15 September 2019, Volume 59 Issue 9

    

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AEROSPACE AND ENGINEERING MECHANICS
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  Review of rover dynamics modeling methods

  LENG Shu, JU Hehua

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 689-698. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.010

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  Modeling of rover dynamics in real-time is key to controlling a rover for planetary exploration. The rover is typically modeled as a rigid body system because of its slow movement and the bogie-rocker structure. This paper introduces various rigid body dynamics analysis approaches and the advantages and disadvantages of these algorithms. Terramechanics is used to describe of the external forces on the rover system. The steel wheel and soft soil model is shown to be the most suitable model for describing the dynamics for real-time modeling of the rover dynamics.
  
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  Fast simulation algorithm for area target visibility using remote sensing satellites

  E Zhibo, LI Junfeng

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 699-704. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.020

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  Fast calculations of area target visibility are important for satellite mission scheduling. This paper presents a fast simulation algorithm to compute the visibility of area targets by remote sensing satellites. A method was developed to judge the visibility of a ground point from a satellite based on the satellite's view field and the geometric relation between the satellite and the ground point. Then, the in-view period of a single point was obtained using large time step discretization and a binary search. The in-view period of all the discrete boundary points was then used to determine the visible time window for an area target. Numerical results show that the algorithm is efficient and accurate with the error between the algorithm and satellite tool kit (STK) predictions of aboutt 0.01 s.
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  Analytical correction for a linear model of inter-satellite collision probabilities

  JIANG Wei, JIANG Fanghua, LI Junfeng

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 705-711. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.026

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  An analytical method was developed to correct linear model results for the inter-satellite collision probability. The traditional linear collision probability model is simple, but has many limitations, and is not applicable to calculations of inter-satellite collision probabilities with small orbital differences. An analytical expression is given to relate the differences between the linear model and Monte Carlo simulation results according to the maximum and minimum relative distances between the satellites. This analytical expression can be used to improve the linear model results to be closer to the true collision probability. Simulations show that the method extends the application range of the linear model and further improves the accuracy.
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  Multi-constraint compound reentry guidance based on onboard model identification

  CHENG Lin, ZHANG Qingzhen, JIANG Fanghua

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 712-719. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.018

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  A period-crossing feasible trajectory planning algorithm for reentry guidance was developed based on control variable parameterization, integral transformations, and onboard model identification. A compound height velocity (HV) corridor simplifies the reentry guidance problem into a root-searching problem. A Gauss integral is introduced to improve the time efficiency of the range prediction with the original integral problem converted into a function calculation problem. The recursive least squares estimation method was used to develop functions for on-board information mining and model identification. The reliable, explicit solution model can easily correct the weight coefficients using the period-crossing Newton-Raphson method. Numerical simulations show that the reentry guidance method based on on-board model identification is much faster, more autonomous and more adaptable than the reference trajectory tracking design method.
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  Assessing spacecraft agility

  YIN Mingwei, WANG Xianyu, LI Jingyang, BAOYIN Hexi

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 720-728. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.012

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  When increasing the spacecraft agility by optimal control, risk and benefits coexist. A method was developed to assess spacecraft agility and estimate the agility enhancement. The model analyzes the conventional eigenaxis rotation and the time-optimal rotation. Then, the reorientation time was used to define an angular acceleration envelope and an equivalent agility envelope with an agility factor and an agility curve introduced to quantitatively assess the spacecraft agility. Numerical simulations validate the reliabilities of the estimates. The results are consistent with previous research and the average reorientation time is well predicted. Thus, this method is suitable for assessing spacecraft agility.
  
ELECTRONIC ENGINEERING
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  Time domain fractional diversity method in single-carrier ultra-wideband systems

  LI Bohua, PEI Yukui

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 729-736. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.028

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  Long delays and dense paths seriously affect the performance of single-carrier ultra-wideband (SC-UWB) systems. Traditional channel equalization methods are not sufficient for these systems and the traditional diversity methods are very complex with excessive data redundancy. This paper presents a joint design method for single-carrier frequency domain equalization (SC-FDE) with time domain fractional diversity to improve the complex multipath characteristics of ultra-wideband (UWB) systems. At the transmitting end, this method uses one diversity branch for two or more information branches. At the receiving end, the system first performs the SC-FDE and then combines operations based on the posterior probabilities combination or the low complexity equal gain combination of the equalization results obtained by the information branches and the diversity branch. The system then finally recovers the original information. This time domain fractional diversity method has outstanding performance with a single antenna and low redundancy. The system needs only 1.5 or less applications of the diversity mode and obtains excellent diversity gains. Simulations show that this scheme has a 3 dB diversity gain over the SC-FDE scheme for a bit error rate of 10^-3.
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  Spatially coupled LDPC-BCH codes in quantum secure direct communications

  WANG Ping, SUN Zhen, YIN Liuguo, LU Jianhua

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 737-743. https://doi.org/10.16511/j.cnki.qhdxxb.2019.21.025

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  Quantum secure direct communication systems are limited by high erasure probabilities and low receiving probabilities due to the high light source losses in the optical elements and optical fibers. A channel mitigation method based on spatially coupled LDPC-BCH codes is presented here to ensure reliable information transformations in quantum secure direct communication systems to resolve the problems of low-rate and Shannon limit-approaching code designs. Spatially coupled LDPC-BCH codes are constructed by coupling multiple identical block LDPC-BCH codes with edge spreading. The extrinsic information transfer functions of the spatially coupled LDPC-BCH code are analyzed to derive the decoding thresholds for the code ensembles. Simulations show that the spatially coupled LDPC-BCH codes have decoding thresholds closer to the Shannon limit with lower bit error rates than block LDPC-BCH codes. The spatially coupled LDPC-BCH code channel mitigation scheme has a transmitting efficiency of 0.001 and a bit error rate less than 10^-6 when the channel receiving probability is set at 0.4%.
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  Miniaturized quantum key distribution transmitterbased on silicon PN junction emissions

  HUANG Weishao, ZHANG Wei, HUANG Yidong

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 744-749. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.017

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  This paper describes a miniaturized quantum key distribution (QKD) transmitter for the BB84 protocol for short-range QKD applications between portable devices and hosts. The transmitter uses silicon PN junctions to generate photons and wire-grid polarizers for the polarization encoding. It also uses a convex lens and an aperture for collimation as well as to eliminate the photon spatial information. Simulations indicate that the transmitter can support a raw key generation rate of 3 kb/s and the polarization extinction ratio of the generated photons in each polarization state can reach 20 dB. The transmitter height can be as small as 5 mm, so the device can be integrated into portable devices such as mobile phones.
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  Dependency tree based Tibetan semantic dependency analysis

  XIA Wuji, HUAQUE Cairang

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 750-756. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.003

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  Tibetan semantic dependence analysis is a deep semantic study based on Tibetan-dependent syntactic analysis. This paper starts from the shallow research of lexical analysis and syntactic analysis, and combines the characteristics of Tibetan grammatical structure and semantic unit to realize the semantic dependence analysis of Tibetan for the first time. Under the premise of formulating the Tibetan semantic dependency labeling specification and designing the Tibetan semantic dependency feature template, the perceptual machine is used to train the Tibetan semantic dependence analysis model. The experimental results show that the root accuracy, dependency arc accuracy, dependent arc type accuracy and complete accuracy of the model on manual labeling test corpus reached 89.56%, 78.63%, 71.67% and 32.32%, respectively, which confirmed that the model has good performance in Tibetan semantic dependence analysis tasks.
MECHANICAL ENGINEERING
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  Simulation of the effects of shoulder-pole parameters on the waist torque during biped load walking

  XU Yuning, HUANG Jing, CHEN Ken, FU Chenglong

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 757-764. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.014

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  Shoulder poles are commonly used to carry bulky loads with the weight distributed on the front and rear ends of the pole. The load transmission path reduces the torque on the waist needed to maintain the torso vertical. Traditional load carrying models have focused on the vertical direction while ignoring the influence of the waist torque to maintain the vertical alignment of the torso during walking. This paper presents a shoulder pole model for the torso. Simulations show that the waist torque is influenced by the walking speed, pole length, and suspension rope length. The simulations show that the effects of the torque on the waist should be considered while walking and longer shoulder poles, less load swing and proper position of the centroid of loads can reduce the waist torque. In practice, the shoulder pole length and suspension rope length should be chosen and the load swing should be controlled by hands to reduce the maximum torque on the waist and the energy consumption of the torque on the waist during walking.
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  Effect of microstructure refinement on the corrosion of AZ91D magnesium alloys

  LIU Qu, CHEN Gaoqiang, ZENG Shenbo, ZHANG Shuai, PAN Jiluan, SHI Qingyu

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 765-771. https://doi.org/10.16511/j.cnki.qhdxxb.2019.25.003

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  The poor corrosion resistance of magnesium alloys in aqueous solution containing electrolyte significantly restricts their engineering application. In this paper, with the aim of developing a new processing technique to efficiently improve the corrosion resistance of magnesium alloy, friction stir processing (FSP) was applied to modify the corrosion resistance of cast AZ91 magnesium alloy. And the effect of microstructural refinement on the corrosion resistance was investigated. The electrochemical measurements reveal that the polarization impendence R_p of processed alloy in NaCl solution with a mass fraction of 3.5% is nearly doubled. The results of microstructure analysis show that it is mainly attributed to the alteration of corrosion process induced by modification on the morphology and distribution of β-Mg₁₇Al₁₂ phase in magnesium alloys. On the one hand, the refinement of cathodic β-Mg₁₇Al₁₂ phase increases the area ratio of anode and cathode, which mitigates the severe galvanic corrosion in magnesium alloys. On the other hand, a large amount of fine β-Mg₁₇Al₁₂ particles could redeposit on the surface of magnesium alloy to form protective layer during corrosion and significantly inhibit the propagation of corrosion into α-Mg matrix.
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  Nonlinear dynamic characteristics of bolted joints for various pretension forces

  ZHAO Tong, GUO Junjie, LÜ Yuhong

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 772-779. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.014

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  The influence of the bolt pretension force on the dynamic characteristics of bolted joints was studied using a double bolt joint with an equivalent single DOF model based on the lumped parameter method near the first mode. Then, the equivalent dynamic parameters were obtained for various bolt preloadings using modal and excitation tests. The equivalent nonlinear dynamic parameters for the joint were then identified using the force-state mapping method for various pretension forces. The results show that the joint nonlinearity increases with decreasing pretension force. Therefore, the nonlinear joint dynamics must be considered for low pretension forces.
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  Rolling-type micro-vibration isolation mechanisms based on elastic contact

  GUAN Gaofeng, XU Dengfeng, ZHU Yu, YU Qiang, LI Qiang

  Journal of Tsinghua University(Science and Technology). 2019, 59(9): 780-784. https://doi.org/10.16511/j.cnki.qhdxxb.2018.25.063

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  Modeling of rolling-type mechanisms for micro-vibration isolation is usually based on the assumption of rigid contact and pure rolling motion that leads to large errors in the measured and theoretical natural frequencies. This study presents a model using elastic contact with elastic deformation and micro-sliding between the contact surfaces that leads to a differential equation for the system motion based on Lagrange's equation to predict the natural frequency. Previous models have only contained structural parameters while this one also includes related material parameters. The analysis predicts natural frequencies that are larger than those in previous models because the elastic potential energy stored in the contact interfaces increases the total potential energy. The theoretical results agree much better with measurements with errors of less than 25% compared to previous models that had errors of 70%.