15 June 2018, Volume 58 Issue 6

    

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MECHANICAL ENGINEERING
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  Multi-objective optimization of loaded spiral bevel and hypoid gears

  WANG Qi, ZHOU Chi, GUI Liangjin, FAN Zijie

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 529-538. https://doi.org/10.16511/j.cnki.qhdxxb.2018.22.023

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  The loaded stresses in the tooth flank of spiral bevel and hypoid gears were optimized without a trial-and-error procedure in a multi-objective optimization model. The multi-objective optimization model was developed considering the maximum contact pressure, load transmission error, load contact pattern and the maximum bending stress. A semi-analytical loaded tooth contact analysis model for the root bending stress was developed to calculate the objectives and constraints. Kriging surrogate models of the objectives and constraints together with a multi-objective genetic algorithm were used to solve the optimization model. The model was then used to optimize a spiral bevel gear pair for a drive axle. The loaded contact pattern was completely in the ideal zone with the maximum contact pressure decreased by 11.7%, the loaded transmission error decreased by 27.9% and the maximum bending stresses in the wheel and pinion decreased by 2.0% and 12.6%. The optimal design was installed in a drive axle for loaded contact tests that showed that the actual loaded contact pattern coincided well with the predicted loaded contact pattern, which verified the accuracy of the optimization approach.
  
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  Prediction of oil temperature variations in a wind turbine gearbox based on PCA and an SPC-dynamic neural network hybrid

  HUANG Zhongshan, TIAN Ling, XIANG Dong, WEI Yaozhong

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 539-546. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.026

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  The oil temperatures in wind turbine gearboxes are difficult to predict due to the strong nonlinearities. Multivariable correlations have been developed, but they are difficult to use due to the data redundancy between the correlation variables and the shortage of off-line training data for the artificial neural networks This paper presents a gearbox oil temperature prediction model based on a principal component analysis (PCA) and a dynamic neural network. The model uses online learning based on statistical process control (SPC). The PCA method deals with the data redundancy problem for the variables affecting the oil temperature. The nonlinear autoregressive with external input (NARX) dynamic neural network is then used to model the oil temperature. The SPC method analyzes the residual distribution to control the online learning behavior. Tests show that the method is stable and can accurately predict the oil temperature variations.
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  Brake pad image classification algorithm basedon color segmentation and information entropy weighted feature matching

  ZHAO Lei, ZHANG Wen, SUN Zhenguo, CHEN Qiang

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 547-552. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.025

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  In appearance inspection of car brake pads based on machine vision, the segmentation of the brake pad image and the extraction and matching of the shape features are keys of brake pads classification. In order to realize high-precision shape classification, this paper proposes a brake pad image classification algorithm including foreground segmentation, geometric feature selection and template matching. First the RGB image captured by the camera is converted to HSV color space and using saturation channel the brake pad is segmented from the dark belt background. Then the multi-dimensional geometric features of the area are extracted. Finally, an improved feature matching algorithm based on information entropy weighting is proposed, in which Manhattan distance of the feature space is weighted by the information entropy of the brake pad features. The experimental results show that the accuracy of the algorithm is 95.00%, and the average processing time is 110 ms. It can be applied to the real-time automatic classification procedure during brake manufacturing.
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  Ultra-short baseline positioning basing on error equalization in a five-element stereo array

  WANG Hongru, WANG Zhongqiu, QI Suiping, HU Tong, ZOU Jing, CAO Zili

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 553-557. https://doi.org/10.16511/j.cnki.qhdxxb.2018.21.009

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  Ultra-short baseline positioning using a single-plane base array will lead to increased positioning errors when the acoustic source is close to the plane of the base array. This paper presents a method to reduce the error using different array planes which can be localized in one stereo five-element array. The stereo array can be divided into several sub-arrays having three basic elements in different orthogonal directions. The positioning data in the local coordinate system of the sub-array is first translated into the global coordinate system of the entire stereo array by a transformation matrix with a weighted average then calculated with the weighting based on the positioning error which is related to the change in the elevation angle in the local coordinate system of the sub-array. Simulations in MATLAB showed that this method can realize positioning localization of a spatially azimuthal target sound source. The positioning accuracy is more than 4 times higher than for a single array because the relative positioning error from all directions of the stereo array is smoothed due to the different elevation angles. Thus, this method significantly reduces the positioning error when the pitch angle of the sound source is close to the single base array plane.
NUCLEAR ENERGY AND NEW ENERGY
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  Scintillation neutron detectors based on a wavelength-shifting optical fiber using ²³²Th+ZnS(Ag)

  BAI Zhaole, ZHOU Qi, YANG Nan, LIU Feng, YANG Zhongjian, CHEN Baowei, WANG Jianlong

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 558-562. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.028

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  Optical fiber scintillator neutron detectors have developed rapidly in recent years because these detectors are small (so that they can be extended into narrow spaces) and give real-time measurements. However, the probe detection efficiency is low due to the small size of the probe's effective detection material. This study presents a theoretical and experimental study of a ²³²Th+ ZnS(Ag) neutron detector with a wavelength-shifting fiber was used to improve the detection efficiency. This detector was used to measure the neutron distribution of the ADS sub-critical experimental assembly with the measurement results being consistent with the theoretical model which indicates that the high detection efficiency is very good and the wavelength-shifting fiber neutron detector is accurate.
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  An optimization method of brachistochrone problem with viscous friction and its application in ADS design

  LI Shengqiang, TAN Ming, ZHANG Zhanbo

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 563-569. https://doi.org/10.16511/j.cnki.qhdxxb.2018.22.029

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  The brachistochrone problem with viscous friction was studied to optimize the beam window (BW) shape of an accelerator driven sub-critical system (ADS) target. A set of nonlinear integral equations was derived using the variational method. The Newton iteration method could not get a convergent solution or an approximate solution for the highly viscous conditions, so an optimization method was developed. The particle swarm optimization (PSO) algorithm was found to be more efficient for the highly viscous conditions than other three heuristic algorithms with approximately linear convergence. The flow separation is later and the stagnation region is smaller for the brachistochrone BW instead of the semielliptical BW, which enhances the heat transfer.
PHYSICS AND ENGINEERING PHYSICS
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  Comprehensive disaster-bearing capacity assessment of the urban area using grid cells based on an evidential reasoning method

  CHEN Tao, CHEN Zhichao

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 570-575. https://doi.org/10.16511/j.cnki.qhdxxb.2018.22.024

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  Most studies on regional comprehensive disaster-bearing capacity have been based on large city scenarios with few for urban scenarios with most based on large administrative units, which leads to rough assessment results. This paper describes an index system for urban disaster-bearing capacity assessments based on the actual characteristics of an urban area in eastern China. Evidential reasoning is combined with an analytic hierarchy process to deal with qualitative and quantitative data with 1 km×1 km scale grids to assess the disaster-bearing capacity of this urban area. This paper describes the method to accurately assess the disaster-bearing capacity of the urban area with high resolution assessment results. The index of "anti-disaster ability→spatial factors→major hazard sources" plays a very important role in the whole assessment. These results are in accord with the actual situation in this area. Thus, this assessment method can be used for urban-level emergency management and to improve disaster prevention and mitigation capacities.
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  Online-offline associated surveillance system based on E-V fusion

  TANG Shiyang, SHU Xueming, HU Jun, WU Jinjin, SHEN Shifei

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 576-580. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.019

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  Recent mass disturbances and terrorist attacks have presented new challenges for public management. Thus, more research is need on how to cope with serous public security incident with limited police forces. An novel online-offline public security incident surveillance system is described here with an E-V information fusion system. The system collects surveillance video data and cellphone data to identify human locations and IDs. Tests show that the system can combine human locations in videos, cellphone MAC addresses and cellphone RSS data in situations with few people by combing online and offline information.
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  Video behavior recognition based on locally compressive sensing

  WANG Wenfeng, CHEN Xi, WANG Haiyang, PENG Wei, QIAN Jing, ZHENG Hongwei

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 581-586. https://doi.org/10.16511/j.cnki.qhdxxb.2018.25.024

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  Compressive sensing has been successfully applied in the field of target tracking but not for behavior recognition. This paper presents a locally compressive sensing algorithm for behavior analysis which combines compressive tracking and centroid localization for recognition of video object behavior. Local compressive sensing selects a behavior-sensitive area for compressive tracking which characterizes target behavior based on classification of the object trajectory and local centroid velocity. Tests show that locally compressive sensing can accurately recognize global behavior such as running and falling and local behavior such as smiles and blinking. Therefore, the locally compressive sensing method is of great value that can be used for video surveillance and behavior recognition.
CIVIL ENGINEERING
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  Space and MEP topology extraction and application based on BIM

  ZHANG Jianping, HE Tianfeng, LIN Jiarui, CHEN Xingyu, ZHANG Yongli

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 587-592. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.018

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  Topologies of space and mechanical, electrical and plumbing (MEP) are crucial to a building's design and maintenance. Traditional information storage is often fragmented and difficult to extract and utilize. Building information modeling (BIM) provides the uniform descriptions, storage, and management of various professional information to significantly improve the extraction and application efficiency of topology information. This paper introduces the industry foundation classes (IFC), proposes an integrated topology model of space and MEP, and studies the technology of extracting and integrating the topological information. Tests show that this model eliminates re-modeling, effectively utilize the rich BIM information, and enables integrated applications.
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  Properties of the reaction products of alkali-activated electric furnace nickel slag

  WANG Qiang, YANG Jun, WANG Dengquan

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 593-597,602. https://doi.org/10.16511/j.cnki.qhdxxb.2018.22.028

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  Electric furnace nickel slag was alkali-activated by sodium hydroxide and water glass to study the reaction product properties with measurements of reaction heat of the binder, the mortar compressive strength, and the morphology and structure of the reaction products. The results show that the mortar compressive strength is the greatest for a sodium hydroxide content of 5%. The optimum modulus of the alkali-activated electric furnace nickel slag is 0.5 for a water glass content of 10%. The high alkalinity improves the initial strength, while the silicate ions improve the later strength. The reaction products are amorphous regardless of the type of alkali used and are denser with sodium hydroxide. The reaction products have more polymerized silica-oxygen connections than the raw materials. No new aluminum-oxygen configurations are found.
ELECTRONIC ENGINEERING
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  Electronic invoice management and public service system

  CHAI Yueting, YU Xiao, HUANG Yadong

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 598-602. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.020

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  E-commerce needs secure electronic invoices to eliminate paper-based invoices and problems of not billing, virtual billing, false invoices and other issues. This paper presents an electronic invoice concept with electronic invoice management and a public service model for automatic generation, centralized management, online inspection and intelligent service of electronic invoices. This paper presents the overall structure of the electronic invoice management and public service system that includes a secure electronic invoice issuing system, a regional electronic invoice information management and service platform, an electronic invoice information exchange and a sharing platform. This paper also describes the system implementation and verification as a foundation for a tax management and service method to meet e-commerce development needs.
  
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  Beamforming and maximum likelihood estimation for speech enhancement using dual closely-spaced microphones

  GONG Qin, ZHENG Shuo

  Journal of Tsinghua University(Science and Technology). 2018, 58(6): 603-608. https://doi.org/10.16511/j.cnki.qhdxxb.2018.26.027

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  Traditional beamforming systems using dual closely-spaced microphones have various problems such as low-frequency-roll-off and limitations in suppressing competitive speech noises from multiple directions. This paper presents a two-step beamforming and maximum likelihood estimation algorithm. The algorithm first uses a WOLA filter for the time-frequency analysis for the speech mixture and then sets mask values to suppress background noise without low-frequency-roll-off based on the ratio of the two beamforming patterns, which have zeros at 0ånd 180°. A statistical model and the maximum likelihood estimation are then used to further enhance the speech. Tests indicate that the algorithm effectively recovers the energy distribution of the target signal and improves the signal-to-noise ratio without a low-pass filter or broadband compensation when the signal-to-ratio is low or multiple kinds of noises exist.