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Abstract There are 9 major coal-accumulating periods during geological history in China, including the Early Carboniferous, Late Carboniferous-Early Permian, Middle Permian, Late Permian, Late Triassic, Early-Middle Jurassic, Early Cretaceous, Paleogene and Neogene. The coal formed in these periods were developed in different coal-accumulating areas (CAA) including the North China, South China, Northwest China, Northeast China, the Qinghai–Tibet area, and China offshore area. In this paper, we investigated depositional environments, sequence stratigraphy, lithofacies paleogeography and coal accumulation pattern of five major coal-accumulating periods including the Late Carboniferous to Middle Permian of the North China CAA, the Late Permian of the South China CAA, the Late Triassic of the South China CAA, the Early-Middle Jurassic of the North and Northwest China CAA, and the Early Cretaceous in the Northeast China CAA. According to distribution of the coal-bearing strata and the regional tectonic outlines, we have identified distribution range of the coal-forming basins, sedimentary facies types and coal-accumulating models. The sequence stratigraphic frameworks of the major coal-accumulating periods were established based on recognition of a variety of sequence boundaries. The distribution of thick coals and migration patterns of the coal-accumulating centers in the sequence stratigraphic framework were analyzed. The lithofacies paleogeography maps based on third-order sequences were reconstructed and the distribution of coal accumulation centers and coal-rich belts were predicted.

This article presents a systematically study through experimental and theoretical methods to better understand the mechanism of the geometric deformation produced during selective laser melting (SLM) treatment of the Ti6Al4V blade. Ti6Al4V blade was prepared by SLM. Microstructure, dimensional deviation and residual stress were investigated. The microstructure observation illustrates that the acicular α′ martensite formed in prior β grain, in addition, the smaller the grain size, the larger the dimensional deviation. The geometric deviation demonstrates that the directions of dimensional deviation at the leading and trailing edges of the blade are opposite to those in the middle. The distribution of dimensional deviation exhibits a parabolic change at the trailing edge of blade. The thermal stresses along the edges are much larger than that of the blade body, which cause the severe deformation of the edges toward the suction side of the blade. This conclusion is further verified by the XRD method. The residual stress distribution measured through X-ray diffraction is consistent with the simulation results.

Abstract The solar system boundary exploration mission has the characteristics of long flight time, high fuel consumption, complex gravity-assist sequence and strict constraints. Therefore, the number of decision variables and the search space of the transfer trajectory are very large, resulting in poor convergence and efficiency of the global search of the metaheuristic algorithm. Moreover, the existing gravity assist space pruning algorithm is no longer applicable for solar system boundary exploration. To effectively reduce the search space and improve the effect of trajectory optimization, an improved gravity assist space pruning algorithm is proposed. In this algorithm, a unique pruning procedure is used to effectively prune the search space, a shape of solution space box bounds combining rectangle and rhombus is adopted, and a method to automatically determine the solution space box bounds is presented. To verify the effectiveness of the improved gravity assist space pruning algorithm, the sensitivity of pruning effect to parameters is analyzed and the optimization effects of three typical metaheuristics are compared. The optimization results of 50 repeated runs of the differential evolution algorithm in the entire search space and the solution space box bounds are compared. Simulation results show that the performance of differential evolution algorithm is better than bat algorithm and firefly algorithm. And the improved pruning algorithm can increase the efficiency of subsequent optimization by more than eleven times and the convergence probability of the objective function by fifty of times. The applicability and efficiency of the proposed method for the solar system boundary exploration are demonstrated.

The capture control of test mass by means of the electrostatic suspensions is crucial for drag-free spacecraft. The test mass must be released to the cage center of the inertial sensor accurately and quickly. This paper proposes a minimum-time capture control method for the test mass release phase of drag-free spacecraft. An analytical solution of optimal control is derived based on Pontryagin’s minimum principle and the linearized dynamics model of the test mass during the release phase. The parameters of the analytical solution are initially guessed with an approximate linear solution of the test mass dynamics model and are slightly modified by using differential correction. Compared with the exact numerical solution by the hp-adaptive pseudospectral method, the analytical solution is proved to be minimum-time. Numerical simulation shows that the proposed control method quickly captures the test mass to the cage center of the inertial sensor. The capture time to stabilization is only half that of the traditional controller.

This paper aims to propose a switching rule to improve the efficiency and stability of the mode switching process for the drag-free satellite. The switching rules will ensure the stability of the different controllers during the switching process. Unlike traditional satellite switching control, the inner loop and the outer loop of the drag-free satellite are strongly coupled. The drag-free satellite not only needs to consider the controller design in the inner loop but also the controller design of the outer loop. In the outer loop, a Proportion Integration Differentiation control method is adopted to design the controller. In the inner loop, considering the release error effect of the test mass, a nonlinear sliding-mode control is employed as a controller before the mode switch. The H∞ mixed-sensitivity controller, to improve the robustness of the system and solve the problem of controller saturation, is designed after the mode switch. In the stability analysis of the switching system, the piecewise continuous Lyapunov function method is adopted. The region of attraction, which is used as the switching rule, is calculated based on the sum of squares. The obtained results demonstrate that the proposed switching rule satisfies the control accuracy and system stability.

The poem "Fold/Unfold" by Mingpei Li, is presented. First Line: I lived several lives wrong. In one, I sat; Last Line: like a knife for any living thing.

This paper proposes a robust control allocation for the capture control of the space inertial sensor’s test mass under overcritical conditions. Uncertainty factors of the test mass control system under the overcritical condition are analyzed first, and a 6-DOF test mass dynamics model with system uncertainty is established. Subsequently, a time-varying weight function is designed to coordinate the allocation of 6-DOF generalized forces. Moreover, a robust control allocation method is proposed to distribute the commanded forces and torques into individual electrodes in an optimal manner, which takes into account the system uncertainties. This method transforms the robust control allocation problem into a second-order cone optimization problem, and its dual problem is introduced to simplify the computational complexity and improve the solving efficiency. Numerical simulation results are presented to illustrate and highlight the fine performance benefits obtained using the proposed robust control allocation method, which improves capture efficiency, increases the security margin and reduces allocation errors.

Proximate spacecraft formation flying has many applications in high accurate earth observation and astronomy. In comparison to conventional or electronic thrust, Coulomb thrust has obvious advantages in close-formation control, such as fast throttling, nearly propellantless features and no thruster plume impingement. This paper presents the concept of charged-spacecraft formation and deployment, also investigates the Coulomb-force control for desired spacecraft formations. It is assumed that the charged chief spacecraft has several controllable charged spheres distributed around it and deputies are charged spheres. The deputies are deployed from the chief spacecraft to the desired formation orbits under the active charge control. In order to deploy these deputies subject to the constraints on the limited controllable charges, the transition trajectories are planned by using pseudo-spectral discretization method. Then a charge feedback controller is designed to track the transition trajectories and the desired formation. Numerical simulation results show that one or more deputies can be deployed by controlling the limited charge of the chief spacecraft.