面向火星着陆器触地模拟试验的重力卸载系统

A designed gravity compensation system for landing preview of Mars lander

在地面环境下开展可靠且快速的验证试验是保障探测器成功着陆的关键。该文基于零自由长度弹簧模拟“天问一号”火星着陆的重力卸载系统,在触地阶段为探测器提供火星表面的重力环境,破解卸载力在触地及剧烈碰撞过程中平稳、恒定输出的技术难题,利用弹簧对冲击力等高频干扰的截断能力,使重力卸载系统在剧烈动态过程中保持平稳的输出。试验结果表明:系统的恒力输出最大平均误差为1.5%,可有效保障大行程、重载、高速和冲击条件下恒定卸载力的施加。系统的负载可调设计可适配不同目标行星的重力环境,提高了相关触地模拟的试验效率。

[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.