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Investigation on the Soft-landing Dynamics and Semi-active Control for Lunar Lander

Author: WangAnLiu
Tutor: NieHong
School: Nanjing University of Aeronautics and Astronautics
Course: Aircraft design
Keywords: lunar lander soft landing dynamics magneto-rheological semi-active control
CLC: V476.3
Type: PhD thesis
Year: 2012
Downloads: 227
Quote: 2
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Abstract


Lunar exploration in future requires to land anywhere to be landed and to observe anything tobe observed. That means lunar lander should be able to land and return safely and reliably even intough lunar environments and emergency landing conditions. However, as an important part oflander, the buffer system still uses passive control technologies and cannot meet the requirementsof the lander of next generation. The main reason is that the buffer system does not changeits characteristics to respond timely to external excitation after it is located andparameters are fixed. Passive control technologies should continue to be improved. At the sametime, semi-active control technologies have also become research direction. The main researchcontents are as follows:A type of lunar lander with four cantilever hinged legs is selected for research and a softlanding dynamic model is developed for a single set of landing leg, the aluminumhoneycomb cushioning materials and the general buffering materials are also put intoconsideration. The soft landing dynamic model for a single set of landing leg is simulated andvalidated by the corresponding drop test. As a result, the model is proved accurate and effective. Inthe research process, the author also analyzed the impacts on soft landing of lunar lander from theaspects of initial velocity of lunar lander, the friction on the surface, the slope of lunar surface, thebearing friction of the primary strut, and the thrust of the retro-rocket. It is proved that the initialvelocity should be reduced as much as possible. When the friction coefficient is determined, therange of the opening angle should not be designed too small. The impact of downhill landing ismuch more than uphill landing. The bearings friction can protect the primary and secondarybuffering systems. The retro-rocket with engine power is significant to reduce overload duringlanding, and it is appropriate to choose0.6as the thrust-weight ratio.The2-2landing mode of lunar lander is selected and a soft landing dynamic model with9DOF is established. Through the corresponding drop test, the mode is proved accurate andeffective. The soft landing performance impacts of the initial velocity of lander, the angle of lunarslope, the angle of initial landing attitude and the thrust of the rocket are analyzed. By doing so,the concept of safe angle scope is proposed to analyze the coupling effects of the slope of lunarsurface and the attitude of lunar lander. The results show that the horizontal speed will reducethe body’s maximum overload and landing stability. When downhill landing, the lunar anglereduces the stability. In the range of0to0.16rad, the initial attitude angle will reducethe maximum overload. The stability of landing with engine power is higher; the stability of uphillwith engine power is the best. The larger the angle between the initial attitude of lunar lander and the slope of lunar surface, the greater the overload of lunar lander, and the worse thelanding stability. If the maximum overload is critical, the safe angle scope is small. When the lunarsurface and the body attitude are both level, the maximum overload is much larger.A configuration of bypass-type MR damper is designed for lunar lander and the dynamicmodel of the bypass damper is established, the impacts of turbulent state and the local resistanceof damping channel are also put into consideration. A sketch of the bypass-type damper is givenwith structural parameters, and a drop test simulation of the damper is built. The impact of thefluid behavior index and corrected model are analyzed in simulation. A model with two differentcurrent buffers operated by the limits of buffer stroke is designed. Study showsthat MR dampers can meet the requirements of lunar soft landing in the buffer stroke, maximumoverload and buffer efficiency. The maximum buffer stroke can be adjusted and the maximumoverload can be changed by current intensity, which provides the controllability and feasibility ofdamping of lunar lander. Shear-thinning will reduce the output force and buffer efficiency of thedamper. The corrected model of MR damper has worse buffer performance, lowerbuffer efficiency and larger maximum overload. When designed legitimately, A MR damper withtwo buffers can also have the advantages of both low current intensity and high current intensity.The buffer control method of soft landing is proposed and the mathematical model of aquarter of lunar lander is established. The first step is to design the state feedback optimal controlsof voltage and force, and the second step is to design the three semi-active control strategies,including jump-state, multistate and fuzzy logic control, and their controllers. The soft landingbuffer controls of lunar lander are simulated and their control effects and the sensitivities to initialcondition are analyzed. The results show that the optimal control of voltage can provide the bestcushioning performance, and reduce vibration during post-landing, but its online computation timeis too long. The optimal control of force is best for the reduction of the maximum overload, but itis difficult to completely fit the optimal force. It is very important to choose the structuralresponse limits for jump state control. It is better to adjust parameter of multistate control strategyfrom0.5to0.8. The fuzzy control is very effective. Off-line calculation can be used to reduce thecomputation time.The stability control method of soft landing is proposed and the mathematical model of a1-1landing mode with fixed legs is established. The process of soft landing is refined as twophases and the landing stability is analyzed. Three strategies, such as Jump-state control strategy,multi-state control strategy and fuzzy logic control strategy, are established, and their controllersare designed. The stability controls for soft landing are simulated and their control effects and thesensitivities to initial conditions are analyzed. Research shows that jump-state control, multi-statecontrol and fuzzy logic control can largely reduce the maximum overload and the maximum attitude angle, improve the stability of soft landing, the sensitivities to initial velocity and safetyangle scope. Fuzzy logic control is the best in integrated control effects, and then the multi-statecontrol and jump-state control. These three control methods have strong adaptability to landingconditions and lunar environments, so that they can meet the requirements of lunar lander even inextreme landing conditions and tough lunar environments.

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CLC: > Aviation, aerospace > Aerospace ( Astronauts ) > Spacecraft and their means of delivery > Space station and space probes > Lunar probe
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