Acceleration and singularity analyses of a parallel manipulator with a particular topology

In this work, two essential steps, the kinematics and the singularity analysis, dealing with the design process of a parallel manipulator are investigated by means of the theory of screws. The proposed mechanism for the analysis is a parallel manipulator with three degrees of freedom. A simple and compact expression is derived here for the computation of the reduced acceleration state of the moving platform, w.r.t. the fixed platform, by taking advantage of the properties of reciprocal screws, via the Klein form of the Lie algebra e(3). To this end, the reduced acceleration state of the moving platform is written in screw form through each one of the three actuator limbs of the manipulator. Afterwards, the acceleration analysis is completed by taking into proper account the decoupled motion of the parallel manipulator. Of course, as an intermediate step this contribution also provides the velocity analysis of the parallel manipulator. The expressions obtained via screw theory are compact and can be easily translated into computer codes. A numerical example is provided to demonstrate the efficacy of screw theory to efficiently analyze the kinematics of the chosen parallel manipulator. Finally, the numerical results from the kinematic analysis are compared with results produced with a commercially available dynamic and kinematic simulation program.     

Optimization algorithm for the pump structure of diode side-pumped solid-state lasers

The pump structure greatly influences the characteristics of a diode side-pumped solid-state laser (DSPSSL). To achieve high absorption efficiency and a homogeneous pump-beam distribution simultaneously, a systemic algorithm has been established to optimize the pump structure, where multiple reflections occur on the internal wall of the reflector inside the pump chamber. The absorption efficiency and the deviation of the absorption distribution are both evaluated for a typical solid-state laser medium. A scheme for identifying an ideal pump configuration was finally obtained from several determination procedures, in which all pump structure parameters were taken into account. The experimental results demonstrate that the approach described in this paper is effective for the design of DSPSSL systems with both high output power and good beam quality.     

螺杆式压缩机转子螺齿压缩侧齿槽面积的通用算法

本文给出了螺杆压缩机转子螺齿压缩侧齿槽面积的数值计算方法。与以前类似的方法相比较,本文所述方法不仅具有较强的通用性,而且简单、实用,特别适用于在计算机上用数值方法进行计算。     

Control of wavepacket dynamics in mixed alkali metal clusters by optimally shaped fs pulses

We have performed adaptive feedback optimization of phase-shaped femtosecond laser pulses to control the wavepacket dynamics of small mixed alkali-metal clusters. An optimization algorithm based on Evolutionary Strategies was used to maximize the ion intensities. The optimized pulses for NaK and Na₂K converged to pulse trains consisting of numerous peaks. The timing of the elements of the pulse trains corresponds to integer and half integer numbers of the vibrational periods of the molecules, reflecting the wavepacket dynamics in their excited states. Received 4 December 2001     

Cracking characteristics of mixed mode dislocations near a lip-like mode crack

This work is concerned with the cracking characteristics of mixed mode dislocations near a lip-like mode crack, stress intensity and strain energy density factor are obtained by using conformal mapping, singularity analysis and Cauchy integrals. Shielding effect generated by screw dislocation near a lip-like mode crack decreases with the increment of the distance between screw dislocation and crack tip. Larger distance between two faces of the crack leads to the shielding effect waning. The strain energy density factor of mode III decreases with the increment of the distance between dislocation and crack tip. Larger distance between two faces of lip-like mode crack also leads to the strain energy density factor waning and encourages crack initiation; the shielding effects generated by edge dislocation near the crack decrease with the increment of the distance between edge dislocation and crack tip.     

Screw dislocation in semi-infinite non-local hexagonal medium

Abstract

Nonlocal stresses of a screw dislocation near a free surface in a semi-infinite hexagonal medium are investigated by a surface dislocation model. The nonlocal image force on the screw dislocation due to the existing free surface is also obtained. All classical singularities for the stress and image force are eliminated. The maxima of the stress and image force are evaluated. A zero point of the stress is found, which predicts that different states of the shear stress exist simultaneously near the dislocation. The appearance of a zero value at the free surface and a maximum of the dislocation image force can be used to explain the existence of the dislocation free zone.     

Theoretical study of the generation of screw dislocations for Rayleigh and Lamb waves in isotropic solids

Phase singularities are generic structures which occur in all wave fields, and they are characterised by an inability to assign a value to the phase. Screw dislocations are a particular kind of phase singularity where the phase possesses a helical structure, with the singularity at the centre of the helix. In this paper we show that it is possible to generate screw dislocations on the surface of elastic isotropic solids by means of the interference of three Rayleigh waves or three Lamb waves. The dispersive character of Lamb waves leads to more complicated behaviour, which may in turn result in greater potential for applications.     

双螺杆转子的受力分析

从双螺杆转子螺旋曲面的参数方程出发，利用空间解析几何理论将转子三维螺旋面坐标映象为对应不同压力腔的二维积分区域，在整个积分区域积分可以计算作用在双螺杆压缩机，双螺杆泵螺旋面转子上的力和力矩。本文以一种双螺杆泵的转子为例，利用本方法进行了受力分析，解析出转子受力，力矩随转子转角的变化关系。