Reliability analysis of the flat-topped array beam FSO communication link

Free-space optical communication systems are affected by turbulent atmosphere. The atmospheric transmission is affected by absorption, scattering and turbulence. In this paper, the effects of absorption and scattering are taken into account using Beer’s law and the effects of turbulence are considered in calculating the average intensity distribution. An analytical expression for the average intensity distribution of a partially coherent flat-topped array (PCFTA) beam in turbulent atmosphere is derived based on the extended Huygens–Fresnel principle. The average intensity, power in bucket, signal to noise ratio, and bit error rate of this kind of beam are investigated in details. It is shown by numerical results and analytical methods that the average intensity and link parameters of PCFTA beams change during propagation and these changes are dependent upon both source parameters and weather conditions.     

High Efficiency Time Redundant Hardened Latch for Reliable Circuit Design

In a modern high density VLSI design, with higher operating frequency and technology scaling, small critical charge in circuit nodes significantly increases susceptibility to radiation induced transient faults. In this paper, we propose a high efficiency hardened latch using the undesired delay of Schmitt trigger circuit and a special feedback loop to a comparator to build a low overhead time redundancy scheme. The proposed structure masks internal node transient faults also improves the recovery of the output node by transferring the faulty output in two different paths to the comparison circuit’s inputs. Experimental results, simulated in 45 nm CMOS technology, show an acceptable increase in the critical charge compared with the previous hardened latches, with a fair increase in power, delay and area. Monte Carlo simulations have also confirmed the proposed latch resistance to the process, voltage and temperature variations.     

Design considerations and experimental studies on semi-active smart pin joint

Hostile dynamic loadings such as severe wind storms, earthquakes, and sudden impacts can cause severe damage to many civil engineering structures. An intelligent structural system equipped with smart structural members that are controllable in real-time is an effective solution to structural damage and failure during such situations. Civil intelligent structures with controllable properties to adapt to any changes due to dynamic loadings can lead to effective protection of structures and their occupants. In this paper, design and testing of a semi-active magnetorheological (MR) pin joint, in which the moment resistance can be controlled in real-time by altering the magnetic field, is reported with the view of using it as a potential candidate for smart members in the development of intelligent structures. Design of prototype smart pin joints includes theoretical analysis related to the radius of the rotary plate, the property of MR fluids and the gap between the rotary plate and the casing based on the requirements of the dynamics of MR pin joints. FEM analysis was deployed to study the distribution of the magnetic field along the gap. It is found, from the theoretical analysis and experimental verification, that the MR pin joint with a diameter of 180 mm can produce a torque of up to 30 Nm, which meets requirements for semi-active members in a multi-storey prototype building model in the next stage of research and development.     

Thermal and crystallization behavior of SiO2-PbF2 glass system in the presence of ErF3 and Al2O3

Oxyfluoride glass-ceramics have been introduced as one of the best bulk hosts for upconversion process of rare-earth ions. However, less attention to the technological topics in the process of glass crystallization has hindered the development of them in industrial scale applications. In this research, SiO₂–PbF₂ glasses were investigated to clarify the ambiguous role of rare-earth ions and alumina content in the microstructure and crystallization behavior. Results indicated that Er³⁺ addition caused a liquid-liquid phase separation via nucleation and growth mechanism, which led to single phase crystallization of β-PbF₂:Er³⁺ solid solution. Moreover, Er³⁺ had a significant effect on the crystallite size, size distribution, and PbF₂ crystallization temperature. On the other hand, increasing Al₂O₃ content enhanced the transparency and thermal stability of glass samples, whereas it reduced the amount of fluorine loss and increased the dissolution of Er³⁺ in fluoride crystalline structure. These results address some of the most controversial issues about crystallization behavior of rare-earth-doped oxyfluoride glasses.     

A flexure-based mechanism and control methodology for ultra-precision turning operation

This paper presents the methodology for modeling and control of a high precision flexure-based mechanism for ultra-precision turning operation. A high performance piezoelectric actuator is used to driven the flexure-based mechanism. A parallel flexure hinge mechanism is utilized to guide the moving platform and to preload the piezoelectric actuator. A high resolution capacitive sensor is used to measure the displacement of the flexure-based mechanism for closed-loop control. With consideration of the driving circuit, the dynamic model of the flexure-based mechanism has been established. The effect of the driving circuit on the dynamic response of the precision mechanism is investigated. Experimental tests have been carried out to verify the established model and the performance of the flexure-based mechanism.     

Development of a high precision flexure-based microgripper

This paper presents the design and development of a high precision microgripper for micromanipulation. The design is based on a hybrid flexure-based compliant mechanism and a bias spring structure which render high fidelity and inherent mechanical advantages. Finite element analysis (FEA) was conducted to evaluate responses of the model under specified load and displacement to investigate optimum design of the model. The prototype of the proposed microgripper was fabricated using electro-discharge machining (EDM) process. An experimental study of the performance was carried out and the results are presented. The experimental results are also compared with the computational analysis results. The results show that a high level of displacement amplification and a maximum stroke of 100 μm can be achieved.     

Characterization of low range GaAs

We present the results of a study of GaAs material grown at substrate temperatures below 250°C (low range GaAs) by molecular beam epitaxy. This material is amorphous and highly resistive and can be converted to single crystal through annealing process. The crystallization process is investigated by transmission electron microscopy, reflection high-energy electron diffraction, and double-crystal x-ray diffraction techniques.     

Local and global instabilities associated with continuing crack extension in dissipative solids

The model developed here links microstructural and continuum mechanics aspects of the early stages of the fracture process occurring in dissipative solids. A variable size damage zone, endowed with a structure of its own determined by the microstructural parameters related to the material ductility and grain size, is incorporated into a moving dominant crack. Prior to and during the course of crack extension, the energy dissipation mechanisms of diverse nature are activated within the volume of the localized damage zone, providing a substantial contribution to the effective material toughness. A criterion for quasi-static crack based on self-similarity of the crack-tip region is used to set up a governing equation of motion. The stress-transferring ability of the damage zone depends on the separation distance created between the two opposite boundaries of the fractures zone, and it determines the history of a quasi-static crack development including the attainment of the terminal instability point. Although detailed information regarding the distribution of stress prevailing within the nonlinear zone is lacking, it is shown that certain plausible governing equations may be constructed and employed to define material resistance curve in sufficiently large specimens (within the so-called ssy range), which is then used to predict the onset of catastrophic fracture. Six different specimen configurations are considered and the pertinent macro-mechanical stability analysis is presented in detail. The class of materials susceptible to this type of analysis is not restricted by the usual LEFM or EPFM constraint. This revised version was published online in August 2006 with corrections to the Cover Date.