Characteristics Of Echo Signal Research Articles

Pretightening stress measurement method of aero-engine fastening components based on acoustic time difference separation of the coupling layers

Accurate measurement of the pretightening stress of the fastening components in the rotor system is crucial for ensuring assembly accuracy and operational safety in aero-engines. The acoustic time difference (ATD, ultrasonic propagation time difference) of the coupling layer reduces the pretightening stress measurement accuracy of the fastening components. This paper proposes a method for measuring the pretightening stress of the fastening components of the aero-engine based on the separation of the ATD of the coupling layer. Firstly, the echo signal characteristics of the bottom surface of the fastening components are analyzed by simulation, and the influence of the coupling layer ATD on the stress measurement error isstudied. Then, based on the ultrasonic multi-layer medium propagation model, a scheme of adding a transition layer to construct the first echo is proposed. The ATD of the coupling layer is fed back by the first echo propagation time difference, and the ATD caused by the coupling layer and the stress is solved by the second echo. On this basis, the ATD separation method of the coupling layer is proposed and the pretightening stress measurement model of the fastening components based on the ATD separation of the coupling layer is established. The experimental results show that after the ATD of the coupling layer is separated, the maximum measurement error of the pretightening stress of the fastening component is reduced from 13.016 MPa to 4.738 MPa, which effectively improves the measurement accuracy.

System Design and Signal Processing in Spaceborne Squint Sliding Spotlight SAR with Sub-Aperture Block-Varying PRF

To tackle the problems of Doppler spectrum, aliasing caused by azimuth beam scanning and azimuthal serious non-uniform sampling in squint sliding spotlight synthetic aperture radar (SAR) with varying repetition frequency technology, the azimuth sampling method of sub-aperture block-varying pulse repetition frequency (SBV-PRF) is proposed, where the sub-aperture division judgement makes the azimuth acquisition time of each sub-block small enough so that the Doppler bandwidth caused by the Doppler center change can be ignored. Based on the echo signal characteristics of a SBV-PRF transmission scheme, an azimuth pre-processing method combining SBV-PRF transmission scheme with sub-aperture division is proposed. Using this method, de-skewing is first performed on each set of sub-aperture data to eliminate the additional Doppler bandwidth introduced by the squint angle, and then the azimuth signal resampling is performed to ensure different sub-aperture data have the same sampling rate. The SBV-PRF technology reduces the difficulty of azimuth signal pre-processing while ensuring the complete acquisition of the complete echo data of the squint sliding spotlight mode. The effectiveness of the SBV-PRF system design and the signal processing method is verified by the point target echo simulation and imaging simulation results.

Research on Reflection Mechanism of Nonuniform Plasma

The surface of the near-space hypersonic vehicle is covered with a plasma sheath (PSh) with fluid properties. During the radar detection of hypersonic vehicles, the spatial distribution characteristics of PSh parameters affect the reflected wave intensity and the position of the maximum reflection layer of plasma in the vehicle’s different areas, resulting in significant differences in the intensity and frequency offset of echo signals in various areas. In order to further study the reflection characteristics of nonuniform plasma, this article uses the equivalent wave impedance method to calculate the reflection and transmission coefficients of each layer of plasma, analyzes the reflection intensity distribution mechanism of nonuniform plasma, and reveals the EM shielding effect (ES-effect) of plasma. Based on the typical carrier frequency, collision frequency, electron density, and electron density distribution gradient, the variation law of maximum reflection position and maximum reflection intensity in nonuniform plasma is revealed. The research results lay a theoretical foundation for analyzing echo signal characteristics of the PSh-covered target, radar detection anomaly suppression, and EM stealth.

Radar Moving Target Detection Method Based on SET2 and AlexNet

Aiming at the nonstationary characteristics of echo signal for a high-speed maneuvering target, a signal feature extraction method is proposed by combining the time-frequency analysis and convolution neural network, and then the automatic detection of radar moving target in a noisy environment is realized. Firstly, the echo signal is modelled as a more accurate Gaussian modulation-linear frequency modulation (GM-LFM) signal and converted into the time-frequency image by a second-order synchroextracting transform (SET2). Then, ridge extraction is applied to extract the maximum energy ridge from the time-frequency distribution, and the data set is constructed by the maximum energy ridge. Finally, the data set is input into AlexNet for training, and the deep-level features of echo signal are extracted to realize the automatic moving targets detection. Simulation results show that SET2 and RE can effectively enhance the time-frequency characteristics of echo signal under the noisy environment, and the detection accuracy and noise robustness of the proposed method are better than that of SET1 and smooth pseudo-Wigner–Ville distribution (SPWVD).

ZnO piezoelectric coatings used for the ultrasonic excitation of longitudinal-transverse waves and their application for smart bolts

The accurate measurement of the prestress for bolt has been intensively investigated in recent years. Many research studies have been conducted to improve the accuracy of prestress measurement using ultrasound techniques; in particular, the real-time prestress of bolt can be obtained without the requirements to calibrate the original load when ultrasonic longitudinal and transverse waves are applied simultaneously. However, there are few studies that have focused on ultrasonic transducers that can generate longitudinal and transverse waves simultaneously. Furthermore, research on coatings deposited directly on the top of the bolts for the excitation of longitudinal and transverse waves has not been reported. ZnO coating is widely used for the ultrasonic transducer due to its excellent piezoelectric property and stable structure in high temperatures. Therefore, it is an outstanding candidate for the excitation of both longitudinal and transverse waves and has a good application prospect in the field of bolt prestress measurement by ultrasonic sound wave with high accuracy. In this paper, ZnO piezoelectric coatings were deposited on (100)-oriented Si substrates by radio frequency sputtering techniques. The morphology, structure, and echo signal characteristics of the coatings were characterized by SEM, AFM, XRD, and ultrasonic measurement instruments. The experimental results showed that Ar/O2 ratio and deposition position exhibited significant effects on the excitation waveform of the ZnO piezoelectric coating. When the Ar/O2 ratio reduced to 1:3, the coating could excite both transverse and longitudinal waves. However, when the deposition position gradually moved away from the sputtering center, the transverse wave gradually enhanced. The coating could excite a pure transverse wave when the sample was at the edge of the effective sputtering area, which showed that the type of the excitation wave and the relative intensity of each wave could be well controlled by the Ar/O2 ratio and deposition position. These research results have a good application prospect in bolt stress measurement.

The methodology of choosing the distribution model of echoes for ground penetrating radar with broadband signals

The paper presents an approach to the solution of the problem of assessing the probabilistic characteristics of echo signals of ground penetration radar (GPR). This allows performing the synthesis of decisive statistical rules in the problems of detection and recognition of subsurface objects.

Features of classification of the underlying earth surfaces according to the characteristics of echo signals in on-board radars

Features of classification of the underlying earth surfaces according to the characteristics of echo signals in on-board radars

Evaluation of the bonded ratio of TC4 diffusion bonded joints based on ultrasonic C-scan

The immersion ultrasonic C-scan ((IUCT) test was conducted for evaluating the bonded ratio of the TC4 diffusion bonded joints. The signal characteristics reflected by the typical bonding states of the joint, gap, kissing bond, microvoids and sound bond were investigated in the time and frequency domain. Moreover, as for the bonding state of microvoids, its local bonded ratio exhibited a linear relationship with the relative amplitude of its echo signal in the time domain. Based on the echo signal characteristics and the linear relationship about microvoids, a MATLAB algorithm program was developed to calculate the global bonded ratio of bonding interface directly from echo signal of IUCT. The results demonstrated that joint bonded ratio and mechanical properties, especially the tensile strength, could be quantitively evaluated by the combination of IUCT test and the developed MATLAB program.

Feature Extraction and Reconstruction by Using 2D-VMD Based on Carrier-Free UWB Radar Application in Human Motion Recognition.

As the size of the radar hardware platform becomes smaller and smaller, the cost becomes lower and lower. The application of indoor radar-based human motion recognition has become a reality, which can be realized in a low-cost device with simple architecture. Compared with narrow-band radar (such as continuous wave radar, etc.), the human motion echo signal of the carrier-free ultra-wideband (UWB) radar contains more abundant characteristic information of human motion, which is helpful for identifying different types of human motion. In this paper, a novel feature extraction method by two-dimensional variational mode decomposition (2D-VMD) algorithm is proposed. And it is used for extracting the primary features of human motion. The 2D-VMD algorithm is an adaptive non-recursive multiscale decomposition method for nonlinear and nonstationary signals. Firstly, the original 2D radar echo signals are decomposed by the 2D-VMD algorithm to capture several 2D intrinsic mode function (BIMFs) which represent different groups of central frequency components of a certain type of human motion. Secondly, original echo signals are reconstructed according to the several BIMFs, which not only have a certain inhibitory effect on the clutter in the echo signal, but can also further demonstrate that the BIMFs obtained by the 2D-VMD algorithm can represent the original 2D echo signal well. Finally, based on the measured ten different types of UWB radar human motion 2D echo analysis signals, the characteristics of these different types of human motion are extracted and the original echo signal are reconstructed. Then, the three indicators of the PCC, UQI, and PSNR between the original echo signals and extraction/reconstruction 2D signals are analyzed, which illustrate the effectiveness of 2D-VMD algorithm to extract feature of human motion 2D echo signals of the carrier-free UWB radar. Experimental results show that BIMFs by 2D-VMD algorithm can well represent the echo signal characteristics of this type of human motion, which is a very effective tool for human motion radar echo signal feature extraction.

Verification of echo amplitude envelope analysis method in skin tissues for quantitative follow-up of healing ulcers

We aim to develop an ultrasonic tissue characterization method for the follow-up of healing ulcers by diagnosing collagen fibers properties. In this paper, we demonstrated a computer simulation with simulation phantoms reflecting irregularly distributed collagen fibers to evaluate the relationship between physical properties, such as number density and periodicity, and the estimated characteristics of the echo amplitude envelope using the homodyned-K distribution. Moreover, the consistency between echo signal characteristics and the structures of ex vivo human tissues was verified from the measured data of normal skin and nonhealed ulcers. In the simulation study, speckle or coherent signal characteristics are identified as periodically or uniformly distributed collagen fibers with high number density and high periodicity. This result shows the effectiveness of the analysis using the homodyned-K distribution for tissues with complicated structures. Normal skin analysis results are characterized as including speckle or low-coherence signal components, and a nonhealed ulcer is different from normal skin with respect to the physical properties of collagen fibers.

Changes in echo-pulse instantaneous frequency when scanning the transducer

A theoretical model has been proposed that makes it possible to explain peculiarities in the frequency characteristics of echo signals at the output of a receiving transducer depending on the geometrical parameters of a reflecting spherical surface when the transducer axis is displaced with respect to the surface’s center of curvature, that is, during scanning. It has been demonstrated that both with and without displacement of the transducer’s geometrical axis with respect to the center of curvature of the reflecting surface, the type of changes in the temporal dependences of the instantaneous frequency of signals at the output of the receiving transducer essentially depends on the geometrical parameters of a wavefront reflected from the surface: the smaller the radius of wavefront curvature at the aperture of the receiving transducer, the stronger the changes of the instantaneous frequency in the echo pulse. It has also been shown that the larger the curvature of this surface, the stronger the changes in the frequency characteristics of the echo pulse as the transducer axis is displaced with respect to the center of curvature of the reflecting spherical surface.

Study of the phase characteristics of echo signals for vertical sounding of water with signals with linear frequency modulation

We study the phase characteristics of echo signals during vertical acoustic sounding of a water medium with narrowly directed emission. We experimentally demonstrate the possibility of measuring the dependence of the phase of echo signals on the delay for a probe signal with linear frequency modulation. We substantiate the possibility of remote determination of sound velocity variations in water.

Single Borehole Radar for Well Logging in a Limestone Formation: Experiments and Simulations

A borehole radar (BHR) prototype system was developed for the exploration of complicated oil and gas reservoirs. To verify the performance of the system, single-hole reflection imaging experiments were carried out in an abandoned limestone mine. In the physical experiments, the cliff wall and a metal plate were selected as the imaging targets to evaluate the detection performance of the prototype system. The average filter method was used to remove the background noise, then the frequency–wave number (F-K) imaging algorithm was adopted for radar imaging. The unknown fractures surrounding the borehole produced complex reflections that were not beneficial to effectively extract the target echo when the down-hole sensor was shifted along the borehole. However, by fixing the down-hole sensor and shifting the target, the detection range of the radar system extended up to about 10 m in the limestone formation. A 2-D finite-difference time-domain (FDTD) modeling method was also implemented to simulate the experimental procedure, and demonstrated that the prototype system can provide enough accuracy to predict the echo signal characteristics and reproduce the radar response in the formation. The combination of field experiment, theoretical analysis, and numerical simulation not only objectively validated the fundamental performance of the radar prototype, but also generated some new concepts for further improvement on the radar system design.

Экспериментальные исследования метода дефектоcкопии при ручном ультразвуковом контроле элементов подвижного состава железных дорог

Introduction. The work aim was to carry out experimental studies of dual-frequency method reliability for defect type determination at manual ultrasonic testing of railway rolling stock elements.
 Method. The method of discontinuity type determination includes: measuring amplitude characteristics of echo signals from the detected defect on the frequency of 2.5 and 5.0 MHz; calculation a defect form-factor; comparing the form-factor with the certain limit values, one of which corresponds to a planar defect and the other corresponds to a volume defect. The defect is considered planar/volume, if its form-factor differs from the appropriate limit value by no more than 30%. The method gives an opportunity to determine types for both point and extensive defects.
 For the experiment a sample was manufactured, in which various artificial reflectors of different types and dimensions were made. The type of each reflector was determined with dual-frequency method. The relative error and reliability of type determination were calculated.
 Results. Experimental studies have shown that reliability of defect type determination with dual-frequency method is quite high. The average accuracy of point defect determination has been 97,389%; extensive defect determination has been 93,588%.

A Robust Imaging Algorithm for Squint Mode Multi-Channel High-Resolution and Wide-Swath SAR With Hybrid Baseline and Fluctuant Terrain

In this paper, the squint mode multi-channel (MC) synthetic aperture radar (SAR) with hybrid baseline and fluctuant terrain is proposed and studied for high-resolution and wide-swath (HRWS) imaging. During the imaging process, due to the cross-track baseline and fluctuant terrain, the azimuth signal reconstruction is the kernel problem for this imaging mode. To deal with this problem, in this paper a robust azimuth signal reconstruction approach is proposed, where terrain elevation of scene is considered. At first, the pre-processing of the linear range cell migration correction (RCMC) and topography-independent phase compensation is implemented in the azimuth time domain. After that, combining the azimuth echo signal characteristics, the local polynomial Fourier transform (LPFT) is utilized to obtain the coarse-focused SAR image. Then, based on joint pixel pair vector and robust Capon beamforming (RCB), a Doppler ambiguity suppression approach is proposed to reconstruct the Doppler ambiguity-free azimuth signal in LPFT frequency domain, during which the influence of the cross-track baseline component and fluctuant terrain is eliminated using the coarse digital elevation model (DEM) for the imaging scene. At last, the chirp scaling imaging algorithm is utilized to focus the SAR image. The effectiveness of the proposed imaging approach is demonstrated via simulated and real measured squint mode MC-HRWS SAR data.

Laser detection method of ship wake bubbles based on multiple scattering intensity and polarization characteristics

It is the research foundation of ship wake detection by laser and new-generation optical homing torpedo to investigate the influence of multiple scattering effect on light scattering intensity and polarization characteristics of the ship wake bubbles. The simulation model of laser back-scattering detection by ship wake bubbles is based on vector Monte Carlo method, and the multiple scattering mechanism is studied. The influences of multiple scattering effect and the bubble density in ship wake on the light scattering intensity and polarization characteristics of echo signal are analyzed. The echo photon polarization contribution reception method and the echo signal polarization statistical method are proposed to solve the problem that the low photon return probability cannot form the echo energy in the system with small receiver field of view. These methods are based on the basic idea of the particle collision importance sampling and the traditional energy receiving method. The polarization detection experimental platform for the simulated wake bubbles is built and the accuracy of the simulation results is verified in experiment. The consistence of the experimental and simulation results shows that the bubble distance and density information can be characterized by echo intensity, polarization information and the echo signal intensity, and the polarization characteristics can be used to detect and distinguish the ship wake bubbles, or even a low density wake bubbles with high precision.

Echo Separation in Multidimensional Waveform Encoding SAR Remote Sensing Using an Advanced Null-Steering Beamformer

In order to reap the potential benefits that waveform diversity can provide for spaceborne synthetic aperture radar remote sensing, echoes from different subpulses constituting a complete transmit waveform should be effectively separated at first. This paper presents a new separation approach implemented by an advanced null-steering beamformer on satellite. Compared with common null-steering beamforming, our approach will take into account the characteristics of echo signal from the scene and accordingly embed the finite-impulse response (FIR) filtering process into the modified null-steering beamformer to deal with the issue of pulse extension. In this paper, echo signals generated by multidimensional encoded waveform will be analyzed in detail; based on this analysis, FIR filter and the new null-steering beamformer are derived. Simulation results show that much better separation performance can be obtained by our approach than by conventional null-steering beamforming.

Application of Laser Technology in Evaporation Duct Detection

Aiming at how to detect evaporation duct environment accurately in real time, method of detecting atmospheric duct by using laser technique was put forward. From the model of air refractive index of radar band, theoretical relationship between laser scattering properties of aerosol over the ocean and vertical change characteristics of the air humidity was established; judge whether to produce evaporation duct, through using laser to detect the echo signal characteristics of aerosol. The research content is an important complement for methods of atmospheric duct detection, and it can solve several common problems that atmospheric duct forecasting methods needs rigorous measuring conditions, its measurement results delay, it need be estimated by experience and detecting precision is not high, which has higher military application value.

On the calculation of characteristics of echo signals of transverse and longitudinal waves from reflectors with flat surfaces

On the basis of an earlier developed model on scattering of transverse waves at a crack in the form of a strip, exact and approximate (estimated) scattering indicatrices for transverse and longitudinal elastic waves are studied, taking into account the influence of their transformation at the flat free surface of a reflector into other wave types on the change in the echo-signal amplitude. The limits of applicability of the estimated indicatrices for mirror and back reflections are numerically studied. The exact results for longitudinal waves are compared to those obtained by the Kirchhoff method.

An experimental study of the fluctuations of acoustic signals reflected from ships

Results of an experimental study of amplitude fluctuations in echo signals received from ships and their wakes are presented for the cases of vertical and horizontal sounding in a wide range of high sonic and ultrasonic frequencies. Dependences of the amplitude variation coefficients of echo signals on the speed of ship motion, the sounding frequency, the duration of sounding pulses, and the height of wind waves on the sea surface are analyzed. The probability characteristics of echo signals reflected from ship hulls and from wakes at different depths are discussed.