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Parametric Resonance Analyses for Spar Platform in Irregular Waves

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China Ocean Engineering 2018年第2期32卷 236-244页

作者： YANG He-zhen xu pei-ji School of Engineering University of GlasgowGlasgow G128QQUnited Kingdom School of Naval Architecture Ocean and Civil EngineeringShanghai Jiao Tong UniversityShanghai 200240China

The parametric instability of a spar platform in irregular waves is analyzed. Parametric resonance is a phenomenon that may occur when a mechanical system parameter varies over time. When it occurs, a spar platform will have excessive pitch motion and may capsize. Therefore, avoiding parametric resonance is an important design requirement. The traditional methodology includes only a prediction of the Mathieu stability with harmonic excitation in regular waves. However, real sea conditions are irregular, and it has been observed that parametric resonance also occurs in non-harmonic excitations. Thus, it is imperative to predict the parametric resonance of a spar platform in irregular waves. A Hill equation is derived in this work, which can be used to analyze the parametric resonance under multi-frequency excitations. The derived Hill equation for predicting the instability of a spar can include non-harmonic excitation and random phases. The stability charts for multi-frequency excitation in irregular waves are given and compared with that for single frequency excitation in regular waves. Simulations of the pitch dynamic responses are carried out to check the stability. Three-dimensional stability charts with various damping coefficients for irregular waves are also investigated. The results show that the stability property in irregular waves has notable differences compared with that in case of regular waves. In addition, using the Hill equation to obtain the stability chart is an effective method to predict the parametric instability of spar platforms. Moreover, some suggestions for designing spar platforms to avoid parametric resonance are presented, such as increasing the damping coefficient, using an appropriate RAO and increasing the metacentric height.

关键词： spar platform instability parametric resonance Mathieu equation irregular waves dynamic responses

A novel shape memory alloy actuated soft gripper imitated hand behavior

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Frontiers of Mechanical Engineering 2022年第4期17卷 261-272页

作者： Jie PAN Jingjun YU xu pei School of Mechanical Engineering and Automation Beihang UniversityBeijing 100191China

The limited length shrinkage of shape memory alloy(SMA)wire seriously limits the motion range of SMA-based *** this paper,a new soft finger without silicone gel was designed based on pre bent SMA wire,and the finger was back to its original shape by heating SMA wire,rather than relying only on heat exchange with the *** imitating palm movement,a structure with adjustable spacing between fingers was made using SMA spring and rigid *** hook structure design at the fingertip can form self-locking to further improve the load capacity of *** the long thin rod model,the relationship of the initial pre bent angle on the bending angle and output force of the finger was *** stress-strain model of SMA spring was established for the selection of rigid *** grasping modes were proposed to adapt to the weight of the *** the test of the gripper,it was proved that the gripper had large bending amplitude,bending force,and response *** design provides a new idea for the lightweight design and convenient design of soft gripper based on SMA.

关键词： shape memory alloy(SMA) pre bent wire gripper grasping mode lightweight

Cusp points and assembly changing motions in the PRR-PR-PRR planar parallel manipulator

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Frontiers of Mechanical Engineering 2023年第2期18卷 177-187页

作者： Chengwei SHEN Jingjun YU xu pei School of Mechanical Engineering&Automation Beihang UniversityBeijing 100191China

Most parallel manipulators have multiple solutions to the direct kinematic *** ability to perform assembly changing motions has received the attention of a few *** points play an important role in the kinematic *** study investigates the cusp points and assembly changing motions in a two degrees of freedom planar parallel *** direct kinematic problem of the manipulator yields a quartic polynomial *** root in the equation determines the assembly configuration,and four solutions are obtained for a given set of actuated joint *** regarding the discriminant of the repeated roots of the quartic equation as an implicit function of two actuated joint coordinates,the direct kinematic singularity loci in the joint space are determined by the implicit *** points are then obtained by the intersection of a quadratic curve and a cubic *** assembly changing motions by encircling different cusp points are highlighted,for each pair of solutions with the same sign of the determinants of the direct Jacobian matrices.

关键词： planar parallel manipulator assembly changing motions cusp points quartic polynomial discriminant of repeated roots

The runoff characteristics under simulated rainfall on purple soil sloping cropland

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Chinese Journal Of Geochemistry 2011年第3期30卷 317-322页

作者： xu pei FU Bin Key Laboratory of Mountain Surface Process and Hazards Chinese Academy of Sciences Chengdu 610041 China Institute of Mountain Hazards and Environment Chinese Academy of Sciences Chengdu 610041 China

Rainfall runoff is a critical hydrological process related to soil erosion and agricultural non-point pollu-tion. In this study, 25 simulation experiments on rainfall were carried out in five runoff plots. Rape (Brassica campestris) was planted on the downslope of the plots. Experiments were conducted when the vegetation coverage reached 80%. Each plot was subjected to five rainfall events differing in intensity. The results showed: (1) the runoff coefficients of overland flow and subsurface flow were less than 0.6 and 0.005, respectively; (2) the discharge of overland flow was the quadratic function of time; (3) runoff coefficient was the function of slope gradient and rain-fall intensity. When the slope gradient increased from 8.7% to 46.6%, the runoff coefficient of overland flow first increased and then decreased. The runoff coefficient reached the maximum when the slope gradient was within the range of 17.6%-36.4%; and (4) the process of subsurface flow generation included the increasing phase and reces-sion phase. Discharge was a logarithm function of time in the increasing phase, and an exponential function in the recession phase. Runoff coefficient of subsurface flow decreased first and then increased when the slope gradient varied from 8.7% to 46.6% and was not correlated with rainfall intensity.

关键词：模拟降雨径流特性紫色土坡耕地农业非点源污染径流系数降雨强度径流小区

A Coordinate-Free Approach to the Design of Generalized Griffis-Duffy Platforms

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Chinese Journal of Mechanical Engineering 2024年第3期37卷 95-103页

作者： Chengwei Shen xu pei Lubin Hang Jingjun Yu College of Optical Mechanical and Electrical EngineeringZhejiang A&F UniversityHangzhou 311300China School of Mechanical Engineering and Automation Beihang UniversityBeijing 100191China School of Mechanical and Automotive Engineering Shanghai University of Engineering ScienceShanghai 201620China

Architectural singularity belongs to the Type II singularity,in which a parallel manipulator(PM)gains one or more degrees of freedom and becomes *** remaining permanently in a singularity are beneficial for linearto-rotary motion ***-Duffy(GD)platform is a mobile structure admitting a Bricard *** this paper,we present a coordinate-free approach to the design of generalized GD platforms,which consists in determining the shape and attachment of both the moving platform and the fixed *** generalized GD platform is treated as a combination of six coaxial single-loop mechanisms under the same *** to the inversion,hidden in the geometric structure of these single-loop mechanisms,the mapping from a line to a circle establishes the geometric transformation between the fixed base and the moving platform based on the center of inversion,and describes the shape and attachment of the generalized GD ***,the center of inversion not only identifies the location of rotation axis,but also affects the shape of the platform mechanism.A graphical construction of generalized GD platforms using inversion,proposed in the paper,provides geometrically feasible solutions of the manipulator design for the requirement of the location of rotation axis.

关键词： Generalized Griffis-Duffy platforms Self-motion Inversion Coordinate-free determination Manipulator design

;ast forward kinematics algorithm for real-time and ligh-precision control of the 3-RPS parallel mechanism

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Frontiers of Mechanical Engineering 2018年第3期13卷 368-375页

作者： Yue WANG Jingjun YU xu pei Robotics Institute Beihang University Beijing 100191 China Department of Mechanical Design Beihang University Beijing 100191 China

A new forward kinematics algorithm for the mechanism of 3-RPS （R： Revolute; P： Prismatic; S： Spherical） parallel manipulators is proposed in this study. This algorithm is primarily based on the special geometric conditions of the 3-RPS parallel mechanism, and it eliminates the errors produced by parasitic motions to improve and ensure accuracy. Specifically, the errors can be less than 10 6. In this method, only the group of solutions that is consistent with the actual situation of the platform is obtained rapidly. This algorithm substantially improves calculation efficiency because the selected initial values are reasonable, and all the formulas in the calculation are analytical. This novel forward kinematics algorithm is well suited for real-time and high-precision control of the 3-RPS parallel mechanism.

关键词： 3-RPS parallel mechanism forward kinematics numerical algorithm parasitic motion

Design of Compliant Straight-line Mechanisms Using Flexural Joints

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Chinese Journal of Mechanical Engineering 2014年第1期27卷 146-153页

作者： pei xu YU Jingjun ZONG Guanghua BI Shusheng School of Mechanical Engineering and Automation Beihang University

Straight-line compliant mechanisms are important building blocks to design a linear-motion stage, which is very useful in precision applications. However, only a few configurations of straight-line compliant mechanisms are applicable. To construct more kinds of them, an approach to design large-displacement straight-line flexural mechanisms with rotational flexural joints is proposed, which is based on a viewpoint that the straight-line motion is regarded as a compromise of rigid and compliant parasitic motion of a rotational flexural joint. An analytical design method based on the Taylor series expansion is proposed to quickly obtain an approximate solution. To illustrate and verify the proposed method, two kinds of flexural joints, cross-axis hinge and leaf-type isosceles-trapezoidal flexural（LITF） pivot are used to reconstruct straight-line flexural mechanisms. Their performances are obtained by analytic and FEA method respectively. The comparisons of the results show the accuracy of the approach. Both examples show that the proposed approach can convert a large-deflection flexural joint into approximate straight-line mechanism with a high linearity that is higher than 5 000 within 5 man displacement. This can lead to a new way to design, analyze or optimize straight-line flexure mechanisms.

关键词： flexure mechanism straight-line mechanism flexural joint center-shift

A novel obstacle avoidance heuristic algorithm of continuum robot based on FABRIK

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Science China(Technological Sciences) 2022年第12期65卷 2952-2966页

作者： WU HaoRan YU JingJun PAN Jie pei xu Robotics Institute Beihang UniversityBeijing 100191China Department of Mechanical Design Beihang UniversityBeijing 100191China

Obstacle avoidance and path planning of continuum robots are challenging tasks due to the hyper-redundant degree of freedoms(DOFs)and restricted working ***,most current heuristic algorithm-based obstacle avoidance algorithms exist with low computational efficiency,complex solution process,and inability to add global *** paper proposes a novel obstacle avoidance heuristic algorithm based on the forward and backward reaching inverse kinematics(FABRIK)*** update of key nodes in this algorithm is modeled as the movement of charges in an electric field,avoiding complex nonlinear *** algorithm achieves the robustness of inverse kinematics and path tracking in complex environments by imposing constraints on key nodes and determining the location of obstacles in *** algorithm is characterized by a high convergence rate,low computational cost,and can be used for real-time *** proposed approach also has wide applicability and can be applied to both mobile and fixed-base continuum *** it can be further extended to the field of hyper-redundant *** algorithm's effectiveness is further validated by simulating the path tracking and obstacle avoidance of a five-segment continuum robot in various environments and comparisons with classical methods.

关键词： continuum robot FABRIK algorithm obstacle avoidance motion planning

Isolation and identification of fungi found in contaminated fermented milk and antifungal activity of vanillin

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Food Science and Human Wellness 2021年第2期10卷 214-220页

作者： xu pei Mekonen Tekliye Mingsheng Dong College of Food Science and Technology Nanjing Agricultural University1 Weigang RoadNanjingJiangsu 210095China

Fungi contamination is an important factor affecting the quality and commercial value of fermented *** is known as a safe and efficient natural antimicrobial additive with great potential value as a food *** this study,the microbiological quality of fermented milk of different brands retailed in China and the antimicrobial activity of vanillin were investigated.A total of 27 fermented milk samples were purchased and *** isolates obtained were characterized by DNA sequencing of the internal transcribed spacer(ITS)*** effects of vanillin on the growth of fungi were studied by plate dilution method,radical colony diameter,and mycelium *** result showed that 14.8%of the samples were contaminated with three different species of fungi with Cladosporium *** the most predominant fungal *** study further showed that vanillin had significant inhibitory activity against the growth of the fungi *** cell number,radical colony diameter,and the mycelium biomass of the fungal contaminant were significantly reduced by the inhibitory action of the *** study thus provides reliable data evidence that will help relevant stakeholders in the fermented milk industries to improve their quality control strategies and consider vanillin as a bio-preservative with the potential to increase the shelf-life and safety of fermented milk.

关键词： Fermented milk FungiInternal transcribed spacer Vanillin Antimicrobial activity