10,166 research outputs found
Improvement of the dynamic properties of a steel-composite hybrid flexspline of a harmonic drive
No full textThe harmonic drive is a special gear-drive speed reduction system whose operation principle is based on elastic deformation rather than rigid-body motion of the general gearing system. I;rom the components of the harmonic drive, the flexspline is the key element for the transmission of motion. It must be flexible in the radial direction, but must be stiff in the torsional direction to accurately transmit rotational motion. Because the contradictory dual role of the flexspline cannot be satisfied effectively with conventional isotropic materials, but can be achieved with anisotropic composite materials, in this paper the cup section of the flexspline was hybridly manufactured by laying-up composite material on the inside surface of the steel cup section. The static and dynamic characteristics of the hybrid flexspline were investigated with respect to the fibre volume fraction, stacking sequence and the mass ratio of the composite to steel. (C) 1997 Elsevier Science Ltd
Improvement of the Dynamic Properties of a Steel-Composite Hybrid Flexspline of a Harmonic Drive
No full textPRIORITIZED CHANNEL ASSIGNMENT IN A CELLULAR RADIO NETWORK
No full textDimensioning procedures for prioritized channel assignment in a cellular radio network are considered. Under the cutoff priority discipline, the prioritized channel assignment procedures for a single cell and multicell system are formulated as nonlinear discrete capacity allocation problems. Exact incremental algorithms which efficiently solve the proposed problems are devised based on the properties of the blocking probabilities of new calls and handoff calls. Given the number of available frequency channels together with the arrival rates and the GOS for both types of calls in each cell, algorithm SP1 generates an optimal channel assignment which ensures priority for handoff calls. Given the arrival rates and distinct GOS's for new and handoff calls, algorithm SP2 finds the minimum number of channels required in each cell. Algorithm MP extends algorithm SP1 to a multicell system and provides the prioritized channel assignment for all cells in the system. Due to their simplicity, our algorithms run very fast and can be implemented adoptively in the practical systems. It is also shown that they are appropriate for the fair allocation of frequency channels among the cells throughout the entire system
Development of an efficient algorithm for global optimization by simplex elimination
No full textAn efficient multi-start algorithm for global optimization is developed by introducing multi-dimensional simplexes as new expression units of attraction regions. The region elimination method generally consists of making a set of eliminated regions called attraction regions, checking adjacency between the current design point and the attraction region, and quitting local optimization for the attracted design points. The efficiency of the elimination method is considerably enhanced by supplementing general simplexes and their neighborhoods to conventional units of attraction regions of points and lines. To show the effectiveness of the proposed algorithm, mathematical problems from the literature are solved and the results are compared with several well-known multi-start algorithms. The present algorithm produces the global optimum in all problems more efficiently than the variants of the multi-start method. Several types of truss, frame, and composite material structures are optimized as engineering applications. Many local optima are found and the differences among the local optima are not negligibly small. These results suggest that an efficient and reliable global optimizer is strongly required in some fields of engineering optimization
Microplasticity phenomena in thermomechanically strained nickel thin films
No full textMagnetron sputtered Ni thin films on both oxidised Si (100) and alpha-Al(2)O(3) (0001) substrates of thickness 150-1000 nm were tested thermomechanically with a wafer curvature system, as well as in situ in a transmission electron microscope. The films on oxidised Si have a {111}-textured columnar microstructure with a mean grain size similar to the film thickness. On (0001) alpha-Al2O3 a near single crystal epitaxy with two growth variants is achieved leading to a significantly larger grain size. The thermo-mechanical testing was analysed in terms of the room temperature/high temperature flow stresses in the films and the observed thermoelastic slopes. It was found that the room temperature flow stresses increased with decreasing film thickness until a plateau of similar to 1100 MPa was reached for films thinner than 400 nm. This plateau is attributed to the present experiments exerting insufficient thermal strain to induce yielding in these thinner films. At 500 degrees C the compressive flow stresses of the films show a competition between dislocation and diffusion mediated plasticity. A size effect is also observed in the thermoelastic slopes of the films, with thinner films coming closer to the slope predicted by mismatch in thermal expansion coefficients. It is put forward here that this is due to a highly inhomogeneous stress distribution in the films arising from the grain size distribution.X1156Nsciescopu
Microstructure-dependent DC set switching behaviors of Ge-Sb-Te-based phase-change random access memory devices accessed by in situ TEM
No full textPhase-change random access memory (PCRAM) is one of the most promising nonvolatile memory devices. However, inability to secure consistent and reliable switching operations in nanometer-scale programing volumes limits its practical use for highdensity applications. Here, we report in situ transmission electron microscopy investigation of the DC set switching of Ge-Sb-Te (GST)-based vertical PCRAM cells. We demonstrate that the microstructure of GST, particularly the passive component surrounding the dome-shaped active switching volume, plays a critical role in determining the local temperature distribution and is therefore responsible for inconsistent cell-to-cell switching behaviors. As demonstrated by a PCRAM cell with a highly crystallized GST matrix, the excessive Joule heat can cause melting and evaporation of the switching volume, resulting in device failure. The failure occurred via two-step void formation due to accelerated phase separation in the molten GST by the polaritydependent atomic migration of constituent elements. The presented real-time observations contribute to the understanding of inconsistent switching and premature failure of GST-based PCRAM cells and can guide future design of reliable PCRAM.1176Ysciescopu
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