15 November 2020, Volume 60 Issue 11

    

• Select all
  |

AUTOMOTIVE ENGINEERING
• Select
  Dynamic braking torque characteristics of a band brake

  LI Fei, SONG Jian, FANG Shengnan, LU Zhenghong, NGUYEN Truong Sinh

  Journal of Tsinghua University(Science and Technology). 2020, 60(11): 887-894. https://doi.org/10.16511/j.cnki.qhdxxb.2020.21.005

  Abstract ( ) Download PDF ( )   Knowledge map   Save

  Band brakes are used as shift actuators in uninterrupted powertrain automatic mechanical transmissions (AMT). The dynamic characteristics of the brake band then strongly affect the shift quality. The dynamic torque characteristics of a band brake were studied using a finite element model of the band brake with experimental tests for various working conditions. The results show that the dynamic braking torque is not only related to the brake drum speed, braking force, and braking force radius, but also depends on the braking force loading speed. When the other conditions are kept the same, a faster braking force loading leads to a smaller braking torque on the band. The effect was described using a qualitative analysis of the hydrodynamic lift force. The results were used to define an equivalent dynamic friction coefficient, an equivalent torque input coefficient and a compensation coefficient with an empirical equation for the dynamic braking torque that integrates the braking force loading speed into the dynamic braking torque.
  
• Select
  Aging experimental investigation and mechanism analysis for zinc-air fuel cells

  CHEN Dongfang, PEI Pucheng, SONG Xin

  Journal of Tsinghua University(Science and Technology). 2020, 60(11): 895-901. https://doi.org/10.16511/j.cnki.qhdxxb.2020.25.022

  Abstract ( ) Download PDF ( )   Knowledge map   Save

  The commercial development of zinc-air fuel cells is limited by their lack of durability. A zinc-air fuel cell durability test evaluation method was developed based on the working conditions for short term stable operation, long term continuous stable operation, shutdown storage and periodic start-stop cycles. The aging characteristics of zinc-air fuel cells were then experimentally investigated for various operating conditions to study the aging mechanism. Finally, several methods were developed to slow the aging through electrolyte management, methods to extend the air cathode lifetime, and zinc-air fuel cell management. The durability test evaluation method presented in this paper can be used to study the aging characteristics and durability of zinc-air fuel cells.
• Select
  Numerical investigation of vehicle occupant injury risks in underbody blast events

  WANG Bo, HE Yangyang, NIE Bingbing, XU Shucai, ZHANG Jinhuan

  Journal of Tsinghua University(Science and Technology). 2020, 60(11): 902-909. https://doi.org/10.16511/j.cnki.qhdxxb.2020.26.021

  Abstract ( ) Download PDF ( )   Knowledge map   Save

  Landmines and improvised explosive devices have caused many casualties in recent conflicts. A model including the occupant, the vehicle and the explosion flow field was developed to analyze occupant injuries resulting from underbody blast. The fluid-structure coupling algorithm of LS-DYNA was used to simulate the blast effect on the vehicle hull, the transmission path of the blast load and the occupant kinematic response. Head, spine, pelvis and lower extremity injury risks were investigated for 6 kg and 8 kg TNT explosion cases using a finite element human body model. The simulations show that for belted occupants, the ankle-foot complex, tibia and fibula have the highest risk of injury, followed by the pelvis and thoracolumbar spine. These results can be used to design vehicle protection systems to improve occupant safety.
• Select
  Modeling and control of gear shifting of a non-synchronizer motor-transmission drive system

  ZENG Yuanfan, CHEN Hongxu, WANG Lijun, TIAN Guangyu, ZHOU Weibo

  Journal of Tsinghua University(Science and Technology). 2020, 60(11): 910-919. https://doi.org/10.16511/j.cnki.qhdxxb.2020.26.018

  Abstract ( ) Download PDF ( )   Knowledge map   Save

  Power interruptions and impacts during shifting are the main reasons restricting large-scale applications of multi-gear transmissions in electric vehicles. This paper presents a non-synchronizer motor-transmission drive system. A hybrid automaton model was used to describe the shifting characteristics using multi-body dynamics. The dynamic characteristics of the model at each stage were then used to design a shifting control strategy with active synchronization of the angular speed and the rotation angle, such that the angular speeds and the angles of the sleeve and the gear ring are precisely synchronized while shifting. Simulations and tests show that the hybrid automaton shifting model predictions are consistent with the actual process and that the shift control strategy enables the non-synchronizer motor-transmission drive system to provide fast, non-impact shifting and shortens the power interruption time to less than 350 ms.
MECHANICAL ENGINEERING
• Select
  Individualized channel-selection strategy-based brain-machine interaction rehabilitation training: A pilot study of clinical experiments

  JIA Tianyu, QIAN Chao, LI Chong, JI Linhong, LIU Aixian, FANG Boyan

  Journal of Tsinghua University(Science and Technology). 2020, 60(11): 920-926. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.017

  Abstract ( ) Download PDF ( )   Knowledge map   Save

  Brain-machine interactions (BMI) can recognize motor intentions and transfer the intentions to control a rehabilitation robot assisting paralyzed limb movement, which can effectively stimulate the neural pathways for motor control and sensory feedback. Lesion types, lesion degrees and lesion sites vary from patient to patient. Motor intention expression may then shift to different brain regions depending on the lesion characteristics. This study presents an individualized channel-selection strategy-based BMI rehabilitation training method which uses the individualized motor expression encephalic area as the electroencephalogram (EEG) collection site. Three stroke patients were recruited for this study and were trained for 10 days using individualized channel-selection strategy-based BMI rehabilitation training. All the patients were assessed using a clinical evaluation scale and EEG tests before and after the 10 day training. The results show that after the training the subjects have improved Fugl-Meyer assessments and one of the subjects regains motor evoked potential which indicates the activation of the neural pathway. This individualized channel-selection strategy-based BMI rehabilitation training method should be further validated with a large number of stroke patients in the future.
• Select
  Energy management of servo press lines based on flywheel speed adaptive planning

  PENG Fazhong, ZHANG Peng, WANG Liping, SHAO Zhufeng, YANG Di, YANG Kuai

  Journal of Tsinghua University(Science and Technology). 2020, 60(11): 927-933. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.014

  Abstract ( ) Download PDF ( )   Knowledge map   Save

  The main drive motors in large servo press lines often draw megawatts of power with frequent acceleration and deceleration of the slider, which strongly affects the power grid and power usage. Thus, effective energy management is indispensable to ensure smooth, efficient operation of large servo press lines. This paper studies how to improve the energy management of large servo press lines using flywheel energy storage. A gradient project algorithm and B-spline speed planning are used to develop a half-period flywheel speed adaptive planning algorithm. This algorithm better optimizes the power usage by introducing an approximately sinusoidal disturbance which avoids sudden changes in the flywheel speed. Finally, the servo press line model is analyzed theoretically in Simulink. The simulation results show that the half-period adaptive flywheel speed planning algorithm significantly improves the incoming line power and bus voltage fluctuations as an effective method for energy management of large servo press lines.
• Select
  Distribution of the center of mass work and metabolism during stair climbing

  LAN Jian, ZHANG Jiwen, FU Chenglong, CHEN Ken

  Journal of Tsinghua University(Science and Technology). 2020, 60(11): 934-942. https://doi.org/10.16511/j.cnki.qhdxxb.2020.26.017

  Abstract ( ) Download PDF ( )   Knowledge map   Save

  The distribution of the center of mass (CoM) work and the metabolism are of great significance for studies of the mechanism of climbing stairs. This paper presents a stair climbing model, defines key parameters reflecting the distribution of the CoM work and metabolism and presents the mathematical relationships between these parameters and the vertical ground reaction force (VGRF). Tests and analyses show that the average power in a single leg is less than the average CoM work rate in one step during the double support and single support phases. During the double support phase, the work rates by the leading and trailing legs are nearly balanced with slightly more work done by the leading leg. The CoM work distribution has little effect on the VGRF curve shape but protects the knee joint during the single support phase. Analysis of the metabolism during the CoM work shows that walking upstairs naturally saves energy, but is not the most energy efficient method. The slightly concave shape of the VGRF curve reduces the probability of early ankle joint plantarflexion and excessive plantarflexion.
HYDRAULIC ENGINEERING
• Select
  Human resource management policies in project-based organizationswith project work churn

  AN Nan, QIANG Maoshan, ZHENG Junping, WEN Qi

  Journal of Tsinghua University(Science and Technology). 2020, 60(11): 943-950. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.012

  Abstract ( ) Download PDF ( )   Knowledge map   Save

  Human resource management policies in project-based organizations should consider the churn caused by frequent project initiation and termination. A questionnaire survey of 155 Chinese project-based organizations shows the impacts of project work churn on human resource policies. Well established training, motivation and advancement opportunities provide coping mechanisms that help employees deal with project work churn. However, project work churn can negatively affect the implementation of human resource policies, especially for project-based organizations.
• Select
  Analysis of features and factors controlling typical drainage networks in the Tibetan Plateau

  LI Minhui, CHEN Yi, WU Baosheng

  Journal of Tsinghua University(Science and Technology). 2020, 60(11): 951-957. https://doi.org/10.16511/j.cnki.qhdxxb.2020.25.030

  Abstract ( ) Download PDF ( )   Knowledge map   Save

  The active tectonic movements and the unique climate on the Tibetan Plateau cause significant spatial differences in the drainage network on the Plateau. The drainage network and the longitudinal profiles of the main streams of three river basins were extracted from 90 m resolution SRTM DEM using DEMRiver to study the factors controlling the drainage network in the Tibetan Plateau. The Yalong River had a bifurcation ratio of 4.46 and a length ratio of 2.35, the Tao River had a bifurcation ratio of 5.00 and a length ratio of 2.71 and the Lasa River had a bifurcation ratio of 4.37 and a length ratio of 2.30. The normalized concavity index for the Tao River was -0.129, that for the Lasa River was -0.082 and that for the Yalong River was 0.009, indicating that the profiles of the first two rivers are concave-up while that of the Yalong River is convex-up. The Horton ratio obtained using the original definition and the fitting method and the climatic conditions and tectonic activities in the basin show that the climate effect is well reflected in the structure of the low-level river network. Strong tectonic movement destroyed the network of the Yalong River Basin with the network now maintaining balance through river capture, indicating that the tectonic movement controls the structure of the high-level river network.
  
AEROSPACE AND ENGINEERING MECHANICS
• Select
  Numerical study on influence of supply inlet air parameter distribution on a membrane-type total heat exchanger

  SHEN Zhijie, MIN Jingchun, DUAN Jiangfei

  Journal of Tsinghua University(Science and Technology). 2020, 60(11): 958-966. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.010

  Abstract ( ) Download PDF ( )   Knowledge map   Save

  Membrane-type total heat exchangers (THX) are air-to-air heat exchangers with a core made of a water vapor permeable membrane that transfers both heat and moisture. These heat exchangers are often used to reduce building energy consumption and improve indoor air quality. However, non-uniform internal channel structures and external disturbances can lead to maldistributions of the supply inlet air parameters into the heat exchanger. This research numerically examines the effects of various supply inlet air parameter variations on the heat and moisture transfer characteristics, including linear and parabolic distributions of the inlet air temperature, moisture ratio and velocity. The model then simulates the heat and mass transfer in the heat exchanger to predict the temperature, moisture and enthalpy effectiveness distributions as well as the temperature difference and humidity ratio difference contours between the two sides of the membrane for the various inlet conditions. The results show that the linear distribution significantly affects the effectiveness while the parabolic distribution has little effect, with both affecting the temperature difference and humidity ratio difference contours between the two sides of the membrane. The heat transfer and the mass transfer affect each other, with the heat transfer having only a weak effect on the mass transfer. More nonuniform supply inlet air temperature and moisture ratio distributions increase the probability that the heat and mass transfer will be in opposite directions, which reduces both the heat and mass transfer.