Effects of Hybrid Voltages on Oxide Formation on 6061 Al-alloys During Plasma Electrolytic Oxidation

Plasma electrolytic oxidation (PEO) is carried out on 6061 Al-alloys in a weak alkaline electrolyte containing NaOH, Na₂SiO₃ and NaCl. Centered on the correlation of composition and structure, analyses by means of X-ray diffration (XRD), scanning electron microscope (SEM) and energy dispersive spectrometry (EDS) are conducted on the specimens, which have been PEO-treated under hybrid voltages of different direct current (DC) values (140-280 V) with constant alternate current (AC) amplitude (200 V). Attention is paid to the composition, properties and growth mechanism of oxide layers formed with hybrid voltages. Moreover, the main effects of DC value are discussed. Ceramic layers with a double-layer structure which combines hard outer and soft inner layers are found to be consist of α-Al₂O₃, γ-Al₂O₃ and mullite. With the DC values increasing, the growth of the ceramic layers tends to have increasingly obvious three-stage feature.     

Control system design of the Korean lunar lander demonstrator

In preparation for the lunar exploration program scheduled to be launched during the early 2020s in Korea, a lunar lander demonstrator, which will be used for developing and demonstrating lunar landing technologies, is being developed. The control configuration of the lunar lander demonstrator is determined with the consideration of available technologies and flight requirements. It is suggested that altitude control be achieved by clustering five 200 N monopropellant thrusters and attitude control with eight 3 N thrusters. A control algorithm designed to follow a predefined trajectory is developed using quaternion feedback. Control system configuration and control logic are verified by using computer simulations. Simulation results show that a soft landing with a touchdown velocity of less than 3 m/s is achieved. Attitude control performance is also verified using computer simulations. The developed control configuration will be further tested by hardware in the loop simulations and ground based firing tests during the next phase of the study.     

Time variations of cosmic-ray helium isotopes with BESS-Polar I

The Balloon-borne Experiment with a Superconducting Spectrometer (BESS) is configured with a solenoidal superconducting magnet and a suite of precision particle detectors, including time-of-flight hodoscopes based on plastic scintillators, a silica-aerogel Cherenkov detector, and a high resolution tracking system with a central jet-type drift chamber. The charges of incident particles are determined from energy losses in the scintillators. Their magnetic rigidities (momentum/charge) are measured by reconstructing each particle trajectory in the magnetic field, and their velocities are obtained by using the time-of-flight system. Together, these measurements can accurately identify helium isotopes among the incoming cosmic-ray helium nuclei up to energies in the GeV per nucleon region. The BESS-Polar I instrument flew for 8.5 days over Antarctica from December 13th to December 21st, 2004. Its long-duration flight and large geometric acceptance allow the time variations of isotopic fluxes to be studied for the first time. The time variations of helium isotope fluxes are presented here for rigidities from 1.2 to 2.5 GV and results are compared to previously reported proton data and neutron monitor data.     

The Improvement of a Low-Cost Inertial Navigator Through Conditional Feedback

A concept for providing an accurate low-cost inertial reference was presented in a previous paper. Low-cost platform gyros, poor in terms of their drift rates, furnish the space-fixed reference frame. A single high-accuracy gyro, mounted on the platform, monitors the performance of the platform gyros. Consequently, drift rates are estimated, predicted, and applied as signals to correct the platform for gyro uncertainties. This paper features new estimation and predictive techniques.     

Accuracy Limitations of Hyperbolic Multilateration Systems

A recently developed procedure [1] for assessing the accuracy of hyperbolic multilateration systems makes it easy to determine basic limitations on accuracy. This paper illustrates how such bounds can be derived. The results include bounds for a variety of geometries that are representative of practical ground-based and satellite-based hyperbolic systems. The results are applicable whenever the ranging errors can be treated as uncorrelated zeromean random variables. In some cases the bounds quantify general knowledge (e. g., the directional dependence of errors). In other cases the bounds represent entirely new limitations (e. g., optimum accuracies for sector-restricted and cone-restricted transmitter/receiver configurations).     

Optimal Digital Beam Splitting for Radar Beacon Systems

Beam splitting for a radar beacon system that scans in azimuth is investigated from a theoretical viewpoint. The video output is quantized into two levels corresponding to a detection and no detection. Further, the antenna sensitivity profile is described by sin ?/?. For this system, a digital method of beam splitting that minimizes angular error is developed. Prior information about the probable location of targets can be included. Both Bayesian and minimax approaches are developed. The minimax estimate of the target azimuth is the average of the angles at which beacon signals are detected. The antenna beam can be interpolated by a factor of 10 when the signal power is 3 dB above the noise and without prior azimuth information The interaction of beam splitting and detection is discussed.     

Large-signal stability analysis of solar array power system

A large-signal stability analysis of the solar array regulator system is performed to facilitate the design and analysis of a low-earth-orbit (LEO) satellite power system. The effective load characteristics of various control methods in the solar array regulator system, such as the constant power load, variable power load, constant voltage load, constant current load, and constant resistive load, are classified to analyze the large-signal stability. Then, using the state plane analysis technique, the large-signal behavior of the solar array system is portrayed and the stability of various equilibrium points is analyzed. Thus, this approach can be contributed to organize the optimal controller structure of the system by representing the relationship between the control method of the solar array regulator and the large-signal stability. For the verification of the proposed large-signal analysis, a solar array regulator system that consists of two 100 W parallel module buck converters has been built and tested using a real 200 W solar array.     

Stochastic Acceleration of Energetic Particles in the Heliosphere

In this paper we assess possible roles of stochastic acceleration by random electric field and plasma motion in the production and transport of energetic particles in the heliosphere. Stochastic acceleration can occur in the presence of multiple small-scale magnetohydrodynamic waves propagating in different directions. Usually, this type of stochastic acceleration is closely related to particle pitch angle scattering or parallel diffusion. Given the values of the parallel diffusion coefficient inferred from the observations of cosmic ray modulation or other energetic particle phenomena in the heliosphere, stochastic acceleration by small-scale waves is much slower than acceleration by shock waves and it is also much slower than adiabatic cooling by the expansion of the solar wind; thus it is considered as inefficient for producing heliospheric energetic particles or for the modulation of cosmic rays. Another type of stochastic acceleration occurs when particles go through random compressions or expansions due to large-scale plasma motion. This acceleration mechanism could be very fast when the correlation time of the fluctuations in plasma compression is short compared to the diffusion time. Particle acceleration by an ensemble of small shock waves or intermittent long wavelength compressible turbulence belongs to this category. It tends to establish an asymptotic p ^?3 universal distribution function quickly if there is no or little large-scale adiabatic cooling. Such a particle distribution will contain an infinite amount of pressure. Back reaction from the pressure is expected to modify the amplitude of plasma waves to an equilibrium state. At that point, the pressure of accelerated particles must remain finite and the accelerated particles could approach a p ^?5 distribution function.     

Geysers spawn hot new nanotechnology

The article reports on efforts by NASA scientists to build nanostructures from thermophilic cyanobacteria living in hot springs. Heat shock protein 60 (HSP60) is purified and engineered to serve as a substrate for quantum dots that are about 1-10 nm across, much smaller than the typical 130 nm found in standard computer chips.