Here, we present a numerical investigation of the mechanical behavior of ellipsoids under triaxial com- pression for a range of aspect ratios. Our simulations use a multi-sphere approach in a three-dimensional discret...Here, we present a numerical investigation of the mechanical behavior of ellipsoids under triaxial com- pression for a range of aspect ratios. Our simulations use a multi-sphere approach in a three-dimensional discrete element method. All assemblies were prepared at their densest condition, and triaxial compres- sion tests were performed up to extremely large strains, until a critical state was reached. The stress-strain relationship and the void ratio-strain behavior were evaluated. We found that the stress-dilatancy rela- tionship of ellipsoids with different aspect ratios could be expressed as a linear equation. In particular, the aspect ratio influenced the position of the critical state lines for these assemblies. Particle-scale char- acteristics at the critical state indicate that particles tend to be flat lying, and the obstruction of particle rotation that occurs with longer particles affects their contact mechanics. Lastly, anisotropic coefficients related to aspect ratio were investigated to probe the microscopic origins of the macroscopic behavior. A detailed analysis of geometrical and mechanical anisotropies revealed the microscopic mechanisms underlying the dependency of peak and residual strengths on aspect ratio.展开更多
A macro-micro dual-drive positioning system showing good potential for high acceleration and high precision positioning required in IC packaging applications is devised in this paper. The dual-drive positioning stage ...A macro-micro dual-drive positioning system showing good potential for high acceleration and high precision positioning required in IC packaging applications is devised in this paper. The dual-drive positioning stage uses a VCM (voice coil motor) driven macro positioning stage and a PZT piezo-electric driven micro positioning stage. The coupling characteristics of the system are analyzed to produce a control structure with a micro positioning stage that can dynamically compensate for the positioning error produced by the macro positioning stage. Models of the two positioning stages are described. The models cover both the mechanism and the actuator. For the macro positioning stage, friction characteristics are taken into account, and a controller with an LQG (linear-quadratic-Gaussian) control algorithm combining a feed-forward compensation algorithm is derived. A PID controller is used to control the micro positioning stage. Detailed designs are derived for the proposed approach, and the performance is validated by simulation.展开更多
With ongoing economic,scientific,and technological developments,the electronic devices used in daily lives are developing toward precision and miniaturization,and so the demand for high-precision manufacturing machine...With ongoing economic,scientific,and technological developments,the electronic devices used in daily lives are developing toward precision and miniaturization,and so the demand for high-precision manufacturing machinery is expanding.The most important piece of equipment in modern high-precision manufacturing is the macro-micro motion platform(M3P),which offers high speed,precision,and efficiency and has macro-micro motion coupling characteristics due to its mechanical design and composition of its driving components.Therefore,the design of the control system is crucial for the overall precision of the platform;conventional proportional–integral–derivative control cannot meet the system requirements,and so M3Ps are the subject of a growing range of modern control strategies.This paper begins by describing the development history of M3Ps,followed by their platform structure and motion control system components,and then in-depth assessments of the macro,micro,and macro-micro control systems.In addition to examining the advantages and disadvantages of current macro-micro motion control,recent technological breakthroughs are noted.Finally,based on existing problems,future directions for M3P control systems are given,and the present conclusions offer guidelines for future work on M3Ps.展开更多
By use of a three-dimensional compressible non-hydrostatic convective cloud model with detailed microphysics featuring spectral bins of cloud condensation nuclei （CCN）, liquid droplets, ice crystals, snow and graupe...By use of a three-dimensional compressible non-hydrostatic convective cloud model with detailed microphysics featuring spectral bins of cloud condensation nuclei （CCN）, liquid droplets, ice crystals, snow and graupel particles, the spatial and temporal distributions of hydrometeors in a supercell observed by the （Severe Thunderstorm Electrification and Precipitation Study） STEPS triple-radar network are simulated and analyzed. The bin model is also employed to study the effect of CCN concentration on the evolution characteristics of the supercell. It is found that the CCN concentration not only affects the concentration and spectral distribution of water droplets, but also influences the characteristics of ice crystals and graupel particles. With a larger number of CCN, more water droplets and ice crystals are produced and the growth of graupel is restrained. With a small quantity of CCN the production of large size water droplets are promoted by initially small concentrations of water droplets and ice crystals, leading to earlier formation of small size graupel and restraining the recycling growth of graupel, and thus inhibiting the formation of large size graupel （or small size hail）. It can be concluded that both the macroscopic airflow and microphysical processes influence the formation and growth of large size graupel （or small size hail）. In regions with heavy pollution, a high concentration of CCN may restrain the formation of graupel and hail, and in extremely clean regions, excessively low concentrations of CCN may also limit the formation of large size graupel （hail）.展开更多
It’s the basic premise of promoting the healthy development of rural finance and strengthen-ing macro-prudential supervision to measure the systemic risk of rural finance accurately.We establish the dynamic factor CA...It’s the basic premise of promoting the healthy development of rural finance and strengthen-ing macro-prudential supervision to measure the systemic risk of rural finance accurately.We establish the dynamic factor CAPM and make an all-round and multi-angle quantitative study on the systemic risk of rural finance in China by constructing macro-micro index system and using machine learning to reduce the dimension of high-dimensional data.Our results show that the dynamic factor CAPM of using macro-micro big data can evaluate systemic risk of rural finance more comprehensively and systematically,and machine learning performs well in processing high-dimensional data.In addition,China’s rural financial systemic risk is stable compared with the Shanghai and Shenzhen main markets,but it is also susceptible to macro and micro influ-enced factors.Finally,it is pointed out that the early warning system of rural financial systemic risk could be constructed at macro and micro level,respectively.展开更多
In nature,shells exhibit remarkable high toughness and impact resistance to the external load despite their brittle main constituent and simple hierarchical structure.In this work,the structure of the mussel shell Hyr...In nature,shells exhibit remarkable high toughness and impact resistance to the external load despite their brittle main constituent and simple hierarchical structure.In this work,the structure of the mussel shell Hyriopsis cumingii is analyzed by scanning electron micro scope and atomic force microscope,and the macro/micro compression and impact tests are perfonned.Results show that the shell has a three-layer structure:an outer cuticle layer,a prismatic layer,and a nacreous layer.The stiffer and load-dependent prismatic layer is conducive to improve the impact resistance of shell structure.Fracture morphology after failure proves that cracks are transgranularly propagated inside the prism and aragonite platelet,and the crack deflection and platelet pullout can effectively lock the stress,thereby eventually improving the impact-resistance and toughness of the shell.展开更多
55NiCrMoV7 hot-work die steel is mainly used to manufacture heavy forgings in the fields of aerospace and automobile.This study aims to clarify the effects of heat treatment on the microstructural evolution and mechan...55NiCrMoV7 hot-work die steel is mainly used to manufacture heavy forgings in the fields of aerospace and automobile.This study aims to clarify the effects of heat treatment on the microstructural evolution and mechanical properties of the steel,in order to find out an optimal heat treatment scheme to obtain an excellent balance of strength,ductility and toughness.The steel was quenched at temperature from 790℃ to 910℃ followed by tempering treatments of 100–650℃ for 5 h.The mechanical property tests were carried out by tensile,impact toughness and hardness.Optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM)were used to observe the austenite grains,lath martensite,carbides and fracture morphology.The results show that the quenching temperature mainly influences the austenite grain size and the volume fraction of undissolved carbides(UCs),while the tempering temperature mainly influences the size and morphology of the martensite with a body centered cubic(BCC)and the carbides with a face centered cubic(FCC).The mechanical properties of the steel,including yield and tensile strength,ductility,impact toughness and hardness,get an excellent balance at a quenching range of 850–870C.As the tempering temperature increases,the yield and tensile strength and hardness decrease,while the ductility and impact toughness increase.These variation trends can be further verified by fracture SEM observation and analysis.Combined with a macro-micro coupled finite element(MMFE)modeling technique,the cooling rate,microstructural evolution and yield strength of the steel were predicted and compared with the tested data.展开更多
In this paper, a composite control scheme for macro-micro dual-drive positioning stage with high accel- eration and high precision is proposed. The objective of control is to improve the precision by reducing the infl...In this paper, a composite control scheme for macro-micro dual-drive positioning stage with high accel- eration and high precision is proposed. The objective of control is to improve the precision by reducing the influence of system vibration and external noise. The positioning stage is composed of voice coil motor （VCM） as macro driver and piezoelectric actuator （PEA） as micro driver. The precision of the macro drive positioning stage is improved by the com- bined PID control with adaptive Kalman filter （AKF）. AKF is used to compensate VCM vibration （as the virtual noise） and the external noise. The control scheme of the micro drive positioning stage is presented as the integrated one with PID and intelligent adaptive inverse control approach to compensate the positioning error caused by macro drive positioning stage. A dynamic recurrent neural networks （DRNN） based inverse control approach is proposed to offset the hysteresis nonlinearity of PEA. Simulations show the positioning precision of macro-micro dual-drive stage is clearly improved via the proposed control scheme.展开更多
This paper deals with a flexible macro-micro manipulator system, which includes a long flexible manipulator and a relatively short rigid manipulator attached to the tip of the macro manipulator. A flexible macro manip...This paper deals with a flexible macro-micro manipulator system, which includes a long flexible manipulator and a relatively short rigid manipulator attached to the tip of the macro manipulator. A flexible macro manipulator possesses the advantages of wide operating range, high speed, and low energy consumption, but the disadvantage of a low tracking precision. The macro-micro manipulator system improves tracking performance by compensating for the endpoint tracking error while maintaining the advantages of the flexible macro manipulator. A trajectory planning scheme was built utilizing the task space division method. The division point is chosen to optimize the error compensation and energy consumption for the whole system. Then movements of the macro-micro manipulator can be determined using separate inverse kinematic models. Simulation results for a planar 4-DOF macro-micro manipulator system are presented to show the effectiveness of the control system.展开更多
基金This research was supported by the National Natural Science Foundation of China （51479027, 51539008）.
文摘Here, we present a numerical investigation of the mechanical behavior of ellipsoids under triaxial com- pression for a range of aspect ratios. Our simulations use a multi-sphere approach in a three-dimensional discrete element method. All assemblies were prepared at their densest condition, and triaxial compres- sion tests were performed up to extremely large strains, until a critical state was reached. The stress-strain relationship and the void ratio-strain behavior were evaluated. We found that the stress-dilatancy rela- tionship of ellipsoids with different aspect ratios could be expressed as a linear equation. In particular, the aspect ratio influenced the position of the critical state lines for these assemblies. Particle-scale char- acteristics at the critical state indicate that particles tend to be flat lying, and the obstruction of particle rotation that occurs with longer particles affects their contact mechanics. Lastly, anisotropic coefficients related to aspect ratio were investigated to probe the microscopic origins of the macroscopic behavior. A detailed analysis of geometrical and mechanical anisotropies revealed the microscopic mechanisms underlying the dependency of peak and residual strengths on aspect ratio.
基金Supported by the National Natural Science Foundation of China(Grant No.50705027)the National High Technology Research and Development Program of China("863"Program)(Grant No.2007AA04Z315)Self-Planned Task of State Key Laboratory of Robotics and System(HIT)(Grant No.SKLRS200804B)
文摘A macro-micro dual-drive positioning system showing good potential for high acceleration and high precision positioning required in IC packaging applications is devised in this paper. The dual-drive positioning stage uses a VCM (voice coil motor) driven macro positioning stage and a PZT piezo-electric driven micro positioning stage. The coupling characteristics of the system are analyzed to produce a control structure with a micro positioning stage that can dynamically compensate for the positioning error produced by the macro positioning stage. Models of the two positioning stages are described. The models cover both the mechanism and the actuator. For the macro positioning stage, friction characteristics are taken into account, and a controller with an LQG (linear-quadratic-Gaussian) control algorithm combining a feed-forward compensation algorithm is derived. A PID controller is used to control the micro positioning stage. Detailed designs are derived for the proposed approach, and the performance is validated by simulation.
基金This research was supported financially by the China Postdoctoral Science Foundation,the National Natural Science Foundation of China(Grant No.51705132)the Young Backbone Teacher Training Program in Henan University of Technology,the Education Department of Henan Province Natural Science Project(Grant No.21A460006)the Natural Science Project of Henan Provincial Department of Science and Technology(Grant No.222102220088).
文摘With ongoing economic,scientific,and technological developments,the electronic devices used in daily lives are developing toward precision and miniaturization,and so the demand for high-precision manufacturing machinery is expanding.The most important piece of equipment in modern high-precision manufacturing is the macro-micro motion platform(M3P),which offers high speed,precision,and efficiency and has macro-micro motion coupling characteristics due to its mechanical design and composition of its driving components.Therefore,the design of the control system is crucial for the overall precision of the platform;conventional proportional–integral–derivative control cannot meet the system requirements,and so M3Ps are the subject of a growing range of modern control strategies.This paper begins by describing the development history of M3Ps,followed by their platform structure and motion control system components,and then in-depth assessments of the macro,micro,and macro-micro control systems.In addition to examining the advantages and disadvantages of current macro-micro motion control,recent technological breakthroughs are noted.Finally,based on existing problems,future directions for M3P control systems are given,and the present conclusions offer guidelines for future work on M3Ps.
基金supported by the National Natural Science Foundation of China (Grant Nos.40537034, 40805057)Jiangsu Province Qinglan Project"cloud fog precipitation and aerosol research group", Foun-dation of Key Laboratory for Cloud Physics and Weather Modification of CMA (Grant No. 2009Z0036)Foun-dation of Nanjing University of Information Science &Technology
文摘By use of a three-dimensional compressible non-hydrostatic convective cloud model with detailed microphysics featuring spectral bins of cloud condensation nuclei （CCN）, liquid droplets, ice crystals, snow and graupel particles, the spatial and temporal distributions of hydrometeors in a supercell observed by the （Severe Thunderstorm Electrification and Precipitation Study） STEPS triple-radar network are simulated and analyzed. The bin model is also employed to study the effect of CCN concentration on the evolution characteristics of the supercell. It is found that the CCN concentration not only affects the concentration and spectral distribution of water droplets, but also influences the characteristics of ice crystals and graupel particles. With a larger number of CCN, more water droplets and ice crystals are produced and the growth of graupel is restrained. With a small quantity of CCN the production of large size water droplets are promoted by initially small concentrations of water droplets and ice crystals, leading to earlier formation of small size graupel and restraining the recycling growth of graupel, and thus inhibiting the formation of large size graupel （or small size hail）. It can be concluded that both the macroscopic airflow and microphysical processes influence the formation and growth of large size graupel （or small size hail）. In regions with heavy pollution, a high concentration of CCN may restrain the formation of graupel and hail, and in extremely clean regions, excessively low concentrations of CCN may also limit the formation of large size graupel （hail）.
文摘It’s the basic premise of promoting the healthy development of rural finance and strengthen-ing macro-prudential supervision to measure the systemic risk of rural finance accurately.We establish the dynamic factor CAPM and make an all-round and multi-angle quantitative study on the systemic risk of rural finance in China by constructing macro-micro index system and using machine learning to reduce the dimension of high-dimensional data.Our results show that the dynamic factor CAPM of using macro-micro big data can evaluate systemic risk of rural finance more comprehensively and systematically,and machine learning performs well in processing high-dimensional data.In addition,China’s rural financial systemic risk is stable compared with the Shanghai and Shenzhen main markets,but it is also susceptible to macro and micro influ-enced factors.Finally,it is pointed out that the early warning system of rural financial systemic risk could be constructed at macro and micro level,respectively.
基金The authors are grateful for the financial support from the National Defense Innovation Projectthe National Natural Science Foundation of China(No.51801098)+1 种基金the Natural Science General Foundation of Jiangsu Province(No.18KJB130003)the High-level Scientific Research Foundation of Nanjing Institute of Technology(No.YKJ201708).
文摘In nature,shells exhibit remarkable high toughness and impact resistance to the external load despite their brittle main constituent and simple hierarchical structure.In this work,the structure of the mussel shell Hyriopsis cumingii is analyzed by scanning electron micro scope and atomic force microscope,and the macro/micro compression and impact tests are perfonned.Results show that the shell has a three-layer structure:an outer cuticle layer,a prismatic layer,and a nacreous layer.The stiffer and load-dependent prismatic layer is conducive to improve the impact resistance of shell structure.Fracture morphology after failure proves that cracks are transgranularly propagated inside the prism and aragonite platelet,and the crack deflection and platelet pullout can effectively lock the stress,thereby eventually improving the impact-resistance and toughness of the shell.
基金the fellowship of China Postdoctoral Science Foundation(Grant No.2020M672309)。
文摘55NiCrMoV7 hot-work die steel is mainly used to manufacture heavy forgings in the fields of aerospace and automobile.This study aims to clarify the effects of heat treatment on the microstructural evolution and mechanical properties of the steel,in order to find out an optimal heat treatment scheme to obtain an excellent balance of strength,ductility and toughness.The steel was quenched at temperature from 790℃ to 910℃ followed by tempering treatments of 100–650℃ for 5 h.The mechanical property tests were carried out by tensile,impact toughness and hardness.Optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM)were used to observe the austenite grains,lath martensite,carbides and fracture morphology.The results show that the quenching temperature mainly influences the austenite grain size and the volume fraction of undissolved carbides(UCs),while the tempering temperature mainly influences the size and morphology of the martensite with a body centered cubic(BCC)and the carbides with a face centered cubic(FCC).The mechanical properties of the steel,including yield and tensile strength,ductility,impact toughness and hardness,get an excellent balance at a quenching range of 850–870C.As the tempering temperature increases,the yield and tensile strength and hardness decrease,while the ductility and impact toughness increase.These variation trends can be further verified by fracture SEM observation and analysis.Combined with a macro-micro coupled finite element(MMFE)modeling technique,the cooling rate,microstructural evolution and yield strength of the steel were predicted and compared with the tested data.
基金partly supported by the National Natural Science Foundation of China(No.61174047)the School Basic Foundation of Northwestern Polytechnical University(No.GCKYI006)the Fundamental Research Funds for the Central Universities(No.HEUCFR1214)
文摘In this paper, a composite control scheme for macro-micro dual-drive positioning stage with high accel- eration and high precision is proposed. The objective of control is to improve the precision by reducing the influence of system vibration and external noise. The positioning stage is composed of voice coil motor （VCM） as macro driver and piezoelectric actuator （PEA） as micro driver. The precision of the macro drive positioning stage is improved by the com- bined PID control with adaptive Kalman filter （AKF）. AKF is used to compensate VCM vibration （as the virtual noise） and the external noise. The control scheme of the micro drive positioning stage is presented as the integrated one with PID and intelligent adaptive inverse control approach to compensate the positioning error caused by macro drive positioning stage. A dynamic recurrent neural networks （DRNN） based inverse control approach is proposed to offset the hysteresis nonlinearity of PEA. Simulations show the positioning precision of macro-micro dual-drive stage is clearly improved via the proposed control scheme.
基金the National Natural Science Foundation of China (No. 60305008)
文摘This paper deals with a flexible macro-micro manipulator system, which includes a long flexible manipulator and a relatively short rigid manipulator attached to the tip of the macro manipulator. A flexible macro manipulator possesses the advantages of wide operating range, high speed, and low energy consumption, but the disadvantage of a low tracking precision. The macro-micro manipulator system improves tracking performance by compensating for the endpoint tracking error while maintaining the advantages of the flexible macro manipulator. A trajectory planning scheme was built utilizing the task space division method. The division point is chosen to optimize the error compensation and energy consumption for the whole system. Then movements of the macro-micro manipulator can be determined using separate inverse kinematic models. Simulation results for a planar 4-DOF macro-micro manipulator system are presented to show the effectiveness of the control system.
