A study on the coaxial aperture electromagnetic sensor and itsapplication in material characterization

An open-ended coaxial line has been studied by many researchers in order to use it as an electromagnetic sensor for the characterization of materials. These studies include static as well as quasi-static formulations under various simplifying assumptions. In this paper, we use a full-wave formulation of the problem and solve an integral equation to evaluate the aperture field distribution. The electromagnetic field distributions in the material medium as well as the aperture admittance are evaluated subsequently. The effects of a conducting flange over the coaxial opening and the finite sample size are also studied. The results are compared with the experimental data as well as the data obtained by other approaches. An application of this coaxial sensor for the characterization of materials up to 40 GHz is demonstrated     

Complex permittivity and moisture measurements of oil palm fruits using an open-ended coaxial sensor

An open-ended coaxial sensor for the determination of complex permittivity and moisture content of oil palm fruits is presented in this paper. The measurement system consisting of the sensor and a PC-controlled vector network analyzer have been tested successfully on a range of oil palm fruits of various degrees of ripeness. The initial values of the complex permittivity were estimated using the admittance model of the sensor. The amount of moisture content was found by matching the values of permittivity from the quasistatic model with the permittivity of a dielectric mixture model using the moisture content values obtained from the standard oven drying method.     

A method for precise RF admittance measurement

An improvement in the theory of the dual admittance bridge with self-calibration ability for precise RF measurement is described. The approach to the bridge function analysis extends utilization of its high resolution and accuracy in immittance standardization into the UHF range. No detailed information about the bridge's internal structure is needed for the theory, which describes it as a multiport device. Thus, the theory has not only contributed to bridge arrangement, but also enables simple and efficient calibration of measurement equipment based on the classical twin-T admittance bridge. Improvement in the accuracy and frequency range of the calibrated equipment should be expected. The theory has been verified on a special coaxial bridge circuit at 100 and 1000 MHz     

Calibration and measurement of dielectric properties of finitethickness composite sheets with open-ended coaxial sensors

The application of open-ended coaxial sensors for dielectric measurement of finite thickness composite sheets is studied. Expressions for calculation of the complex aperture admittance for two geometries are presented. These expressions are used to calculate the dielectric constant of infinite half-space as well as finite thickness slabs. A more efficient method of such calculations, using a personal computer, for low to medium loss dielectrics is demonstrated. The question of when a dielectric layer may be considered as infinitely thick is also addressed, and examples are presented. A different calibration technique (compared to the conventional ones) is described and successfully implemented. This calibration technique utilizes a dielectric sheet with known dielectric properties and thickness. Measurements for different airgaps between the open-ended coaxial line and the dielectric sheet are used to perform and enhance the calibration. The results of this calibration technique and several subsequent measurements are presented and discussed     

Dual layer stacked rectangular microstrip patch antenna for ultra wideband applications

An antenna consisting of a U-slotted rectangular microstrip patch stacked with another patch of a different size on a separate layer is presented and its performance results are investigated. An equivalent circuit model of this stacked patch design structure is also presented based on an extended cavity model to predict the input impedance. The theoretical input impedance is evaluated from this circuit model and the experimental results support the validity of the model. In this case, stacking with a simple patch adds another resonance to the antenna thus providing a wider bandwidth. The dimension of the top patch is optimised to achieve ultra wide bandwidth. A maximum impedance bandwidth of 56.8% is achieved using this structure, and the return loss |S₁₁|of the antenna is less than -10 dB between 3.06 and 5.49 GHz and the radiation patterns are found to be relatively constant throughout the band. A coaxial feed with Gaussian modulated pulse is used for this antenna.     

Friction and heat transfer in the gap between two rotating coaxial cylinders

An attempt is made at a theoretical analysis of the friction and heat transfer in the gap between two coaxial cylinders rotating in one direction. The interaction of a fluid with the channel walls is divided into two stages: 1) formation of roller, annular fluid flows in the gap; and 2) friction and heat transfer with forced flow past the walls.     

Plane-strain deformation of an elastic material compressed in a rough rectangular cavity

Deformation and stress distributions in a linear elastic solid, confined to a rigid cavity with rough walls and subjected to uniform compression from one end, are examined. Wall roughness is modeled by Coulomb friction. At the rigid walls, one boundary condition involves deformation and the other stresses, and this renders the problem non-standard. A Laplace transformation solution is constructed for a semi-infinite cavity, and a computational solution for a cavity with finite length. Agreement between the two solutions is good, and improves with increasing cavity lengths and higher coefficients of friction. There exists a critical value of the coefficient of friction below which the axial displacements decay monotonically with distance from the loaded end and the material points stay in contact with the rough walls. For supercritical values of the coefficient of friction, displacements and stresses on the rough walls exhibit oscillatory behavior in the axial direction. The material loses contact with the walls, and the analytical solution presented here loses validity.     

An improved technique for permittivity measurements using a coaxialprobe

An enhanced model for an open-ended coaxial probe used for making permittivity measurements is presented. A permittivity measurement system consisting of the coaxial probe and a network analyzer is described including details of the error correction and curve fitting techniques. Determination of the percent dissolved solids in fructose solutions from permittivity measurements is presented as example of the usefulness of the coaxial probe/network analyzer measurement system     

基于压电导纳的钢框架螺栓松动检测试验研究

介绍了基于压电导纳的结构健康监测技术基本原理;利用该技术对钢框架结构进行了螺栓松动检测试验研究。即将三个压电陶瓷片粘贴在钢框架节点处的不同构件表面,通过测量框架节点处各压电陶瓷片在螺栓松动前后电导纳的变化来识别损伤.试验结果发现,位于连接板处的压电陶瓷片对该节点处的螺栓松动最敏感,而与螺栓不直接相连的上斜撑上的压电陶瓷电导纳则几乎不受连接板螺栓松动的影响。分别定义了基于电导纳实部和虚部改变的均方根(RMSDR和RMSDI)作为损伤程度识别指标,对框架节点的螺栓松动损伤程度进行了定性识别研究。通过对得到的RMSDR和RMSDI两种损伤指标值对比发现,基于压电导纳实部信息定义的损伤程度识别指标RMSDR能较好地识别框架节点螺栓松动损伤程度,而基于电导纳虚部定义的识别指标RMSDI则不能正确识别。     

Characterization of four-terminal-pair resistance standards: acomparison of measurements and theory

Coaxial straight-wire resistance standards with calculable frequency dependence, link AC measurements to DC measurements of resistance. Several coaxial standards of values 100 Ω and 1000 Ω are compared with each other and with other resistance standards at frequencies of 1592 Hz (ω≅10⁴) and 15920 Hz (ω≅10⁵) using AC bridges. The data are compared with calculations based on the geometry of this resistor design. Our results provide good assurance that the calculations are accurate for the quadrature component of admittance (phase angle), which is directly proportional to frequency. Frequency-dependent contributions to the in-phase component of admittance, which increase as the square of the frequency, are larger than predicted by the calculations     

Coaxial range measurement - current trends for mobile roboticapplications

The useful environmental interaction of a mobile robot is completely dependent on the reliable extraction of information from its immediate surroundings. A review of some of the most commonly used coaxial, active sensing methods in robotics is presented. A coaxial sensor results if the transmitter and receiver are geometrically arranged in a coaxial manner. Certain advantages of such configurations are discussed. First, optical sensors, in which a controlled light signal is transmitted, are reviewed with respect to their applicability to mobile robotics. An overview of the literature describing light detection and ranging (LIDAR) systems for range measurement in robotics is given. A review of mobile robot research using sound navigation and ranging (SONAR) is presented where processing algorithms are demonstrated for the correct interpretation of ultrasonic data recorded in indoor environments. Ultrasonic wave reflection is generally dominated by specular reflections. Methods for the interpretation of SONAR data, for correct target classification, and, hence, range estimation are given, based on the acoustic reflection mechanism     

高静水压下换能器阻抗特性的测量方法研究

针对在小腔体中阻抗分析仪发射连续波无法准确测得换能器阻抗的问题,提出一种在高静水压下使用脉冲正弦信号激励换能器测量阻抗的方法。以采样电阻法为基础,根据腔体尺寸确定发射脉冲个数以及可测频率范围来有效避免腔体边界反射对测量造成的影响。通过设置不同的发射频率,分别采集换能器两端及采样电阻两端的电压波形信号,利用已知频率的三参数正弦曲线拟合法分别得到波形信号的幅值和初始相位角,计算得到换能器的导纳值。改变静水压力,利用脉冲法测得0~10 MPa静水压下换能器导纳特性。实验结果表明,采用脉冲正弦信号激励的方法可在有限空间内准确测量换能器的阻抗特性;且随着静水压力的升高换能器的谐振频率发生偏移,导纳圆直径减小。     

AC Bridge Methods for the Measurement of Three-Terminal Amittances

To avoid uncertainties due to finite lead impedances, a direct admittance is best defined as the transfer admittance of a two-terminal-pair network. The network may contain both voltage and current transformers. Unwanted loop currents may be suppressed by coaxial chokes and the equivalent circuit reduced to that of a three-terminal admittance. Two direct admittances may be compared with a voltage or current ratio, and two such ratios may be combined to give a general form of four-arm bridge. This general bridge network may be balanced in a way which leads to simple relations independent of the ground admittances between the four direct admittances. At least two separate balance conditions must be satisfied, but it is usually best to make three balances since the error in the main balance is dependent on the product of the errors in the other two. There is no restriction on the type of network which may be used to couple the source and the detector to the bridge, provided the ground balance conditions are satisfied. Networks may be chosen to provide for filtering, impedance matching, and the elimination of transformers. Examples are given of four-arm bridge networks which use adjustable decade ratio-transformers as the main balance controls and fixed ratio-transformers for multiplication or inversion.     

Field attenuation as the underlying principle of cryo current comparators

This paper describes both theoretical and experimental investigations of the error behaviour of cryo current comparators (CCC) as a function of the geometric parameters of the superconducting shields. The basis of our calculations has been the attenuation of magnetic fields by a superconducting coaxial cavity. The theoretical results agree with experiments which have been performed on a special test configuration of a CCC. Its main feature is a toroidal shield made from mercury with an elongated gap region forming the coaxial cavity.The results obtained are applicable to all types of CCCs. We found that the superconducting coaxial cavity behaves like a filter which transmits only the useful signal containing information about the net current linkage. All other fields contributing to the error of the comparator are attenuated exponentially.     

Boundary domain integral method for the study of double diffusive natural convection in porous media

The main purpose of this paper is to present a boundary domain integral method (BDIM) for the solution of natural convection in porous media driven by combining thermal and solutal buoyancy forces. The Brinkman extension of the classical Darcy equation is used for the momentum conservation equation. The numerical scheme was tested on a natural convection problem within a square porous cavity, where different temperature and concentration values are applied on the vertical walls, while the horizontal walls are adiabatic and impermeable. The results for different governing parameters (Rayleigh number, Darcy number, buoyancy ratio and Lewis number) are presented and compared with published work. There is a good agreement between those results obtained using the presented numerical scheme and reported studies using other numerical methods.     

Dielectric resonator probe arrays with sectoral hard horn for wideband spatial power combiners/dividers

Analysis of dielectric resonator probe (DRP) arrays operating in an oversized hard waveguide with sectoral hard horn excitation is presented. A single DRP excited by a coaxial probe is first investigated inside a hollow rectangular waveguide and a waveguide loaded with hard walls, respectively. Simulation results are compared with measurement results and show a good agreement. Then, a one-dimensional DRP array is analysed inside a dielectric-loaded, oversized waveguide with sectoral hard horn excitation. A full-wave analysis based on the region-by-region finite-difference time-domain technique is applied for the optimisation of array parameters such as inter-element coupling, active reflection coefficients and uniform excitation. Numerical results are compared with those generated by using commercial software and exhibit a very good agreement.     

A domain-extension method for quantitative detection of cavities by infrared scanning

A domain-extension method for quantitative detection of irregular-shape cavities inside irregular-shape bodies is presented. An auxiliary problem is introduced in the solution of the cavities. In the auxiliary problem, the original body domain at the cavity side is extended so that the original cavity walls become interior points. The position of the cavities can then be found by solving the temperature field in the extended domain and matching the temperatures and heat fluxes at the interior points to the conditions imposed on the cavities. A boundary-element method is used for the solution of the auxiliary problem, and by means of four examples, the accuracy of the domain extension method is established. The paper provides the details for the numerical solution of the cavities. Limitation of the method in the detection of multiple cavities is also explored. The domain-extension method has shown to be highly effective in quantitative detection of cavities in single-cavity bodies.     

A mathematical model of the admittance between tapped windings andtheir shield for ratio instruments

A mathematical model is developed for the equivalent admittance dependent on the measurable admittance distributed between the shield and individual sections of a tapped winding. The model makes possible the calculation of ratio instrument errors, correcting admittances, and the input admittance caused by the admittance to the shield. These parameters strongly depend on both the type of supply and the location of the shield with respect to the winding connection. Experimental results are presented that demonstrate the usefulness of the model for an enhancement of ratio instrument accuracy     

Nondestructive Online In Vitro Monitoring of Pre-Osteoblast Cell Proliferation Within Microporous Polymer Scaffolds

We present a system for the online, in vitro, nondestructive monitoring of tissue growth within microporous polymer scaffolds. The system is based on measuring the admittance of the sample over a frequency range of 10-200 MHz using an open-ended coaxial probe and impedance analyzer. The sample admittance is related to the sample complex permittivity (CP) by a quasi-static model of the probe's aperture admittance. A modified effective medium approximation is then used to relate the CP to the cell volume fraction. The change of cell volume fraction is used as a measure of tissue growth inside the scaffold. The system detected relative cell concentration differences between microporous polymer scaffolds seeded with 0.4, 0.45, 0.5, and 0.6 x 10⁶ pre-osteoblast cells. In addition, the pre-osteoblast proliferation within 56 scaffolds over 14 days was recorded by the system and a concurrent DNA assay. Both techniques produced cell proliferation curves that corresponded to those found in literature. Thus, our data confirmed that the new system can assess relative cell concentration differences in microporous scaffolds enabling online nondestructive tissue growth monitoring.