A Study On Soft-landing Dynamics And Dynamic Substructuring Techniques For Lunar Probe

The safe and steady landing of lunar probe on the surface of the moon is asignificant mark of the successful completion of the second phase of the Chinesemoon-landing project. To ensure the success of this mission, we need to adopt anecessary approach—the analysis of soft-landing dynamics. Confronted with the needfor the design work of lunar probe, with the aim of accurately predicting the dynamicresponses to impact process caused by the soft landing, hence to provide referencesfor designing the subsystem of probe structure and for determining its conditions forconducting the mechanical environmental test, this thesis set up the soft-landingdynamic model of lunar probe using nonlinear finite element method in considerationof various nonlinear factors, and conducted research and analysis on the impactprocess of probe’s landing based on the established model. At the meantime, in orderto accomplish the tasks such as design-oriented structure optimization, and modelmodification, the author carried out technical research on corresponding dynamicsubstructuring techniques.The simulation-based efficiency and accuracy is a pair of contradictions in thefield of the dynamic analysis of lunar probe’s soft landing. In view of this pair ofcontradiction, choosing building equivalent model and finding efficient solvingstrategies as breakthrough points, and combining with the practical problemsoccurring in lunar probe’s structure design, this thesis has conducted research areas infour aspects as follows.(1) In terms of the time consuming problem of solving nonlinear finite elementequations, the author has proposed an equivalent but simplified model for buffersystem, so that tremendously decreases the number of nonlinear DOFs of the system.Adopting implicit integration algorithm to solve the motion equations of the system isable to shorten the computing time so effectively as to cut down the time consumptionof probe’s soft landing dynamic analysis from the usual one day to just one hour withthe simulation precision guaranteed. Compared to the data getting from groundlanding impact test, when examining the key testing point we have found that the acceleration response’s relative peak error is controlled within15%, which proves thevalidity of the simplified model and integration algorithm.(2) With the aim of figuring out the overall nonlinear iteration problem caused bythe local nonlinear feature when using integration algorithm, this thesis defines thelinear part of probe structure as a substructure and conducts the order reduction to thislinear substructure in modal domain. Moreover, a sort of modal truncation criterion isput forward to carry out the second order reduction for reduced model. Through twiceorder reduction, the DOFs of participating the nonlinear motion equation is furtherdecreased, which cuts down the time consumption of probe’s soft landing dynamicanalysis from one hour to kilosecond level. In comparison to the test result of theoriginal model’s landing impact experiment, when examining the key testing point wefind that the acceleration response’s relative peak error is controlled within5%,proving the effectiveness of the modal domain substructuring approach.(3) With regard to the problem that in small damping case modal domainsubstructuring method can’t accurately predict the probe’s acceleration response, thethesis raises a kind of reduced-order iteration format suitable for predictingacceleration based on IBS method, and proves that this format, which has thedistinctive features of guaranteed precision and unconditional numerical stability, isequivalent to Newmark implicit integration algorithm. Using the buffer load obtainedfrom the test of probe’s soft landing as an incentive, the thesis analyzes the lunarprobe’s acceleration responses by modal domain substructuring method and IBSmethod respectively. Numerical sample indicates that the latter method is better thanthe former one both in computational accuracy and efficiency, proving that IBSmethod is more appropriate than modal domain substructuring method for coping withthe lunar probe’s soft landing impact problem.(4) In order to tackle the traditional IBS method’s inability to simulate theflexibility of the connecting joints between the solar wing and the center body of lunarprobe, linear and nonlinear hybrid joint description method (HJDM and NHJDM) isproposed, which is able to simultaneously simulate both the rigidity and flexibility ofthe joints between substructures. These two methods make up for the inappropriatedescriptions of joints’ dynamic behavior given by traditional IBS method, and make the joints independent of substructures. Therefore, when the dynamic parameters (e.g.mass, damping and stiffness) of the joints change, researchers can get the entiresystem’s responses without doing reanalysis on the substructures. In this way, it ismore convenient to identify and revise the dynamic parameters of the joints, and toimprove the dynamic characteristics of such important equipment as solar wingthrough optimizing the dynamic performance of joints. Moreover, NHJDM canreduce the dimensions of effective tangent stiffness matrix applied to Newton iterationfrom the entire substructure’s DOFs to joints’ DOFs. Therefore, this approach hasenormously improved the ability of IBS method to solve local nonlinear problems.