Integrated Model Reduction,Identification Technology And Attitude Control For A Strap-on Launch Vehicle

With the development of aerospace technology,the form of launch vehicle has been changed from the traditional series one to the strap-on one.Especially with the increasing of demand to the carrying capacity,the strap-on launch vehicle presents the trend of seriation,modularization and large scale,which puts forward new requirement for dynamic modeling of the launch vehicle.To conveniently and exactly describe dynamic behavior of the large and flexible system,it should be discrelized as a multi-degree-of-freedom system in the modeling process and the order of dynamic model established is often very high,resulting in that the traditional analytic or semi-analytic methods for dynamic modeling of the vehicle are both not available again.Moreover,the strap-on launch vehicle usually shows the characters of dense low-frequency and obvious coupled vibration,and it is not simple to get rid of noise in modal vibration test.These factors can easily result in false values of structural modal parameters from tests.Therefore,it is necessary to study the model reduction method and modal parameter identification technology for the strap-on launch vehicle to effectively reduce the degrees of freedom of structural dynamic model and improve the identification accuracy of flexible parameters.On the other hand,due to the vibration modes of the strap-on launch vehicle become more and more complicated,attitude dynamic modeling method for the traditional series launch vehicle can’t effectively describe the attitude motion of the strap-on launch vehicle,and the attitude control design for the strap-on launch vehicle becomes more complicated and difficult.Therefore,it is necessary to research on new attitude dynamic modeling method and controller design for the strap-on launch vehicle to improve model accuracy and controller stability.Using a strap-on launch vehicle as research object,five aspects are studied in this dissertation: structural dynamic modeling and model reduction method,modal parameter identification technology,attitude dynamic modeling,multivariable frequency-domain control design,and the relationship of structural dynamics and attitude stability.The research was funded by the National Natural Science Foundation of China(Grant No.’s 11132001,11272202),the Key Scientific Project of Shanghai Municipal Education Commission(Grant No.14ZZ021)and the Natural Science Foundation of Shanghai(Grant No.’s 14ZR1421000).The main research and achievements are as follows:(1)A two-step model reduction method is proposed for the strap-on launch vehicle.In this method,the double-compatible free-interface modal synthesis method is first used for the modeling of the system by preserving the lower-order modes of flexible components of the system;then the modal cost analysis method is applied for the system to further reduce the order of the system.After these two steps,a low-order dynamic model of the strap-on launch vehicle can be obtained.Finally,numerical simulations are carried out to verify the validity of the proposed dynamic model.Simulation results indicate that the accuracy of double-compatible free-interface modal synthesis method is higher than the traditional free-interface modal synthesis method and fixed-interface modal synthesis method,especially in the high-order frequencies;and the low-order dynamic model established by the two-step model reduction method can effectively reflect the characteristics of the original system.(2)Modal parameter identification technology for the strap-on launch vehicle is studied in this dissertation and a modal parameter identification method based on input and output data of the system is given.At first,the Markov parameter of the system is obtained by the observer/Kalman filter identification(OKID).Then the modal parameter identification is performed using the eigensystem realization algorithm(ERA)based on the acceleration signal.In the studies,white-noise,sine and impulse signals are separately used as the input of the system and the output is measured by acceleration sensors.Simulation results indicate that OKID and ERA can efficiently identify the natural frequencies of the strap-on launch vehicle.This identification technology can be used as a supplement of ground test of the launch vehicle.(3)Nonlinear attitude dynamic modeling for the strap-on launch vehicle is studied in this dissertation.Base on the dynamics of flexible multibody system,the attitude dynamic model of the strap-on launch vehicle is derived.At first,the trajectory of launch vehicle is designed and the simulating curves of the trajectory parameters are calculated.Then,the attitude dynamic model of strap-on launch vehicle is derived,and this model is simplified and linearized using the small disturbance theory to obtain the small-disturbance linearization attitude dynamic model.Finally,the small-disturbance linearization attitude dynamic model is compared numerically with the Adams sofeware to verify the reliability of this model.(4)Multivariable frequency domain design is carried out for the strap-on launch vehicle using the small-disturbance linearization attitude dynamic model.At first,due to the high dimension of coefficient matrix of state equations of the attitude dynamic model and the low effective bits of polynomial coefficients of transfer function,a new method for calculating the transfer function with high accuracy is designed and the transfer function matrix of three-channel coupling is obtained.Then the attitude control parameters of every characteristic second during boosting phase of the vehicle are obtained by the inverse Nyquist method.Finally,variable-parameter time-domain simulations are carried by integrating the control parameters of every second to verify the accuracy of transfer function model and the effectiveness of attitude control design.(5)Based on the small-disturbance linearization attitude dynamic model,four factors neglected in the attitude dynamic modeling of the traditional series launch vehicle are considered in this dissertation.Firstly,frequency-domain control design and variable-parameter time-domain simulations are conducted with and without considering the small disturbance factor related to the rigid-flexible coupling inertial force,and system stability for these two cases are illustrated.Secondly,the factor related to the overload is considered in the frequency-domain control design and variable-parameter time-domain simulations,and the influence of overload on system stability is discussed.Thirdly,the longitudinal modal components of the strap-on launch vehicle are taken into account in this study,and the influence of the longitudinal modal components on system stability is studied.Finally,the local modes of booster is considered independently and the influence of local modes of booster on system stability is investigated.