Synchrosqueezing transform (SST) is a currently proposed novel postprocessing time-frequency (TF) analysis tool. It has been widely shown that SST is able to improve TF representation. However, so ...far, how to improve the TF resolution while ensuring the accuracy of signal reconstruction is still an open question, particularly for the vibration signal with time-varying instantaneous frequency (IF) characteristics, due to the fact that the vibration signals of mechanical equipment usually contain many types of noise generated by harsh operating conditions, and the SST will mix these noise into the real signal. Our first contribution is using the Gaussian modulated linear chirp (GMLC) signal model to represent the general nonstationary signals. The GMLC signal model can more accurately represent the time-varying nonstationary signal, compared with the SST signal model composed of linear phase function and constant amplitude. Our second contribution in this work is proposing a method to improve the TF resolution and reconstruction accuracy for nonstationary signals with time-varying IF, which we coined the second-order synchroextracting transform (SET2). In SET2, we apply the GMLC to deduce the nonstationary signal model and then only use the energy at the IF to characterize the TF distribution, which improves the TF while reducing the impact of noise on the real signal.
This paper considers the analysis of impulsive-like signals whose time-frequency ridge curves are nearly perpendicular to the time axis. Although the instantaneous frequency of such a signal is a ...multivalued time-dependent function, its group delay is a single-valued function of frequency, which indicates that a frequency-domain signal model is more suitable for describing impulsive-like signals. Therefore, a new frequency-domain signal model, called Gaussian-modulated linear group delay (GLGD) model, is applied to the second-order time-reassigned transform (TSST2) that achieves the time-frequency representation (TFR) of impulsive-like signals while allowing mode decomposition. Compared to the recently proposed time-reassigned synchrosqueezing transform (TSST), the TSST2 can provide a more accurate group-delay estimator, which is beneficial to obtain a sharper TFR. Numerical signals and experimental signals are employed to validate the effectiveness of the proposed TSST2.
Time−frequency analysis (TFA) is regarded as an efficient technique to reveal the hidden characteristics of the oscillatory signal. At present, the traditional TFA methods always construct the signal ...model in the time domain and assume the instantaneous features of the modes to be continuous. Thus, most of these approaches fail to tackle some specific kinds of impulselike signal, including shock and vibration waves, damped tones, or marine mammals. This article introduces a new method called generalized horizontal synchrosqueezing transform (GHST) to process the transient signal. A signal model defined in the frequency domain is used to deduce the GHST. Next, the new synchrosqueezing operator termed as group delay (GD) is proposed based on high-order Taylor expansions of the signal model. Finally, the modulus around ridge curves is rearranged from the original position to the estimated GD. Numerical results of a simulated signal demonstrate the precision of the GHST in terms of both readability of the time−frequency representation and reconstruction accuracy. Additionally, the proposed method is implemented to diagnose the fault in a rotary machine by analyzing the vibration signal. The validation demonstrates that the GHST performs better than other traditional TFA methods, and it is qualified for the online condition monitoring of the industrial mechanical system.
This article introduces the second-order transient-extracting transform (TET2) to extract transient components from a nonstationary signal. Different from the traditional transient-extracting ...transform (TET1), the proposed method is based on a more general frequency-domain signal model, termed the Gaussian-modulated linear group delay signal model. The first step of the proposed method is the computation of the second-order group delay (GD) estimator that describes the temporal positioning of signal ridges in the time-frequency (TF) plane. Based on this second-order GD estimator, we can then obtain an energy-concentrated TF representation. Finally, the transient component of interest is reconstructed after filtering out the noise in the TF plane. Thus, the TET2 can be considered as an effective TF filtering tool. Numerical experiments on real and synthetic signals show the effectiveness of the proposed method in the TF filtering application.
Advancements in the synchrosqueezing transform as a postprocessing time-frequency method have received considerable attention in the past few decades for the analysis of nonstationary signals. Many ...studies have focused on improving the accuracy of the estimated instantaneous frequency (IF). In some fields, the IF spectra of signals exhibit fast varying behavior or even some times parallel to the frequency axis. Thus, a majority of the existing methods may fail to adequately handle the transient signals such as guided waves and vibration waves. To address this problem, a new method called the horizontal synchrosqueezing transform (HST) is introduced in the present research study. By applying a local estimation of the group delay (GD), the proposed technique can yield a compact time-frequency representation for a transient signal while retaining its invertibility. For the purpose of validation, the proposed approach is implemented in both synthetic and real signals. The results are observed in a convincing manner in terms of the smaller value of the Rényi entropy and better reconstruction accuracy. The HST is supposed to be a reliable and accurate method for the analysis of transient signals with extensive usefulness.
Highlights
The review discusses the key concepts, loss mechanisms and test methods of electromagnetic interference (EMI) shielding.
The research progress of polymer matrix EMI shielding composites ...with different structures is detailedly illustrated, especially their preparation methods and corresponding evaluations.
The key scientific and technical problems for polymer matrix EMI shielding composites with different structures are proposed, and their development trend are prospected.
With the widespread application of electronic communication technology, the resulting electromagnetic radiation pollution has been significantly increased. Metal matrix electromagnetic interference (EMI) shielding materials have disadvantages such as high density, easy corrosion, difficult processing and high price, etc. Polymer matrix EMI shielding composites possess light weight, corrosion resistance and easy processing. However, the current polymer matrix composites present relatively low electrical conductivity and poor EMI shielding performance. This review firstly discusses the key concept, loss mechanism and test method of EMI shielding. Then the current development status of EMI shielding materials is summarized, and the research progress of polymer matrix EMI shielding composites with different structures is illustrated, especially for their preparation methods and evaluation. Finally, the corresponding key scientific and technical problems are proposed, and their development trend is also prospected.
The measuring technique combining a phase-shifting algorithm and Gray-code light has been widely used in three-dimensional (3D) shape measurement for static scenes because of its high robustness and ...anti-noise ability. However, in the high-speed measurement, phase unwrapping errors occur easily on the boundaries of adjacent Gray-code words because of the defocus of the projector, the motion of the objects and the non-uniform reflectivity of the surface. To overcome this challenge, a high-speed 3D shape measurement method based on shifting Gray-code light has been proposed in this paper. Firstly, the average intensity of three captured phase-shifting fringe images are used as a pixel-wise threshold to binarize the Gray codes and to eliminate most phase unwrapping errors caused by the non-uniform reflectivity, ambient light variations, and the defocus of projector. Then, the shifting Gray-code (SGC) coding strategy is proposed to avoid the remaining errors of phase unwrapping on the edge of the code words. In this strategy, no additional patterns are projected, and two sets of decoding words with staggered boundaries are built in the temporal sequences for one wrapped phase. Finally, the simple, efficient, and robust phase unwrapping can be achieved in the high-speed dynamic measurement. This proposed method has been applied to reconstruct 3D shape of randomly collapsing objects in a large depth range, and the experimental results demonstrate that it can reliably obtain high-quality shape and texture information at 310 frames per second.
Due to their high specific surface area, graphene oxide and graphene oxide-base nanoparticles have great potential both in dual-drug delivery and combination chemotherapy. Herein, we developed ...cisplatin (Pt) and doxorubicin (DOX) dual-drug-loaded PEGylated nano-graphene oxide (pGO) to facilitate combined chemotherapy in one system. In this study, nano-sized pGO-Pt/DOX ranged around 161.50 nm was fabricated and characterized using zeta-potential, AFM, TEM, Raman, UV-Vis, and FTIR analyses. The drug delivery efficacy of Pt was enhanced through the introduction of pGO, and the final weight ratio of DOX: Pt: pGO was optimized to 0.376: 0.376: 1. In vitro studies revealed that pGO-Pt/DOX nanoparticles could be effectively delivered into tumor cells, in which they induced prominent cell apoptosis and necrosis and exhibited higher growth inhibition than the single drug delivery system or free drugs. The pGO-Pt/DOX induced the most prominent cancer cell apoptosis and necrosis rate with 18.6%, which was observed almost 2 times higher than that of pGO-Pt or pGO-DOX groups. in the apoptosis and necrotic quadrants In vivo data confirmed that the pGO-Pt/DOX dual-drug delivery system attenuated the toxicity of Pt and DOX to normal organs compared to free drugs. The tumor inhibition data, histopathology observations, and immunohistochemical staining confirmed that the dual-drug delivery system presented a better anticancer effect than free drugs. These results clearly indicated that the pGO-Pt/DOX dual-drug delivery system provided the means for combination drug delivery in cancer treatment.
•This paper provides a detailed comparison of two typical temporal phase unwrapping methods including the two-frequency phase-shifting method and the Gray-coded-based method.•The basic principles and ...recent developments of the six approaches (hierarchical, number-theoretical and heterodyne approaches in two-frequency phase-shifting method; traditional Gray code approach, complementary Gray code approach and Gray code approach with tripartite phase unwrapping (Tri-PU) in Gray-coded-based method) are reviewed.•The performance of six approaches on measuring anti-noise ability, efficiency and robustness to motion are compared in detail based on proposed rigorous noise-induced error model and motion-induced error model.•Theoretical analysis, numerical simulation and experimental results are consistent and demonstrate that (a) Gray-coded-based method has better anti-noise ability than two-frequency phase-shifting method; (b) two-frequency phase-shifting method is more efficient than Gray-coded-based method when the number of encoding fringe periods exceeds 8; (c) hierarchical approach and Gray-coded approach with Tri-PU have best performance to resist motion-induced error in their respective types of methods.•Comparative results can give a helpful guidance to choose the “best” approach in different application scenes and quantitatively determine the projected fringe periods and imaging or projecting speed in dynamic measurement for FPP.
Two-frequency phase-shifting phase unwrapping and Gray-coded-based phase unwrapping are two typical temporal phase unwrapping methods to recover discontinuous or spatially isolated surfaces in fringe projection profilometry (FPP). But in dynamic measurement, environmental noise, required number of patterns and inter-frame motion affect the performance of these two methods on measuring anti-noise ability, efficiency and robustness in varying degrees. So, it is a challenge to quantitatively compare these methods’ abilities and determine the optimum one with appropriate system parameters and under a certain condition. In this paper, a detailed comparative study is conducted for the two-frequency phase-shifting method including hierarchical, number-theoretical and heterodyne approaches and the Gray-coded-based method including traditional Gray code approach, complementary Gray code approach and Gray code approach with tripartite phase unwrapping (Tri-PU). First, the principles of six approaches are reviewed; then, their anti-noise ability was quantitatively compared based on the proposed noise model; next, theoretical comparison of measuring efficiency, comparative review of improved methods and recommendation on usage are given to boost their efficiency; finally, motion-induced error model in phase unwrapping was presented to determine the appropriate system parameters in dynamic measurement. Theoretical analysis, numerical simulation and experimental results are consistent and demonstrate that (i) Gray-coded-based method has better anti-noise ability than two-frequency phase-shifting method; (ii) two-frequency phase-shifting method is more efficient than traditional Gray-coded-based method when the number of encoding fringe periods exceeds 8; (iii) hierarchical approach and Gray-coded approach with Tri-PU have best performance to resist motion-induced error in their respective types of methods. Comparative results can give a helpful guidance to choose the “best” approach in different application scenes and quantitatively determine the projected fringe periods and imaging or projecting speed in dynamic measurement for FPP.
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