Current distribution control for parallel connected converters. I

For pt.II, see ibid., vol.28, no.3, p.841-851 (1992). A master-slave control scheme for a uniform current distribution among converter modules in a parallel connected system is presented. In this technique, inner current loops are introduced to the system to achieve output current equalization. The current distribution error is used as a criterion for judging system performance. Using this control scheme, the current distribution error can be reduced greatly even with nonidentical converters in the system. To optimize system efficiency and facilitate the fault-tolerant algorithm realization, this technique is refined so that only the necessary number of converters are activated for different load conditions     

Direction finding with a four-element Adcock-Butler matrix antennaarray

The conventional analog Adcock-Butler matrix (ABM) antenna array direction finder suffers from systemic errors, component matching problems, and bandwidth limitations. Three digital bearing estimators are developed as candidates to replace the analog signal processing portion of the ABM. Using the same antenna array, they perform all signal processing in the frequency domain, thereby benefitting from the computational efficiency of the fast Fourier transform (FFT) algorithm. The first estimator requires two analog-to-digital converters (A-D) and three antenna elements. It multiplies the difference between the discrete Fourier transforms (DFTs) of the output signals from two antenna elements with that from a third antenna element. At each frequency component, the phase of this product is a function of the bearing. A weighted least squares (LS) fit through all the phase components then gives a bearing estimate. The second estimator is similar to the first but uses three A-D and all four antenna elements. The output signal from the additional antenna element provides an independent estimate of the weights for the LS fit, giving an improvement in accuracy. The third estimator applies the physical constraint existing between the time-difference-of-arrival (TDOA) of a signal intercepted by two perpendicular sets of antenna elements. This yields a better estimator than simple averaging of the bearing from each set of antenna elements. The simulation studies used sinusoids and broadband signals to corroborate the theoretical treatment and demonstrate the accuracy achievable with these estimators. All three direction finders have superior performance in comparison with the analog ABM     

A New Mechanism of Coronal Heating

The interaction between network magnetic fields and emerging intranetwork fields may lead to magnetic reconnection and microflares, which generate fast shocks with an Alfvén Mach number M _A<2. Protons and less abundant ions in the solar corona are then heated and accelerated by fast shocks. Our study of shock heating shows that (a) the nearly nondeflection of ion motion across the shock ramp leads to a large perpendicular thermal velocity (v _th), which is an increasing function of the mass/charge ratio; (b) the heating by subcritical shocks with 1.1 M_A 1.5 leads to a large temperature anisotropy with T/T 50 for O⁵⁺ ions and a mild anisotropy with T_/T 1.2 for protons; (c) the large perpendicular thermal velocity of He⁺⁺ and O⁵⁺ ions can be converted to the radial outflow velocity (u) in the divergent coronal field lines; and (d) the heating and acceleration by shocks with 1.1 M_A 1.5 can lead to u(O⁵⁺) v _th(O⁵⁺) 460 km s^–1 for O⁵⁺ ions, u(He⁺⁺) v _th(He⁺⁺) 360 km s^–1 for He⁺⁺ ions, and u(H⁺) v _th(H⁺) 240 km s^–1 for protons at r=3–4 R . Our results can explain recent SOHO observations of the heating and acceleration of protons and heavier ions in the solar corona.     

超声速磁流体管道流动的数值模拟

应用可同时适用于描述磁流体MHD (MagnetoHydroDynamic)强化超燃冲压发动机磁流体发电器与磁流体加速器的,包含磁流体N-S(Navier-Stokes)方程与磁感应方程的可压缩磁流体方程组对超声速磁流体管道流动进行了数值模拟研究,以验证本项工作所提出的对2组方程进行交替迭代的分裂式算法.给出了算例的计算结果,并与无磁场及恒磁场下相同管道的流动计算结果进行了对比分析.     

Advanced Technologies Demonstrated by the Miniature Integrated Camera and Spectrometer (MICAS) Aboard Deep Space 1

MICAS is an integrated multi-channel instrument that includes an ultraviolet imaging spectrometer (80–185 nm), two high-resolution visible imagers (10–20 μrad/pixel, 400–900 nm), and a short-wavelength infrared imaging spectrometer (1250–2600 nm). The wavelength ranges were chosen to maximize the science data that could be collected using existing semiconductor technologies and avoiding the need for multi-octave spectrometers. It was flown on DS1 to validate technologies derived from the development of PICS (Planetary Imaging Camera Spectrometer). These technologies provided a novel systems approach enabling the miniaturization and integration of four instruments into one entity, spanning a wavelength range from the UV to IR, and from ambient to cryogenic temperatures with optical performance at a fraction of a wavelength. The specific technologies incorporated were: a built-in fly-by sequence; lightweight and ultra-stable, monolithic silicon-carbide construction, which enabled room-temperature alignment for cryogenic (85–140 K) performance, and provided superb optical performance and immunity to thermal distortion; diffraction-limited, shared optics operating from 80 to 2600 nm; advanced detector technologies for the UV, visible and short-wavelength IR; high-performance thermal radiators coupled directly to the short-wave infrared (SWIR) detector optical bench, providing an instrument with a mass less than 10 kg, instrument power less than 10 W, and total instrument cost of less than ten million dollars. The design allows the wavelength range to be extended by at least an octave at the short wavelength end and to ∼50 microns at the long wavelength end. Testing of the completed instrument demonstrated excellent optical performance down to 77 K, which would enable a greatly reduced background for longer wavelength detectors. During the Deep Space 1 Mission, MICAS successfully collected images and spectra for asteroid 9969 Braille, Mars, and comet 19/P Borrelly. The Borrelly encounter was a scientific hallmark providing the first clear, high resolution images and excellent, short-wavelength infrared spectra of the surface of an active comet’s nucleus.     

Current-mode control for multiple-output converters

A small-signal model for multiple-output forward converter with current-mode control is derived. The model can accurately predict the small-signal characteristics for current-mode control. It is observed that the power stage pole-zero relative positions, which are critical to the compensator design, are not affected by introducing current-mode control     

Comments, with reply, on `Parallel resonant converter with LLC-typecommutation'

In a recent paper by C.Q. Lee et al. (ibid., vol.25, no.6, p. 844-7, Nov. 1989), the authors analyzed a DC-DC converter that they termed the LLC-type PRC (parallel resonant converter). Its resonant network contains three active components-two inductances and a parallel capacitance-and as a consequence the converter might be expected to have third-order dynamics. But Lee et al. employed a matrix transformation to show that the behavior of the circuit may be represented as a state-plane trajectory, as for a second-order circuit. The purpose of this contribution is to show that the converter has a zero-frequency eigenvalue, associated with undesirable circulating DC. The second-order dynamics exhibited by the third-order converter are explained by an application of Thevenin's theorem. Some aerospace applications of the LLC-type parallel resonant converter (PRC) are discussed. In their reply, the authors show that the circulating direct current does not exist in the practical converter circuit     

OLTARIS: On-line tool for the assessment of radiation in space

OLTARIS (On-Line Tool for the Assessment of Radiation In Space) is a space radiation analysis tool available on the World Wide Web. It can be used to study the effects of space radiation for various spacecraft and mission scenarios involving humans and electronics. The transport is based on the HZETRN transport code and the input nuclear physics model is NUCFRG. This paper describes the tools behind the web interface and the types of inputs required to obtain results. Typical inputs are mission parameters and slab definitions or vehicle thickness distributions. Radiation environments can be chosen by the user. This paper describes these inputs as well as the output response functions including dose, dose equivalent, whole body effective dose equivalent, LET spectra and detector response models.     

高超声速尖锥边界层转捩数值模拟

边界层转捩对高超声速飞行器气动力和热产生重要影响.通过联立求解Favré平均层流脉动能方程与Favré平均Navier-Stokes方程,开展了高马赫数条件下尖锥边界层转捩位置的数值预测研究.将数值计算结果与可用的实验结果进行对比,结果表明增大单位Reynolds数可使转捩提前发生,攻角效应可使迎风面转捩延迟,背风面转捩提前.