Many methods are proposed to deal with the type synthesis of parallel kinematic mechanisms（PKMs）, but most of them are less intuitive to some extent. Thus, to propose a concise and intuitive type synthesis method fo...Many methods are proposed to deal with the type synthesis of parallel kinematic mechanisms（PKMs）, but most of them are less intuitive to some extent. Thus, to propose a concise and intuitive type synthesis method for engineering application is a very challenging issue, which should be further studied in the field. Grassmann line geometry, which can investigate the dimensions of spatial line-clusters in a concise way, is taken as the mathematic foundation. Atlas method is introduced to visually describe the degrees of freedom（DOFs） and constraints of a mechanism, and the dual rule is brought in to realize the mutual conversion of the freedom-space and constraint-space. Consequently, a systematic method based on Grassmann line geometry and Atlas method is generated and the entire type synthesis process is presented. Three type 4-DOF PKMs, i.e., 1T3R, 2T2R and 3T1R（T： translational DOF; R： rotational DOF）, are classified according to the different combinations of the translational DOFs and rotational DOFs. The type synthesis of 4-DOF PKMs is carried out and the possible configurations are thoroughly investigated. Some new PKMs with useful functions are generated during this procedure. The type synthesis method based on Grassmann line geometry and Atlas method is intuitive and concise, and can reduce the complexity of the PKMs＇ type synthesis. Moreover, this method can provide theoretical guidance for other PKMs＇ type synthesis and engineering application. A novel type synthesis method is proposed, which solves the existing methods＇ problems in terms of complicated, not intuitive and unsuitable for practical application.展开更多
H1-Galerkin nonconforming mixed finite element methods are analyzed for integro-differential equation of parabolic type.By use of the typical characteristic of the elements,we obtain that the Galerkin mixed approximat...H1-Galerkin nonconforming mixed finite element methods are analyzed for integro-differential equation of parabolic type.By use of the typical characteristic of the elements,we obtain that the Galerkin mixed approximations have the same rates of convergence as in the classical mixed method,but without LBB stability condition.展开更多
When a fault occurs in a DC system,the fault current rises rapidly with no zero-crossing point which makes fault-line selection and fault-type identification difficult.In this paper,an online detection and protection ...When a fault occurs in a DC system,the fault current rises rapidly with no zero-crossing point which makes fault-line selection and fault-type identification difficult.In this paper,an online detection and protection method based on graph theory,namely the“double D method”,is proposed for fault-line selection and fault-type identification in DC systems.In the proposed method,the entire distribution network is visualized as a“map”with vertices representing the line convergence points and edges representing the connection lines.A network topology matrix“D”is formed by detecting the current directions as the current directions are altered following a fault,whereas the current directions at the ends of non-fault lines remain the same.In order to prevent misjudgment problems arising from power flow reversal,the rates of change of the fault currents are used to further determine whether a fault has occurred and the“double D method”is introduced to identify the fault type.Simulations results with different fault types verify the effectiveness and reliability of the proposed method.展开更多
基金supported by National Natural Science Foundation of China(Grant No.51135008)National Basic Research Program of China(973 Program,Grant No.2013CB035400)China Postdoctoral Science Foundation(Grant Nos.2012M520256,2013T60107)
文摘Many methods are proposed to deal with the type synthesis of parallel kinematic mechanisms（PKMs）, but most of them are less intuitive to some extent. Thus, to propose a concise and intuitive type synthesis method for engineering application is a very challenging issue, which should be further studied in the field. Grassmann line geometry, which can investigate the dimensions of spatial line-clusters in a concise way, is taken as the mathematic foundation. Atlas method is introduced to visually describe the degrees of freedom（DOFs） and constraints of a mechanism, and the dual rule is brought in to realize the mutual conversion of the freedom-space and constraint-space. Consequently, a systematic method based on Grassmann line geometry and Atlas method is generated and the entire type synthesis process is presented. Three type 4-DOF PKMs, i.e., 1T3R, 2T2R and 3T1R（T： translational DOF; R： rotational DOF）, are classified according to the different combinations of the translational DOFs and rotational DOFs. The type synthesis of 4-DOF PKMs is carried out and the possible configurations are thoroughly investigated. Some new PKMs with useful functions are generated during this procedure. The type synthesis method based on Grassmann line geometry and Atlas method is intuitive and concise, and can reduce the complexity of the PKMs＇ type synthesis. Moreover, this method can provide theoretical guidance for other PKMs＇ type synthesis and engineering application. A novel type synthesis method is proposed, which solves the existing methods＇ problems in terms of complicated, not intuitive and unsuitable for practical application.
基金Foundation item： the National Natural Science Foundation of China （Nos. 10671184 10371113）.
文摘H1-Galerkin nonconforming mixed finite element methods are analyzed for integro-differential equation of parabolic type.By use of the typical characteristic of the elements,we obtain that the Galerkin mixed approximations have the same rates of convergence as in the classical mixed method,but without LBB stability condition.
基金Thanks for the financial support from the following fund projects:Project Supported by National Natural Science Foundation of China(51607070)。
文摘When a fault occurs in a DC system,the fault current rises rapidly with no zero-crossing point which makes fault-line selection and fault-type identification difficult.In this paper,an online detection and protection method based on graph theory,namely the“double D method”,is proposed for fault-line selection and fault-type identification in DC systems.In the proposed method,the entire distribution network is visualized as a“map”with vertices representing the line convergence points and edges representing the connection lines.A network topology matrix“D”is formed by detecting the current directions as the current directions are altered following a fault,whereas the current directions at the ends of non-fault lines remain the same.In order to prevent misjudgment problems arising from power flow reversal,the rates of change of the fault currents are used to further determine whether a fault has occurred and the“double D method”is introduced to identify the fault type.Simulations results with different fault types verify the effectiveness and reliability of the proposed method.
