查看更多>>摘要：The electro-optical payloads on mobile platforms generally suffer undesirable vibrations generated by maneuvers and turbulence.These vibrations are in six degrees of freedom and cause line-of-sight jitters,resulting in image blurring and loss of tracking accuracy.In this paper,a Hex-apod Vibration Isolation System(HVIS)is proposed and optimized to solve this problem.The opti-mization aims to centralize and minimize the natural frequencies of HVIS,for expanding the vibration isolation bandwidth and improving the vibration isolation in the higher frequency band.Considering that the design space for HVIS is limited and interfered with the frames of the mobile platform,a non-collision algorithm is proposed and applied in the optimization to obtain the fea-sible optimal design.The optimization result shows that the natural frequency bandwidth has been reduced by 42.9％,and the maximum natural frequency is reduced by 30.2％.The prototypes of initial and optimal designs are manufactured and tested.Both simulated and experimental results demonstrate the validity of the optimization,and the optimal design provides a maximum of 15 dB more isolation in rotation direction than the initial design.
查看更多>>摘要：A multi-body dynamic rigid-flexible coupling model of landing gear is established to study the gear walk instability caused by the friction characteristics of the brake disc.After validat-ing the model with the experimental results,the influence of the landing gear structure and braking system parameters on gear walk is further investigated.Among the above factors,the slope of the graph for the friction coefficient of the brake disc and the relative velocity of brake stators and rotors is the most influential factor on gear walk instability.Phase trajectory analysis verifies that gear walk occurs when the coupling of multiple factors causes the system to exhibit an equivalent negative damping trend.To consider a more realistic braking case,a back propagation neural net-work method is employed to describe the nonlinear behavior of the friction coefficient of the brake disc.With the realistic nonlinear model of the friction coefficient,the maximum error in predicting the braking torque is less than 10％and the effect of the brake disc temperature on gear walk is performed.The results reveal that a more negative friction slope may contribute to a more severe unstable gear walk,and reducing the braking pressure is an effective approach to avoid gear walk,which provides help for future braking system design.
查看更多>>摘要：In the process of stage separation of recoverable liquid launch vehicles,because of the large amount of residual fuel in the storage tanks,the influence of liquid sloshing on separation safety must be considered.Considering calculation simplicity and operation practicability,the Mov-ing Pulsating Ball Model(MPBM)of large amplitude liquid sloshing is introduced into the calcu-lation of launch vehicle stage separation.Combining the dynamic equation of the model with the energy relationship during"breathing movement",the formula calculating the force of liquid on the rigid body is derived.Compared with the calculations of commercial CFD calculation software,the accuracy of MPBM model is verified.Then,all the external forces and moments are applied to the rigid body of the stages,so that the translational and rotational dynamic equations of the stages are obtained respectively.According to the relative position of the two stages,the geometric shape of the interstage section and the engine of the second stage,the minimum clearance in the separa-tion process can be decided to guarantee that the separation process is safe.
查看更多>>摘要：Stratospheric airships are long-endurance aerostats and have broad applications.All of the energy required for their operation is obtained from solar radiation,which makes accurate cal-culation of the energy output from the solar array crucial to the design and flight planning of the airships.However,the status of each photovoltaic module in the solar array may differ due to the airship curvature,resulting in mismatch losses and lowered output power,which has not been widely studied.In this paper,an irradiation model and a thermal model are established based on the actual arrangement of the modules.The output power model is established considering the non-uniform radiation in the array.The mismatch losses of the array are analyzed under different flight conditions.The output power of the solar array is decreased by up to 31.6％compared to the ideal state.Moreover,the proportion of mismatch losses increases with latitude,but the maximum mismatch loss power occurs at mid-latitudes.Then,an array reconfiguration method is proposed based on the irradiance dispersion index and position dispersion index.The reconfigured array increases output power by 11.5％and can maintain energy balance in continuous flight.The results can be used to correct the overestimation of the output power during the airship design or to guide the configuration of the solar array.
查看更多>>摘要：The appropriate terminal attitude of missile is important to improve the damage effec-tiveness of fragmentation warhead against aircraft targets.In this paper,three missile terminal atti-tude selection methods are proposed to solve the problem of terminal attitude selection in different situations according to their respective evaluation indexes.The MVE-based method uses the dam-age probability of each detonation point around the aircraft at a given attitude of missile to calcu-late the Mean Volume of Effectiveness(MVE)at the corresponding attitude,and then uses the MVE for different attitudes to select the terminal strike attitude.The detonation position-based method addresses the case where the missile detonation position can be assessed in advance.Given the effects of missile guidance errors and fuze activation position errors,Monte Carlo simulations are used to calculate the damage probability of aircraft with different strike attitudes,from which the terminal strike attitude is selected.The BP-ANN model-based method uses the constructed Back Propagation Artificial Neural Network(BP-ANN)model instead of simulation to calculate the evaluation indexes in the corresponding cases of MVE-based method and Detonation position-based method,which can improve the efficiency of attitude selection.Simulations are con-ducted for different scenarios to verify the feasibility and effectiveness of the proposed method.
查看更多>>摘要：Relative positioning is recognized as an important issue for vehicles in urban environ-ments.Multi-vehicle Cooperative Positioning(CP)techniques which fuse the Global Navigation Satellite System(GNSS)and inter-vehicle ranging have attracted attention in improving the perfor-mance of baseline estimation between vehicles.However,current CP methods estimate the baselines separately and ignore the interactions among the positioning information of different baselines.These interactions are called'information coupling'.In this work,we propose a new multi-vehicle precise CP framework using the coupled information in the network based on the Carrier Differential GNSS(CDGNSS)and inter-vehicle ranging.We demonstrate the benefit of the cou-pled information by deriving the Cramer-Rao Lower Bound(CRLB)of the float estimation in CP.To fully use this coupled information,we propose a Whole-Net CP(WN-CP)method which consists of the Whole-Net Extended Kalman Filter(WN-EKF)as the float estimation filter,and the Partial Baseline Fixing(PBF)as the ambiguity resolution part.The WN-EKF fuses the mea-surements of all baselines simultaneously to improve the performance of float estimation,and the PBF strategy fixes the ambiguities of the one baseline to be estimated,instead of full ambiguity resolution,to reduce the computation load of ambiguity resolution.Field tests involving four vehi-cles were conducted in urban environments.The results show that the proposed WN-CP method can achieve better performance and meanwhile maintain a low computation load compared to the existing methods.
查看更多>>摘要：SATech-01 is an experimental satellite for space science exploration and on-orbit demon-stration of advanced technologies.The satellite is equipped with 16 experimental payloads and sup-ports multiple working modes to meet the observation requirements of various payloads.Due to the limitation of platform power supply and data storage systems,proposing reasonable mission plan-ning schemes to improve scientific revenue of the payloads becomes a critical issue.In this article,we formulate the integrated task scheduling of SATech-01 as a multi-objective optimization prob-lem and propose a novel Fair Integrated Scheduling with Proximal Policy Optimization(FIS-PPO)algorithm to solve it.We use multiple decision heads to generate decisions for each task and design the action mask to ensure the schedule meeting the platform constraints.Experimental results show that FIS-PPO could push the capability of the platform to the limit and improve the overall obser-vation efficiency by 31.5％compared to rule-based plans currently used.Moreover,fairness is con-sidered in the reward design and our method achieves much better performance in terms of equal task opportunities.Because of its low computational complexity,our task scheduling algorithm has the potential to be directly deployed on board for real-time task scheduling in future space projects.
查看更多>>摘要：Wind is the primary challenge for low-speed fixed-wing unmanned aerial vehicles to fol-low a predefined flight path.To cope with various wind conditions,this paper proposes a wind dis-turbance compensated path following control strategy where the wind disturbance estimate is incorporated with the nominal guiding vector field to provide the desired airspeed direction for the inner-loop.Since the control input vector for the outer-loop kinematic subsystem needs to sat-isfy a magnitude constraint,a scaling mechanism is introduced to tune the proportions of the com-pensation and nominal components.Moreover,an optimization problem is formulated to pursue a maximum wind compensation in strong winds,which can be solved analytically to yield two scaling factors.A cascaded inner-loop tracking controller is also designed to fulfill the outer-loop wind dis-turbance compensated guiding vector field.High-fidelity simulation results under sensor noises and realistic winds demonstrate that the proposed path following algorithm is less sensitive to sensor noises,achieves promising accuracy in normal winds,and mitigates the deviation from a desired path in wild winds.
查看更多>>摘要：With the great development of Multi-Target Tracking(MTT)technologies,many MTT algorithms have been proposed with their own advantages and disadvantages.Due to the fact that requirements to MTT algorithms vary from the application scenarios,performance evaluation is significant to select an appropriate MTT algorithm for the specific application scenario.In this paper,we propose a performance evaluation method on the sets of trajectories with temporal dimension specifics to compare the estimated trajectories with the true trajectories.The proposed method evaluates the estimate results of an MTT algorithm in terms of tracking accuracy,continu-ity and clarity.Furthermore,its computation is based on a multi-dimensional assignment problem,which is formulated as a computable form using linear programming.To enhance the influence of recent estimated states of the trajectories in the evaluation,an attention function is used to reweight the trajectory errors at different time steps.Finally,simulation results show that the proposed per-formance evaluation method is able to evaluate many aspects of the MTT algorithms.These eval-uations are worthy for selecting suitable MTT algorithms in different application scenarios.
查看更多>>摘要：Deep learning significantly improves the accuracy of remote sensing image scene classi-fication,benefiting from the large-scale datasets.However,annotating the remote sensing images is time-consuming and even tough for experts.Deep neural networks trained using a few labeled sam-ples usually generalize less to new unseen images.In this paper,we propose a semi-supervised approach for remote sensing image scene classification based on the prototype-based consistency,by exploring massive unlabeled images.To this end,we,first,propose a feature enhancement mod-ule to extract discriminative features.This is achieved by focusing the model on the foreground areas.Then,the prototype-based classifier is introduced to the framework,which is used to acquire consistent feature representations.We conduct a series of experiments on NWPU-RESISC45 and Aerial Image Dataset(AID).Our method improves the State-Of-The-Art(SOTA)method on NWPU-RESISC45 from 92.03％to 93.08％and on AID from 94.25％to 95.24％in terms of accu-racy.
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