequence as most space missions landing on other planets (or back on Earth), a process known as Entry, Descent, and Landing. NASA engineers have described the descent of the Mars landing missions as "seven minutes of terror" as it is the most unpredictable. About 20 attempts to land on Mars have been made by different countries so far. Besides a considerably thinner atmosphere, Mars's gravitational field is weaker than that of Earth; thus, on average, it delivers 38% as much downward acceleration. In this paper, a large-scale gravity compensation system is designed to simulate the Martian environment to test the Tianwen-1 lander during the Mars landing.[Methods] Considering severe collisions and abrupt changes of states during the landing, the system uses multiple elastic elements, including springs, to eliminate undesired high-frequency vibrations, thereby enabling the system to maintain a stable output during severe dynamic processes. The compensation system is composed of an adjustment mechanism, springs, guide bar, wire rope, and oscillating bar. To achieve the target stiffness, five springs are used in parallel. By adjusting the adjustment mechanism, the initial preload of springs can be varied to match different loads with masses varying within a certain range. The guide bar can restrain the lateral movement of the spring, thereby ensuring that it maintains a stable state during shortening and elongation. Additionally, it can also offset the weight of the springs. Conversely, the equivalent replacement of the zero-free-length spring enlarges the stroke of the system. To achieve the equivalent replacement of the zero-free-length spring, an additional mechanism and pulley are designed. Then, the mechanical properties are explored from the perspective of energy conservation. Eventually, the relationships among the characteristic values of each component in the system can be determined.[Results] Three major issues had been resolved by the gravity compensation system. 1) With the lander moving at high speed, the system successfully achieved gravity compensation for heavy loads (7 200 N) in a long stroke (800 mm). The error between the experimental and simulation results was within the allowable range. 2) When the lander hit the ground, the system output a constant force (maximum error:7.8%), thereby implying that the system had good adaptability for dynamic processes. 3) During the entire landing process, the tracing accuracy (maximum error:7.8% and average error:1.5%) of the constant-force output from the system had already met the requirements (maximum error:10% and average error:10%).[Conclusions] To fully or partially compensate for the gravity of landers during the landing process, this paper presents a large-scale gravity compensation system. The design, analysis, fabrication, and experimental testing are implemented to investigate the performance in terms of the constant-force output. With the Mars lander descending at high speed, the system successfully achieved gravity compensation for the heavy load (7 200 N). During the landing process, the tracking accuracy of the output force of the system has already met the requirements. Furthermore, the compensation system can be quickly adjusted to suit a target planet.
