Implementation of an Aerostat Positioning System With Cable Control

The capabilities of a multi-tethered aerostat positioning system are investigated using experimental and simulation results. The system consists of a platform supported by a helium-filled aerostat and attached to three anchored ground tethers actuated using computer-controlled winches. The experimental system was designed to perform a proof-of-concept study of a novel large-scale radio telescope requiring a receiver to be positioned accurately at an altitude of up to 500 m. Results from a series of flight tests are presented with a comparison between the passive response of the system and the response using proportional, integral, and derivative (PID) controllers with a position feedback. The motion of the platform is smaller for all cases using the feedback control. To improve on the PID results, a dynamics model of the system is used to develop and simulate optimal and feedforward (FF) control strategies. The optimal linear quadratic Gaussian (LQG) controller offers a 50% improvement over the PID controller, and both the LQG and the PID feedback controllers were shown to benefit considerably from the addition of a FF control term that exploits the measurements of the system's main disturbance force     

Operator dependence of 3-D ultrasound-based computational fluid dynamics for the carotid bifurcation

The association between vascular wall shear stress (WSS) and the local development of atherosclerotic plaque makes estimation of in vivo WSS of considerable interest. Three-dimensional ultrasound (3DUS) combined with computational fluid dynamics (CFD) provides a potentially valuable tool for acquiring subject-specific WSS, but the interoperator and intraoperator variability associated with WSS calculations using this method is not known. Here, the accuracy, reproducibility and operator dependence of 3DUS-based computational fluid dynamics were examined through a phantom and in vivo studies. A carotid phantom was scanned and reconstructed by two operators. In the in vivo study, four operators scanned a healthy subject a total of 11 times, and their scan data were processed by three individuals. The study showed that with some basic training, operators could acquire accurate carotid geometry for flow reconstructions. The variability of measured cross-sectional area and predicted shear stress was 8.17% and 0.193 N/m/sup 2/ respectively for the in vivo study. It was shown that the variability of the examined parameters was more dependent on the scan operators than the image processing operator. The range of variability of geometrical and flow parameters reported here can be used as a reference for future in vivo studies using the 3DUS-based CFD approach.     

A multiple objective selection methodology for strategic industryselection analysis

An application of mathematical programming in the industry selection step of strategic acquisition analysis is presented. Specifically, the management information uses of multiple-objective programming in the strategic planning of organization acquisitions are described. The modeling approach has application to all forms of organization-acquisition analysis (i.e. acquisitions, mergers, joint ventures, etc.). An illustrative application of the modeling approach is presented using data from a regional conglomerate's actual acquisition study. The benefits of the use of the multiple-objective modeling approach include improvements in the efficacy and specificity of information on which acquisition decision making is based, and types of information that are not methodology     

Desiccation and recolonization of phytobenthos in a regulated desert river: Colorado River at Lees Ferry,Arizona, USA

We tested the recolonization of the phytobenthic community in the tailwaters of Glen Canyon Dam following long- and short-term experimentally induced desiccation. The response of Cladophora glomerata, Oscillatoria spp., miscellaneous phytobenthos species and periphyton was studied over 18 weeks using three treatments: (1) undisturbed control cobbles from the submerged zone; (2) cobbles desiccated and replaced into the submerged zone; and (3) cobbles desiccated and replaced into the varial zone. Periphyton density and compositional response resulting from these treatments were also examined. Desiccation treatments were significantly different in biomass from controls throughout the study. The biomass of desiccated and replaced river cobbles averaged <30% of the controls for C. glomerata and periphyton during the 18-week recolonization period. In contrast, the biomass of the control Oscillatoria and miscellaneous phytobenthos species averaged only 3 and 50% of that in the desiccation treatments, respectively. Cladophora, the dominant alga, comprised 77% of the phytobenthic biomass. A significant positive relationship between discharge, Cladophora and periphyton biomass was present in all treatments, while there was a negative relationship with discharge for Oscillatoria and miscellaneous phytobenthos species. There was a significant inverse correlation, as well as a pattern of separate biomass dominance, between C. glomerata and Oscillatoria. Cladophora was the dominant phytobenthos species on cobbles below the baseflow and Oscillatoria was dominant on cobbles in the varial zone. In general, the Cladophora biomass decreased under intermittent drying conditions, while Oscillatoria declined under permanently wet conditions. Diatom composition was not significantly different between treatments; however, their density was lower on desiccated cobbles. Diatom density on desiccated cobbles in the submerged and varial zones averaged 69 and 42% of that of the controls, respectively. Recovery and maintenance of benthic resources are hindered by fluctuating flow regimes driven by electricity and irrigation requirements. Repeated desiccation of the phytobenthos has major effects on the bottom-up interactions in the Colorado River ecosystem. © 1998 John Wiley & Sons, Ltd.     

ICE中调试模块IP软核的设计与实现

开发人员在根据通用仿真器方案开发所需在线仿真器时,由于受ICE机制中调试模块IP软核的限制,并没有一个完整的调试模块方案,使得开发周期加长、成本增加。针对基于FPGA的通用在线仿真器方案缺少调试模块详细设计方案的问题,采用自顶向下划分层次和自底向上模块单独设计相结合的方法,设计出具有调试模块的IPcore,并结合EM78P447S的IP软核,在Xilinx Spartan系列XC3S700AN开发板套件中进行了验证。     

Application of retrodirective beams to achieve low sidelobe levelsin small phased arrays

Synthesis of beams with low sidelobe levels is a difficult problem for the case of small phased arrays with few elements. Mutual coupling between elements means that conventional weighting algorithms are not applicable. A technique is presented that calculates a complex weight vector for a five element linear array, giving a reduced sidelobe beam pattern. Sidelobes are reduced by the addition of retrodirective beams to the quiescent beam pattern. No knowledge of the coupling coefficients or element radiation patterns is required     

星间相干光通信中的光学锁相环

在相干体制下,二进制相移键控(BPSK)信号调制的零差接收机可实现理论上的最高灵敏度,是星间相干光通信的研究和应用重点.零差接收机要求本振波和信号波严格相位同步,常用的相位同步技术是光学锁相环(OPLL).阐述了光学锁相环的基本原理,介绍了近20年来光学锁相环的发展,在此基础上总结了各种类型锁相环的性能和适用范围.回顾了欧洲国家在星间相干光通信计划中通信终端使用光学锁相环的情况.最后对光学锁相环技术进行了总结,对该技术的前景进行了展望.     

用于大面积显示的低成本纳米管场发射显示器

我们验证了一个显示视频图像的阴极阵列,它基于6in对角线、QVGA分辨率的碳纳米管,采用了简单的低成本器件结构。该纳米管是利用选择性化学汽相沉积法生长在特定位置上的,对净化阴极具有良好的可控性。该器件结构只需要三个低分辩率的掩膜工序,开关电压为50V。此外,我们的器件设计为较长的显示寿命创造了条件。在一个较小的全熔接密封的试验显示器上,我们已经记录了超过3000小时的寿命,而电流仅下降了20%。