Availability Allocation Using a Family of Hyperbolic Cost Functions

This paper develops a technique for allocating the parameters, repair time and failure rate, to each component of a system at the lowest possible cost. The allocations are subject to the constraint of a required availability. To make an analytic solution possible, the assumptions of constant failure rates, s-independent components, series system, and no multiple failures were made. Each component is assumed to have a hyperbolic cost function associated with changing its failure rate and its repair time. A Lagrange Multiplier method was used to solve the problem.     

A Cost-Based Availability Allocation Algorithm

This paper develops a cost-based procedure for allocating the availability parameters (repair times and failure rates) to the various components that make up a system. The allocation is handled as a cost minimization problem, subject to the constraint of meeting a system availability requirement. The problem is solved using Lagrange multipliers and an example is stated and solved. This technique is applicable in the early stages of system design to determine the detailed component availability specifications (repair times and failure rates) that will allow a system requirement to be met. Furthermore, the technique is useful in the latter stages of system design when initial specifications have fallen short of the required goal, and modifications and improvements are required. The technique points out the problem components and defines the amount of improvement necessary.     

Imprcved Fast Neutron Tolerance of GaAs JFET's Operating in the Hot Electron Range

The fast neutron induced degradation of the electrical characteristics of n-channel GaAs junction field effect transistors (JFET's) operating in the Shockley mode (thermal equilibrium) was estimated previously. Advances in technology have led to the fabrication of devices with micron sized channel lengths which are capable of response at microwave frequencies. Short channel lengths, however, result in device operation under thermal nonequilibrium conditions and hot electron effects must be considered. The theory of Lehovec and Zuleeg, which provides the characteristics for JFET's operating in the hot electron range, is employed in this paper to estimate the neutron degradation of these devices. It is shown that JFET's operating in the hot electron range are more resistant to neutron exposure than are JFET's operating in the Shockley region. Devices with channel doping concentration of 1017 cm-3 are predicted to survive fluences of 1016 neutrons/cm2. Some preliminary test results are reported.     

Selecting operating points for discrete-time gain scheduling

Gain scheduling is a popular approach for nonlinear control system design. A controller is obtained by designing a set of controllers at operating points and then linearly interpolating controller values between them. However, little guidance has been provided in the literature for the selection of operating points. We use interval mathematics and a classical synthesis design approach to determine a near minimal set of design points and assess the quality of a gain scheduled controller. A sufficient condition for the assignment of the system closed loop poles is developed, and an algorithm for selecting the operating points is provided. An example is given to demonstrate the approach.     

Neutron-Induced Metallic Spike Zones in GaAs

The metallic spike model for neutron damage has been shown to account for the observed anomalous infrared absorption in GaAs. In this paper, the electrical properties of semiconductors containing metallic spikes are explored. The metallic zones are shown to act as deep potential wells which trap carriers from the host semiconductor energy bands. The component of mobility associated with carrier scattering from the depletion region surrounding charged spikes is estimated as a function of temperature, including the temperature dependence of the trapped charge. Hall measurement data taken before and after neutron irradiation of n-type GaAs are compared with theory and good agreement is obtained. It is proposed that the high field trapping and slow release of electrons observed in neutronirradiated Gunn diodes is associated with the presence of metallic spikes. Hot electrons in high field domains penetrate the electrostatic barrier and are trapped within the spikes. When the low field condition is restored, excess electrons return to the host semiconductor matrix. The rate of escape of excess electrons is estimated from considerations of the processes of emission over and tunneling through the electrostatic barrier. Measurements of the temperature - dependent decay rates of the excess charge are obtained from neutron-irradiated Gunn diodes. Two decay rates were obtained at each temperature. The shorter decay time shows a temperature dependence consistent with a quantum tunneling mechanism. The longer decay time shows a stronger temperature dependence which is in qualitative agreement with emission of electrons over the barrier.     

Effect of Fast-Neutron Irradiation on Optical Attenuation in Compound Semiconductors

Recently a model was proposed to account for anomalous infrared attenuation noted experimentally for fast-neutron-irradiated, compound semiconductors. The model represents neutron damage by localized phase transitions to a high-pressure metallic-like state. It is shown that this model not only accounts for the anomalous, continuous optical attenuation at photon energies less than the band gap, but also can explain the absorption edge fuzziness noted for neutron-irradiated GaAs. The optical attenuation produced by embedded metallic zones at frequencies above the absorption edge is derived, and it is shown that the metallic resonance absorption band is altered from the usual Lorentzian shape. It is shown that in GaAs irradiated with fast neutrons at sufficiently high doses the resonant absorption by the metallic phase can dominate the host semiconductor absorption.     

Estimation Under Reliability Growth Assuming Gamma Failure Models

Engineers often make changes during the development of a system in order to correct design weaknesses. If done well, this results in reliability growth (an increase in reliability and mean life) as development continues. The lifetime distribution at each stage in development is assumed to be gamma. Approximate maximum likelihood estimates (MLEs) of the parameters are obtained subject to the conditions that no parameter decreases in the next stage. An iterative procedure involving two constrained nonlinear optimization problems is proposed for obtaining the approximate MLEs. The constrained optimization problems can be computed using isotonic regression; the iterations converge rather quickly. The computations can be performed using common mathematical subroutine packages such as the International Mathematical and Statistical Libraries.     

Simultaneous optical and nuclear magnetic resonance spectroscopy for monitoring cardiac energetics in vivo

There are a number of applications in which it is useful to simultaneously collect data from what are traditionally separate instrumentation modalities. In particular, in vivo physiological investigations in which data from parallel experiments must be correlated would benefit from simultaneous data collection through 1) elimination of subject variability, 2) elimination of treatment variability, and 3) a reduction in the number of animal preparations required. Here we describe the simultaneous collection of fluo-3 optical fluorescence and 31P nuclear magnetic resonance (NMR) spectra to measure intracellular calcium levels and high-energy phosphate metabolism, respectively, in vivo. This work is part of ongoing research into the profound anoxia tolerance exhibited by the hearts of certain turtle species. An NMR compatible optical fluorescence spectrometer was constructed and tested. In the 31-cm bore of a 2 T superconducting magnet, NMR and optical spectra were collected every 10-15 min from the in situ, in vivo hearts of anesthetized turtle subjects prior to and during one to three hours of anoxia. It was found that while PCr stores became significantly depleted during anoxia, beta-adenosine triphosphate (ATP) levels remained within 20% of control values, and intracellular diastolic calcium levels did not vary by more than 10%. The ability to make simultaneous phosphorus and calcium measurements on a single subject is important to understanding the exact relationship between phosphorus energy state and maintenance of calcium homeostasis.     

Operationalizing the Moskos institution-occupation model: An application of Gouldner's cosmopolitan-local research.

Evaluated the problems of developing and validating measures of institutional–occupational orientations in the US Air Force. The measures were developed by modifying A. W. Gouldner's (see PA, Vol 33:4757 and 33:8067) cosmopolitan–local measurement. A questionnaire was administered that contained 8 attitudinal items concerning military and civilian life, a measure of career intent, a job satisfaction measure, and several demographic questions. Factor analysis of the 8 attitudinal items revealed 2 independent dimensions: Institutional and Occupational Orientations. Hypothesized differences between various demographic groups in a survey of 10,687 Air Force personnel of various grades, seniority levels, and educational levels were observed. The Institutional orientation was positively associated with career intent, seniority, and job satisfaction, whereas the Occupational orientation was negatively associated with those criteria. It is concluded that the developed measures provide the means to assess trends longitudinally in the military, as posited by C. Moskos (1977). (24 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Affinity-based turbidity sensor for glucose monitoring by optical coherence tomography: toward the development of an implantable sensor

We investigated the feasibility of constructing an implantable optical-based sensor for seminoninvasive continuous monitoring of analytes. In this novel sensor, analyte concentration-dependent changes induced in the degree of optical turbidity of the sensing element can be accurately monitored by optical coherence tomography (OCT), an interferometric technique. To demonstrate proof-of-concept, we engineered a sensor for monitoring glucose concentration that enabled us to quantitatively monitor the glucose-specific changes induced in bulk scattering (turbidity) of the sensor. The sensor consists of a glucose-permeable membrane housing that contains a suspension of macroporous hydrogel particles and concanavalin A (ConA), a glucose-specific lectin, that are designed to alter the optical scattering of the sensor as a function of glucose concentration. The mechanism of modulation of bulk turbidity in the sensor is based on glucose-specific affinity binding of ConA to pendant glucose residues of macroporous hydrogel particles. The affinity-based modulation of the scattering coefficient was significantly enhanced by optimizing particle size, particle size distribution, and ConA concentration. Successful operation of the sensor was demonstrated under in vitro condition where excellent reversibility and stability (160 days) of prototype sensors with good overall response over the physiological glucose concentration range (2.5-20 mM) and good accuracy (standard deviation 5%) were observed. Furthermore, to assess the feasibility of using the novel sensor as one that can be implanted under skin, the sensor was covered by a 0.4 mm thick tissue phantom where it was demonstrable that the response of the sensor to 10 mM glucose change could still be measured in the presence of a layer of tissue shielding the sensor aiming to simulate in vivo condition. In summary, we have demonstrated that it is feasible to develop an affinity-based turbidity sensor that can exhibit a highly specific optical response as a function of changes in local glucose concentration and such response can be accurately monitored by OCT suggesting that the novel sensor can potentially be engineered to be used as an implantable sensor for in vivo monitoring of analytes.