Based on the analogy between bifurcation of equilibrium paths in structures and kinematic bifurcation of mechanisms, this paper proposes an analogous stiffness method to detect the singularity and kinematic bifurcation of mechanisms. The analogous stiffness in mechanisms is first defined as the derivative of the state variable with respect to the controlling variable. By investigating the value of analogous stiffness, the singularity can be classified into output singularity, input singularity and architectural singularity. And the kinematic characteristics of free joints at corresponding singularity configurations are expounded. The singular and kinematic bifurcation points of mechanisms can then be determined by solving analogous stiffness equations and compatibility equations simultaneously. Following that, the analytical criterion for finite motion of corresponding free joints at singularity configurations is derived from the second-order analysis of compatibility equations. The efficiency of the proposed method is finally illustrated by three typical examples. 相似文献
The low laser induced damage threshold of the KH2PO4 crystal seriously restricts the output power of inertial confinement fusion. The micro-waviness on the KH2PO4 surface processed by single point diamond turning has a significant influence on the damage threshold. In this paper, the influence of micro-waviness on the damage threshold of the KH2PO4 crystal and the chief sources introducing the micro-waviness are analysed based on the combination of the Fourier modal theory and the power spectrum density method. Research results indicate that among the sub-wavinesses with different characteristic spatial frequencies there exists the most dangerous frequency which greatly reduces the damage threshold, although it may not occupy the largest proportion in the original surface. The experimental damage threshold is basically consistent with the theoretical calculation. For the processing parameters used, the leading frequency of micro-waviness which causes the damage threshold to decrease is between 350-1 μ-1 and 30-1 μ-1, especially between 90-1 μ-1 and 200-1 μ-1. Based on the classification study of the time frequencies of micro-waviness, we find that the axial vibration of the spindle is the chief source introducing the micro-waviness, nearly all the leading frequencies are related to the practical spindle frequency (about 6.68 Hz, 400 r/min) and a special middle frequency (between 1.029 Hz and 1.143 Hz). 相似文献
The title compound, K2[VF5(H2O)], was synthesized from potassium antimony tartrate, piperazine, V2O5 and HF under hydrothermal conditions. It is isostructural with K2[FeF5(H2O)] and contains polymeric anion chains held together by strong O—H⋯F bonds. Each V atom is coordinated to five terminal F atoms and the O atom of one water molecule. Pairs of O—H⋯F bonds are formed by two cis‐related F atoms. Twofold axes run along the O—V—F axis of the V‐centred otahedra. 相似文献
Water stable! Radiochemists have recently discovered silicon chemistry as a tool for the introduction of 18F into biomolecules for positron emission tomography (PET) imaging. 18F‐labeled PET tracers must be stable towards defluorination under physiological conditions. Here, a theoretical model of organofluorosilane hydrolysis is developed that correlates with the experimentally determined hydrolytic half‐lives and allows estimation of the stability of newly designed compounds (see scheme).
Highly efficient Michael addition reactions of malonates to nitroalkenes catalyzed by novel chiral thioureas derived from optically pure BINOL and amino acids are reported. Various trans-nitroalkenes reacted with malonates affording the desired products in up to 95% yield with excellent enantioselectivities (up to 97% ee). 相似文献