Kai Zhao

Doppler impact intensity measurement is widely applied in railway transportation and aerospace engineering for enhancing equipment robustness, reliability, and accelerometer accuracy. Data acquisition and processing still present challenges due to high frequency, non-stationarity, and large data volume. This study presents an in-situ transient velocity profile measurement system that segments Doppler intensity in the frequency domain and identifies the effective component on-chip. The system adopts a four channels acquisition structure that achieves dynamic data capture even when a single channel’s sampling rate is relatively low, while its method supports expansion to higher frequencies in future applications. A impact dynamics up to 63 MHz can be captured by data acquisition in four channels of only 31.25 mega sampling per second, and the computations is reduced by a half. For different velocities of mechanical impacts, the proposed method yields a smaller deviation compared with commercial LDV and accelerometer.

Limited projections from actual optical paths yield an underdetermined projection equation for tunable diode laser absorption spectroscopy tomography. An iterative reconstruction method via adaptive grid partitioning is proposed to solve the equation. A partition of grids is used to determine equality constraints on grids for the same pixel value, and the unknowns are decreased. Projection equation of the gridding at high spatial resolution is decomposed into several equations with fewer unknowns. This is verified by using asymptotic point spread functions. The shape proximity of the point spread function to the Dirac function with the proposed method is 21.1% smaller than that of simultaneous algebraic reconstruction technique (SART). In addition, smaller image errors and higher structural similarity index values are achieved in both noise-free and noisy cases compared to SART. In temperature imaging on a high-temperature wind tunnel, a relative deviation of 2.55% from thermocouple values verified the accuracy of the proposed method.

Tunable diode laser absorption spectroscopy (TDLAS) tomography utilizes various laser paths from different views to image combustion flames. Usually, one data acquisition (DAQ) channel is required for the projected intensity along each laser path, and a DAQ hardware with complex channel alignment and huge storage capacity is used for the collection and cache of laser intensities from multiple projections to image 2-D dynamic distributions. A cost-effective tomography system is achieved with an eightfold increase in data storage capacity by on-chip data compression and a half decrease of channel number by channel multiplexing (CM) of time division, and it enables fast response and long-period flame monitoring without hardware modifications. The proposed system can achieve a monitoring time of 40 s at a framerate of 2 k FPS. The complete ignition and extinguishing of a Bunsen burner were captured, and the imaged flame frequency matches that of a photodetector of visible light. Also, nine working cases were monitored at the exit of a two-stage combustion chamber, and relative deviations of measured temperatures by the proposed method and thermocouple are less than 3%, which verifies the robustness of the proposed system in harsh environments.

Wavelength modulation spectroscopy (WMS) is recognized as a good noise immunity method in tunable diode laser absorption spectroscopy (TDLAS). It can obtain high signal-to-noise ratio harmonic intensity signals for precise gas parameter measurement, like temperature and concentration. However, the TDLAS-based velocity is sensitive to the Doppler shift in the laser frequency caused by the high-speed flow, which usually requires complex wavelength calibration and precise signal alignment. This work proposed a novel method for the precise velocity measurement directly by extracting the time interval of the even-symmetric 2f/1f harmonics signal with a self-convolution algorithm. The even-symmetric 2f/1f harmonics signal is obtained from the up-and-down laser optical frequency scanning WMS with lock-in amplification technology. A portable measurement system has been developed to achieve simultaneous velocity and temperature measurement of the gas in turbojet engine exhaust with different operation states. The maximum standard deviation of the velocity derived by the proposed method is 15.6 m/s, while that by traditional DAS and WMS are 69.2 m/s and 26.4 m/s, respectively. Experiment results have verified the improvement of velocity precision by the proposed method. It can provide reliable experimental data for improving the design of combustors and optimizing numerical combustion simulations.

In tunable diode laser absorption spectroscopy (TDLAS) tomography, high resolution images require sufficient projections of absorbed spectral data along line-of-sight laser paths. A reflective TDLAS tomography system was designed to improve the spatial resolved ability of the images, and more than one hundred projected laser paths are available in each projection coverage view. In the numerical simulations, the number of projection views and beams per view were evaluated in terms of the image errors of the flame temperature and H ₂ O concentration. The number of projection views and beams per view are optimized for the filtered back projection (FBP) algorithms, and the proposed reflective TDLAS tomography system yielded a spatial resolution of a grid sizing 1.625 mm. In actual experiments, reconstructed images of different flame distributions in a Mckenna burner achieved a high frame rate of 20 fps, with a small image error for the flame imaging of temperature and H ₂ O concentration.

Laser absorption spectroscopy (LAS) tomography is an effective method for cross-sectional imaging of temperature and gas concentration distributions in combustion diagnosis. In this article, the adaptive basis function fitting and artifact removal method is proposed to realize robust imaging in LAS tomography. Modified Mexican hat functions are introduced as basis functions to depict continuous distributions in the region of interest (ROI). Adaptive basis function fitting is realized by introducing a second reconstruction with core parameters of basis functions, the scale factors, and center points, determined adaptively from the first reconstruction. After obtaining the integral absorbance densities of two different spectral lines by adaptive basis function fitting, the similarity between their ratio and one of them is used to locate and remove the artifacts in reconstructed temperature images. The proposed method yields less artifacts and shows stronger noise immunity. The temperature image error can decrease by 12% at high noise levels. Dynamic flames of a McKenna burner were measured and temperature images with less artifacts were achieved. In an acoustically excited Bunsen burner, the proposed method precisely extracted the fundamental frequency and acoustical excitation frequency and yielded a higher structural similarity index. For a high-temperature wind tunnel, the maximum relative error of temperature in the center of the imaging region was 5.43% and the robustness of the proposed method in practical applications was verified.

Tunable diode laser absorption spectroscopy (TDLAS) is a popular technique for non-contact temperature measurement but is highly susceptible to beam-steering distortion due to the time-varying along-path inhomogeneity of refractive index in dynamic process monitoring. An adaptive beam-steering-distortion compensation method is proposed to eliminate the distortion in TDLAS measurement. A high-frequency modulated laser at a non-absorbing wavelength is coupled with a conventional low-frequency scanning laser at an absorbing wavelength and passes through the target field along an identical path. A synchronous demodulation method is proposed to demodulate the transmitted hybrid laser signal into the distorted absorption and distortion signals. The distortion signal is utilized to adaptively compensate the distortion parasitic on the distorted absorption signal. Experimental results obtained on a vibrated water thermostat show that the maximum deviation of temperature derived by the proposed method is 4.97 K, while that by the traditional DAS method is 34.64 K. Experimental results on a turbulent flame show that the relative standard deviation of temperature obtained by the proposed method has decreased from 36.7 % (DAS) to 4.20 %. Also, the temperature fitting success rate has increased from 88.0 % to 100 % in the turbulent flame case. The results show that the proposed method is more precise and robust than the traditional method. The proposed method will potentially play an important role in laser-based dynamic combustion monitoring and diagnosis.

This paper presents the design and implementation of an online speech driven talking head animation system. The system first recognizes phoneme sequence from the input speech with a Chinese Mandarin speech recognizer. The phoneme sequence is further transformed to a sequence of visemes. The sequence of MPEG-4 facial animation parameters (FAPs) is further derived from the viseme sequence, and is used to drive the facial animations on a 3-dimentional talking head. The architecture and the major features are also presented in the paper, together with the evaluations of the system.