Multiple Echo Signals Research Articles

Intelligent Robot Obstacle Avoidance Perception Method Using a High-Precision Laser Distance Measuring System

Laser distance measuring is increasingly used in large-scale and real-time scanning measurement, including three-dimensional map construction, size measurement of large-scale buildings, and real-time surface information acquisition. Therefore, there are high-precision and high-efficiency requirements for the laser distance measuring system. Based on the phase laser distance measuring, a laser parallel distance measuring system is proposed as per the frequency division multiplexer principle. The system uses multi-channel modulation signals with different frequencies to drive multiple lasers to emit light intensity in parallel, and then the light wave is measured according to the change of the modulation signal. The single-channel modulation signal adopts the multi-scale composite wave to eliminate the uncertainty of the whole wavelength by using the combination of two scales and distance measurement. The multiple echo signals after diffuse reflection of the measured target are mixed and received by the same photodetector and go through signal amplification, clutter filtering, etc., and then are sent to the mixing unit together with the reference signal, followed by down-frequency processing, with the effective signal obtained through the low-frequency filter. In the experiment, the laser distance measuring system is used for obstacle avoidance control of mobile robots, and fuzzy control is used to design the corresponding obstacle avoidance controller. The robot lateral deviation and heading angle are used as the input of the fuzzy controller, and the track variation is the output. The test verifies that the obstacle avoidance control is effective, that is, the laser distance measuring system designed in this research can be used for the obstacle avoidance control of mobile robots.

Time-of-Flight Estimation in the Presence of Outliers. Part II—Multiple Echo Processing

The mean squared error of the classical maximum likelihood time-of-flight (ToF) estimator increases dramatically when the signal-to-noise ratio falls below a certain threshold. For narrow-band signals, this well-known threshold effect occurs largely due to the biased outliers which are induced by the local maxima of the source signal autocorrelation function. In our previous work (Part I), we have described a machine learning biosonar-inspired method for reducing the effect of the outlier bias on the accuracy of a single-echo estimate. In this paper, we extend this approach by introducing a method for combining multiple echo signals into a robust ToF estimator which is resilient to outliers. The individual bias-corrected estimates are combined together using the optimal weighted averaging (OWA) scheme which takes into account the uncertainties associated with inlier and outlier measurements. As in the single-echo case, a weak classifier and a bank of phase-shifted unmatched filters are used for estimating the probabilities of appearance of a specific outlier class in a single-echo estimate. Combined with the previously introduced single-echo bias-reduction method, the OWA scheme results in significant improvement in the ToF estimation accuracy.

A correction method for streak artifacts in gradient-echo EPI using spin-echo EPI reference data

To analyze the streak artifacts in a gradient-echo echo planar imaging (GE-EPI) sequence and to propose a correction method for the Nyquist ghost artifacts that does not cause streak artifacts in the GE-EPI imaging. Several GE-EPI imaging experiments with various reference scans, using both GE-EPI and SE-EPI scan data, were performed to analyze the streak artifacts and to investigate the spin dephasing phenomena of the GE-EPI reference scan. In addition, the analysis based on the spin dephasing was undertaken in order to demonstrate that the SE-EPI reference data can be used for the correction of the GE-EPI main scan data. The experimental results confirmed that the improvement of the reference data using either signal averaging or a large flip angle cannot guarantee perfect correction of the streak artifact if the noise is not completely removed. Due to the main field inhomogeneity, the spins of the GE-EPI reference data were dephased in multiple echo signals. The proposed correction method, which uses a SE-EPI reference scan for the GE-EPI images, eliminates the N/2 ghost artifacts without producing streak artifacts. It is believed that the proposed phase error correction scheme can improve the EPI performance in high field MRIs with higher magnetic field inhomogeneities.

MREIT conductivity imaging of canine head using multi-echo pulse sequence

In magnetic resonance electrical impedance tomography (MREIT), we measure induced magnetic flux densities subject to multiple injection currents to reconstruct cross-sectional conductivity images. Spin echo pulse sequence has been widely used in MREIT and produce postmortem and in vivo conductivity images of animal and human subjects. The image quality depends on the SNR of the measured magnetic flux density image. In order to reduce the scan time and current amplitude while keeping the image quality, we have developed a multi-echo pulse sequence for MREIT. In this study, we show results of canine head MREIT imaging experiments using the multi-echo pulse sequence. Compared to the injection current nonlinear encoding (ICNE) pulse sequence, it provides a higher SNR of MR magnitude images by combining multiple echo signals. Noise in measured magnetic flux density data is significantly reduced due to an increased current injection time over multiple echo signals. These allow us to significantly decrease the total scan time. Reconstructed conductivity images of a canine head show enhanced conductivity contrast between gray and white matter using the multi-echo pulse sequence. In our future work, we will focus on in vivo human and disease model animal experiments using the new MREIT pulse sequence.

Decay of an Intense Polariton Wave in Direct-Gap Semiconductors and Time Dynamics of the Distribution of Scattered Polaritons

The time evolution of an intense polariton wave has been studied in the framework of a microscopic approach. It has been shown that due to polariton–polariton scattering a significant polariton wave depletion takes place in a comparatively short time interval. The condensate decay occurs in the form of multiple echo signals. Distribution-function dynamics of scattered polaritons have been investigated.

Nonmonotonic decay of nonequilibrium polariton condensate in direct-gap semiconductors.

Time evolution of a nonequilibrium polariton condensate has been studied in the framework of a microscopic approach. It has been shown that due to polariton-polariton scattering a significant condensate depletion takes place in a comparatively short time interval. The condensate decay occurs in the form of multiple echo signals. Distribution-function dynamics of noncondensate polaritons have been investigated. It has been shown that at the initial stage of evolution the distribution function has the form of a bell. Then oscillations arise in the contour of the distribution function, which further transform into small chaotic ripples. The appearance of a short-wavelength wing of the distribution function has been demonstrated. We have pointed out the enhancement and then partial extinction of the sharp extra peak arising within the time interval characterized by small values of polariton condensate density and its relatively slow changes.

Mössbauer gamma echo

By applying stepwise phase modulation of recoilless gamma radiation in a coincidence experiment, constructive interference is produced in transmission geometry between the source and the absorber fields. The resulting regenerated decay signal is called a gamma echo. Here it is demonstrated that during the decay of the 14.4 keV state of57Fe multiple echo signals can be generated.