伪随机码超声扩频测距发射与接收电路设计

针对单脉冲回波方式的超声测距方法存在的缺陷,借鉴雷达中的脉冲压缩技术,介绍了基于伪随机二进制序列(m序列)的超声扩频测距实现方法。针对超声换能器带宽特性和单脉冲回波特点,对m序列参数设计及其与超声载波的调制方式作了分析。为提高超声波的作用距离,还研究了发射电路的优化设计问题。     

基于软件无线电的近距离扩频测距算法

对基于软件无线电的近距离扩频测距算法进行了研究.论述了直接序列扩频测距的原理,研究了近距离测距的回波模型.从理论上分析了由于体目标引起的引信回波信号的多普勒展宽和距离展宽问题,以及它们对近距离扩频测距的影响.提出采用正交接收机结构来消除载波随机相位对测距的影响.最后对算法进行了软件仿真,对文章的研究工作进行了总结.     

扩频映射编码中伪随机序列的性能研究

扩频映射编码系统中,伪随机码的纠错性能至关重要.构造平衡Gold序列,循环移位m序列和移位正交m序列,分析比较其相关特性;计算扩频映射码集的最小码距,证明循环移位m序列和移位正交m序列纠错能力更好;建立系统的数学模型,对采用不同码型的系统误码率进行蒙特卡罗仿真,结果与理论分析吻合.证明选取相关特性好、最小码距大的伪随机序列集,适当增加扩频因子,可以提高扩频编码系统的性能.     

基于相关函数包络峰细化的高精度超声测距法

建立了超声测距时延估计模型,由于窄带超声回波参考信号与接收信号的相关函数在极值点附近具有慢衰减高频振荡特性,所以有必要将搜索直接相关函数的极值点转化为搜索相关函数包络的极值点.针对相关峰常规插值方法在多倍插值的情况下存在计算复杂、时延估计精度不高等缺点,结合超声回波信号的窄带带通特性和相关峰细化(Fine Interpolation of Correlation Peak)原理,提出了直接提取相关函数包络及包络峰细化方法,并分析了其计算复杂度.仿真与实验研究表明,该方法能大大提高相关函数包络峰值分辨率,适用于类超声回波信号的带通信号精细时延估计问题.     

基于DSP Builder的混沌扩频序列的生成

扩频通信系统在用的m序列和Gold序列,存在序列长度有限和数量有限等缺陷.连续混沌系统经离散化可以产生长度非常大的序列和数目巨大的码元,这样混沌所产生扩频伪随机码在扩频通信领域表现出比传统扩频码更好的相关特性和误码特性,使频谱资源得到充分利用.我们设计个混沌电路.进行电路实现,经离散化后得到具有良好相关特性的混沌扩频序列.     

扩频法超声测距中的渡越时间的确定

基于对扩频技术在雷达系统中的应用和数字相关器进行的研究,提出一种扩频法超声波测距中渡越时间捕获的方法。适当选择的一个伪随机序列信号,经超声传感器调制发射。然后通过数字相关方法求出发射序列和接收序列的相位差,即渡越时间。在计算自相关函数时用并行数字相关技术代替傅里叶变换算法,这样显著地减少了数据处理时间和现场可编程门阵列（FPGA）资源。系统以Altera公司的quartus Ⅱ软件为开发平台,经逻辑综合、布局布线后,适配到FPGA芯片中,给出了仿真结果。     

时延/时延差组合定位模型抗距离模糊技术研究

基于时延测量的水声跟踪定位系统,当信号传播时延大于脉冲重复周期时,就会出现测距模糊。测距模糊是高帧率定位系统不可避免的问题。研究了基于时延测量的合作声信标定位系统中的抗距离模糊问题,提出了一种新的抗距离模糊技术,基于时延测量定位模型和时延差测距模型,推导了相应的软件抗模糊算法。该方法将两种定位模型的优点相结合,实现了无模糊的、高精度的水下定位。最后海试结果验证了算法有效性和可靠性。该算法不仅具有硬件抗模糊无法具有的优点,而且与以往的硬件、软件抗距离模糊算法相比,有更灵活的运用空间。     

桥式推挽结构超声激励电路性能分析

基于桥式推挽结构,设计了由两路正向单脉冲电压产生双极性电压效果的电路,提高了激励信号的能量,为超声无损检测激励电路的负向耐压低、供电电源复杂等问题提供了解决方案。该方案包括硬件实现和相关实验验证。脉冲回波检测实验结果证明:桥式推挽结构的超声激励可以提高回波的幅度,在相同实验条件下,回波幅度为单极性方波脉冲激励的两倍,等效于双极性方波脉冲激励的效果。该文方法可以提高超声激励电路的性能,为高性能便捷式超声激励系统的设计提供了理论和实验依据。     

卫星信道高动态时延与多普勒模拟系统的研究与设计

本文利用大规模存储器实现大范围时延模拟,基于可变分数时延滤波器实现精确的时延控制,通过数字变频实现对多普勒特性的模拟.同时,设计了一种控制算法,实现对信号多普勒和时延特性的动态实时控制.最后,通过测距信号收发系统验证了系统大范围时延模拟能力;利用扩频信号收发系统验证了系统对多普勒与时延的动态实时模拟能力.     

利用射频信号解卷积估计组织和血流运动速度

基于射频信号解卷积,提出了一种无需MTI（Moving Target Indicator）滤波重建血流及组织运动速度的方法。对射频回波的采样序列进行快速傅里叶变换,在频域对相邻两次射频回波的频谱作解卷积处理,通过分析解卷后的数值序列得到组织和血流各自的运动速度。仿真实验对两组典型的时延参数的超声回波信号进行了血流和组织速度的估算,并与给定值比较,证明了方法的有效。该方法有助提高超声血流成像系统对低速血流的测量精度。     

Flow velocity profile via time-domain correlation: error analysis and computer simulation

An ultrasonic human-blood-flow velocity profile measurement method using time-domain correlation of consecutive echo pairs has been developed. The time shift between a pair of range gated echoes is estimated by searching for the shift that results in the maximum correlation. The time shift indicates the distance a group of scatterers has moved, from which flow velocity is estimated. The basis for the computer simulations and error analyses of the scheme includes a band-passed white Gaussian noise signal model for an echo from a scattering medium, the estimate of flow velocity from both a single scatterer and multiple scatterers, and a derived precision estimation. The error analysis via computer simulation includes an evaluation of errors associated with the correlation method. For a uniform flow velocity profile, beamwidth modulation represents the greatest error source. However, for a nonuniform flow velocity profile, the jitter caused by a small flow velocity gradient can exceed the other error sources. A detailed computer simulation evaluated the interdependencies of window length, beam width, vessel diameter, and viewing angle on the estimation of flow velocity.     

Accurate distance measurement by an autonomous ultrasonic systemcombining time-of-flight and phase-shift methods

This paper presents an efficient algorithm for distance measurement, combining both the pulse time-of-flight method and the CW phase-shift method, It copes with a low-rate sampling technique allowed by the limited bandwidth of two ultrasonic transducers working in air at 30 kHz and with modest software resources of autonomous devices. The measuring system was implemented and tested on a compact Motorola MC68HC16-based platform, with a minimum of attached hardware. Experimental results show an accuracy better than 1 mm for a poor reflecting target at a distance of about 1 m     

基于编码激励超声流速剖面测量的实验研究

相比常规的声学流速剖面仪,用于人体血流速度剖面测量的超声系统需要更高的信噪比增益和空间分辨率.为此,首先采用编码激励技术来改进传统的测量硬件系统,构造出一种具有"零相关窗"的互补码序列激励换能器发射声波,其优良的距离选通特性能提取出每个分辨单元的速度信息,同时大幅度提高回波信噪比.然后分析提取的分辨单元回波信号的特征,提出基于二维频谱图的邻帧差分法来消除相邻单元的干扰,对预处理后的信号进行高分辨率的速度参数估计,从而得到整个血管区域的速度剖面.通过搭建模拟人体血流测量实验装置进行仿血流的流速剖面测量实验.实验结果表明,采用此方法,在发射换能器的中心频率为5 MHz时,能达到0.3 mm的纵向分辨率,取得良好的测量效果.     

基于回波信号相似度的气体超声流量计动态阈值法研究

基于双阈值法的气体超声流量计采用第1阈值检测和过零检测确定回波的到达时刻点,然而由于回波波形易受工况环境因素的影响,会导致第1阈值错误,使回波到达时刻点发生错误偏移,影响测量精度,因此提出了一种基于回波信号相似度的动态阈值法.该方法获取回波信号极大值幅值后,计算设定工况和实际工况下回波信号上升区域部分的欧式距离以评估相...     

Echo decorrelation from displacement gradients in elasticity and velocity estimation

Several ultrasonic techniques for the estimation of blood velocity, tissue motion and elasticity are based on the estimation of displacement through echo time-delay analysis. A common assumption is that tissue displacement is constant within a short observation time used for time delay estimation (TDE). The precision of TDE is mainly limited by noise sources corrupting the echo signals. In addition to electronic and quantization noise, a substantial source of TDE error is the decorrelation of echo signals because of displacement gradients within the observation time. The authors present a theoretical model that describes the mean changes of the crosscorrelation function as a function of observation time and displacement gradient. The gradient is assumed to be small and uniform within the observation time; the decorrelation introduced by the lateral and elevational displacement components is assumed to be small compared with the decorrelation caused by the axial component. The decorrelation model predicts that the expected value of the crosscorrelation function is a low-pass filtered version of the autocorrelation function (i.e., the crosscorrelation obtained without gradients). The filter is a function of the axial gradient and the observation time. This theoretical finding is corroborated experimentally. Limitations imposed by decorrelation in displacement estimation and potential uses of decorrelation in medical ultrasound are discussed.     

面向机器人标定的单激光跟踪仪顺序多站式测量系统建模与分析

为实现大型串联工业机器人的高精度标定,搭建了一种单激光跟踪仪的顺序多站式测量系统。该系统仅需单台激光跟踪仪,先后在不同的基站位置对工业机器人的末端位置进行独立测量,并基于多边测量方法计算机器人的末端位置。计算过程中仅需激光跟踪仪的距离信息,有效地优化了末端位置的测量不确定度。首先,建立了该测量系统的仿真模型,深入地分析了测量点的数量与分布形状、测量距离以及工业机器人定位精度等因素对系统测量精度的影响;然后,依据分析结果确定单激光跟踪仪顺序多站式测量系统的搭建方案。实验结果表明:该系统在2.5 m距离上的测量误差仅为0.023 mm,优于激光跟踪仪的测量精度,满足大型串联工业机器人参数标定的测量精度要求。     

Chirp signal matching and signal power optimization in pulse-echomode ultrasonic nondestructive testing

Chirp pulse compression is a signal correlation technique that uses frequency modulated pulses as transmitted signals. Usually, signals with linear frequency modulation are applied. They can be generated rather easily, but their spectra are not totally matched to the transfer function of ultrasonic systems. In pulse-echo mode operation, with signal duration and consequently the time-bandwidth product being critical parameters, waveforms should be applied which make full use of the available power and bandwidth resources. We report here two methods to improve the overall efficiency of an ultrasonic pulse-echo system. Transmitter signals with constant amplitude level and nonlinear frequency modulation can be generated in such a way that they are spectrally matched to the system. A formula for the calculation of such a matched nonlinear chirp signal is presented. This modulation scheme also leads to a side-lobe level reduction of the compressed pulses. The application of square wave chirps derived from sine type chirps yields an additional gain of echo signal amplitude. Moreover, the complexity of the signal generation hardware is reduced. The methods are illustrated by an example     

Field Programmable Analog Arrays for Conditioning Ultrasonic Sensors

This paper proposes the use of field programmable analog arrays (FPAAs) as adaptive conditioning blocks for ultrasonic sensors. The uncertainty achievable through this technique, in fact, results very sensitive to the measurement conditions, due to the attenuation affecting the echo during its propagation. Indeed, FPAAs emulate analog circuits whose characteristics have to be dynamically tuned according to different operating conditions. Actually, the signal provided by the ultrasonic sensor is properly processed in order to improve the overall measurement accuracy. In this paper, the prototype of a distance meter based on time-of-flight (TOF) measurement is presented in order to evidence the advantages gained by FPAA features in processing the sensor output to compensate echo attenuation and distortion versus target distance. The prototype working is supervised by a digital signal controller (DSC) whose tasks are: 1) driving the ultrasonic transducer; 2) performing the echo acquisition; 3) tuning on the fly the FPAA features; 4) evaluating the TOF; 5) measuring the target distance; and 6) delivering the final result to the end user. This paper is completed by the results achieved in a number of experimental tests allowing interesting considerations to be drawn. In particular, the experiments confirm the prototype reliability and effectiveness also with ultrasonic echoes characterized by very low signal-to-noise ratios.     

Using a Single Transducer Ultrasonic Imaging Method to Eliminate the Effect of Thickness Variation in the Images of Ceramic and Composite Plates

This article describes a single transducer ultrasonic imaging method based on ultrasonic velocity measurement that eliminates the effect of thickness variation in the images of ceramic and composite plate samples. The method is based on using a reflector located behind the sample and acquiring echoes off the sample and reflector surfaces in two scans. As a result of being thickness-independent, the method isolates ultrasonic variations due to material microstructure. Its use can result in significant cost savings because the ultrasonic image can be interpreted correctly without the need for precision thickness machining during nondestructive evaluation stages of material development. Velocity images obtained using the thickness-independent methodology are compared with apparent velocity maps and c-scan echo peak amplitude images for monolithic ceramic (silicon nitride), metal matrix composite and polymer matrix composite materials having thickness and microstructural variations. It was found that the thickness-independent ultrasonic images reveal and quantify correctly areas of global microstructural (pore and fiber volume fraction) variation due to the elimination of thickness effects. A major goal achieved in this study was to move the thickness-independent imaging technology out of the lab prototype environment and into the commercial arena so that it would be available to users worldwide. The method has been implemented on commercially-available ultra-sonic can systems manufactured by Sonix, Inc. via a formal technology transfer agreement between NASA and Sonix.