直流大电流测量电路中电流纹波的研究

介绍了直流大电流测量中纹波对于测量仪器的影响,发现二次仪表的差摸抑制能力对于测量的影响最大.论述了如何从纹波的两个主要来源(电源和同路)治理纹波分量,分析铜排框架的纹波治理效果,并提出一种利用带铁心铜排框架抑制纹波的方案,最后给出一套简易工程设计方法.     

一种新型的纳米巨磁阻抗磁敏传感器

主要介绍纳米微晶材料的巨磁阻抗效应及利用该效应研制的一种新型的磁敏传感器.它以脉冲频率的方式作为输出结果,与传统的磁敏传感器相比,具有高灵敏度、便于数字化测量等优点,在地磁测量、磁航向等方面有良好的应用前景.     

一种直流大电流的间接测量方法

当被检装置输出的直流大电流超出了标准表的测试范围时,使用一种间接的方法进行测量,并对测量结果进行了分析和不确定度评定,证明此种方法能够满足测量要求。     

一种新型压磁薄膜

在硅橡胶基体中复合非晶Fe73.5Cu1Nb3Si13.5B9粉体,制得一种具有压磁特性的复合薄膜,并对一些影响因素进行了初步探讨.实验表明,该复合薄膜具有灵敏的压磁特性,尤其在＜120kPa微应力,测试频率低于400kHz时,压磁效应最好.在相同测试频率下,压应力与应力阻抗值成正比关系;薄膜中非晶粉体越细、粉体含量越高,薄膜越薄,其压磁效应越大.     

电流退火对CoFeSiB非晶丝和薄带的巨磁阻抗效应比较

采用熔融抽拉法和单辊急冷法分别制备了Co68.25Fe4.5Si12.25B15非晶丝和薄带。测量了制备态下两者的巨磁阻抗(GMI)效应,发现非晶丝的GMI比率高于薄带。研究了不同电流密度退火后非晶丝和薄带的GMI效应,结果发现ΔΖ/Ζ=[Z(H)-Z(H=0)]/Z(H=0)都明显上升,且非晶薄带数值更大;当电流密度等于0.96×107A/m2时,薄带的这一比率最大达到410%,磁场灵敏度达到5.1%/(A/m)。分析了出现上述现象的原因。     

钴基软磁非晶合金的巨磁阻抗效应研究进展

介绍了钴基软磁非晶合金丝与薄带的制备方法,重点讨论了频率、磁各向异性等因素对巨磁阻抗效应的影响.对非晶合金的巨磁阻抗效应传感器的应用进行了展望.     

变压器中性点直流电流的三种抑制措施比较研究

直流架构下的输电系统,在大地回线特有的运行路径下,换流站及这一地段的中性点,会产生接地特性的直流分量。对变压器之上的这种分量,预设了三种可用的抑制途径：串联架构内的电阻接地、交流输电关联的串联电容、中性点特有的电容接地。依循实测得来的电流数值,建构了大地电网特有的运算模型,对变压器安设着的中性点,深入辨识了串联电容。调研结论表征着：直流电流特有的最优抑制路径,是在安设好的变压器以内,添加某数值的串联电容。     

巨磁阻抗效应的非对称特性

巨磁阻抗效应的应用需要传感器具有好的线性度和高的灵敏度,实现非对称巨磁阻抗效应可以使传感器在零场附近提高灵敏度和改善线性度.本文对近年来非对称巨磁阻抗效应的研究进展进行了综述,并分析了其产生的机理.     

一种低电流纹波的低压大电流DC-DC变换器的研究

通过n个倍流整流结构交错并联方式用以进一步减小纹波电流。给出了电路的开关信号波形和仿真模型，并使用Pspice仿真软件对该模型进行仿真，取得满意效果。最后通过实验验证。这种结构特别适用于通信设备、计算机、宇航等领域的电源。     

A novel high common mode DC ammeter

A wide-range DC ammeter for laboratory research, capable of operating at high voltage, is described. Low cost, easy construction, versatility, and commonly available components were desired design requirements. The ammeter consists of a series-connected microammeter and milliammeter. Specifications include linear current sensing from 1 μA to 1 mA with a fixed linear scale resolution of 1 μA/div for the microammeter and 20 μA/div for the milliammeter with an accuracy of ±2.5% of full scale for both meters, operating voltage up to 30 kV DC, short circuit, and RF transient protection. The principles of operation and a method for calibrating the ammeter are included     

A novel sensorless control method for brushless DC motor

The authors present the theory and implementation of a novel sensorless control method for the interior permanent magnet (IPM) brushless DC motor (BLDCM). The proposed new sensorless technique can accurately detect the zero-cross point (ZCP) of back electromotive force (BEMF), which is based on a comparison of the terminal voltage of the un conducting phase during the first and second part of a pulse width modulation (PWM) cycle. Compared with the conventional BEMF sensorless approach, the proposed new sensorless method solves the problem of the sensorless BLDCM drives at very low speeds. Experimental results confirm the validity of the new method.     

Analytical evaluation of DC capacitor RMS current and voltage ripple in neutral-point clamped inverters

The sizing of the DC-link capacitor in a three-level inverter is based on the RMS current flowing through it. This paper analyses the DC-link capacitor RMS current in a neutral-point clamped (NPC) inverter and expresses the same as a function of modulation index, line-side current amplitude and power factor. Analytical closed-form expressions are derived for the capacitor RMS current for single-phase half-bridge, single-phase full-bridge and three-phase three-leg topologies of a three-level inverter. The worst-case capacitor current stress is determined for each topology based on the analytical expressions. Further, analytical expressions are derived for the RMS values of low-frequency and high-frequency capacitor currents. These expressions are then used to estimate voltage ripple across the DC capacitor for sinusoidally modulated three-phase NPC inverter. The analytical expressions for the RMS current and voltage ripple are validated experimentally over a wide range of operating points.     

一种实现目标定位与跟踪的新方法

本文介绍了一种采用圆周阵对目标声源进行被动定位和跟踪的方法，与线阵不同，圆四可对目标进行全方位跟踪，且测距精度不受目标方位变化的影响，文中给出了圆周阵的测距，测向公式，并从理论上分析了测距性能，计算机仿真结果验证了理论分析的正确性。     

A novel high yield method for dry functionalization of carbon nanotubes

A novel and high yield (> 80%) dry method to functionalize (dry functionalization) carbon nanotubes (CNTs) using hydrothermal method, is reported here. The hydrothermal solution was prepared with HNO3, H2SO4 and H2O2 (1:3:2 vol. ratios) and reaction was carried out from 120 to 200 degrees C for 24 h. CNTs (multi wall) were kept in a way to avoid the direct contact with the solution. Treatment above 180 degrees C resulted in better functionalization of nanotubes as observed from Fourier transform infrared absorption spectroscopic (FTIR) measurements. Field emission scanning electron microscopic (FESEM) images showed that after functionalization, the nanotubes are seen with open ends, granular surface, twisted and are joined together. These clearly indicate the destruction of the graphite structure on the surface. This indicates that after treatment, CNTs reactivity has increased at the ends as well as at the side walls. X-ray Photoelectron Spectroscopic (XPS) studies show a shift in the C 1s peak position, increase in O 1s peak intensity and appearance of an N 1s peak.     

高精度小电流恒流源的设计与实现

目前以TL431稳压管搭建的恒流源电路存在小电流输出状态下输出精度较低,温度跨度大时工作电路的温漂较大,且不具备抑制温漂能力,在飞行器系统测试过程中无法全面地满足指标需求。该文对TL431恒流源做出改进,加入精密运算放大器OP77及π型滤波电路以保证输出精度,设计温度系数互补的电阻网络,通过调整合适的补偿点降低恒流源整体温漂。实验结果表明:改进后恒流源具有低温漂、高精度的优越性能。     

A novel method for fabrication of biodegradable scaffolds with high compression moduli

It has been previously shown that, when used for meniscal reconstruction, porous copoly(L-lactide/-caprolactone) implants enhanced healing of meniscal lesions owing to their excellent adhesive properties. However, it appeared that the materials had an insufficient compression modulus to accomplish 100% fibrocartilage formation. In addition, to be used for meniscal prosthesis, the compression modulus of the porous materials should be larger than 150 kPa in order to protect the articular cartilage. A technique was developed to prepare stiff porous materials of a high molecular weight 50/50 copoly(L-lactide/-caprolactone) suitable for fibrocartilage regeneration in meniscal implants and meniscal prosthesis. Porous microspheres (50–250 m) were agglutinated in the presence of NaCl crystals (250–300 m). The microspheres were mixed with solid solvent in order to obtain a homogeneous distribution of solvent over the spheres. By changing the amount of solvent and crystals, the density and the compression modulus could be varied over a range of 0.07 g ml-1 to 0.5 g dl-1 and 40–1100 kPa, respectively.     

A current ripple reduction of a high-speed miniature brushless direct current motor using instantaneous voltage control

High-speed miniature brushless direct current motor (BLDCM) is used in robots and medical applications because of its high-torque and high-speed characteristics. When compared with the general BLDCM, a high-speed miniature BLDCM has a low electrical time-constant. The current and torque ripple are very high when compared with the conventional pulse-width modulation (PWM) control scheme in the conduction period because of the inherent electrical characteristics. The authors propose a simple instantaneous source voltage and phase current control for torque ripple reduction of a high-speed miniature BLDCM. To reduce the switching current ripple, instantaneously controlled source voltage is supplied to the inverter system according to the motor speed and the load torque. In addition, a fast hysteresis current controller can keep the phase current within a limited band. Computer simulations and experimental results up to 40 000 rpm show the effectiveness and verification of the proposed control scheme.