基于伪频响函数矩阵法的运行模态分析方法

基于传递率测量的伪频响函数法运行模态分析技术无需白噪声激励假设,具有强\"谐波模态抑制\"能力,但其模态识别结果依赖待分析伪频响函数的选取、稳定性欠佳.为解决该问题,建立了基于伪频响函数矩阵法的运行模态分析方法.该方法首先构造多参考测点、多参考工况的伪频响函数矩阵,然后基于伪频响函数矩阵采用最小二乘复频域法求解模态频率和阻...     

3D视觉结合图像检测与导纳控制的圆轴孔零件机器人装配

面向机器人柔顺装配圆轴与圆孔零件,建立基于3D、单目视觉与导纳控制的机器人自动装配系统,提出基于三维点云的轴线位姿估计算法、图像深度学习目标检测、导纳控制结合的圆轴孔零件的装配策略.针对3D视觉估计圆孔零件位姿问题,重点研究基于三维点云的轴线位姿估计算法.首先,介绍三维点云关键点选取方法;然后,以点云表面法线与轴线的几何约束为基础,提出并分析轴线粗估计的算法;最后,在轴线粗估计的基础上,提出并分析基于迭代鲁棒最小二乘的轴线位姿优化的算法.实验结果表明:轴线位姿估计的角度均方根误差为0.248°,位置均方根误差为0.463 mm,与现有流行的轴线估计方法相比,所提方法的精度更高,使装配策略很好地满足了机器人圆形轴孔零件装配的精度高、稳定可靠的要求.     

金字塔砂带磨损状态的声信号GA-BP识别方法

目的 金字塔砂带连续磨损会引发钝峰、材料去除能力差和产热多等问题,为避免砂带磨损造成加工效率持续降低和工件表面质量逐渐恶化,需提高金字塔砂带磨损预测能力。方法 在配有声音采集系统的力控机器人磨削系统中对钛合金工件进行了砂带磨损试验;基于Archard模型建立了金字塔砂带磨损模型,并对金字塔砂带磨损程度进行量化;然后利用短时傅里叶和小波包分解分析、提取砂带磨损相关的声音特征;基于声音信号特征建立GA-BP模型,并对金字塔砂带磨损状态进行预测。结果 Kr与R0规律相近,随着磨削速度的增大而略微增大。对磨削声音进行小波包分解,DD2频段的声音特征随磨削时间逐渐降低,相较于其他频段更具有规律性。提取DD2频段的声音信号特征建立GA-BP模型,并对金字塔砂带磨损状态进行预测。结果表明,决定系数(R²)大于0.8,平均绝对误差(MAE)小于0.04,平均偏差误差(MBE)在±0.002之间,均方误差(RMSE)小于0.05。结论 随着砂带的磨损,金字塔尖锐的胞体开始磨平,单颗胞体的局部压力逐渐减小,材料去除能力减弱,产生的微振荡越来越弱,高频信号的声音特征逐渐下降。通过DD2频段声音信号特征建立的GA-BP模型对金字塔砂带磨损状态进行预测,具有准确性和稳定性。     

Scale and material effects on flame characteristics in small heat recirculation combustors of a counter-current channel type

Small energy sources have been interested with the recent development of small-scale mechanical systems. With the purpose of developing a basic model of micro-combustors of heat recirculation, small combustors of a counter-current channel type were fabricated, and the premixed flame stabilization characteristics were investigated experimentally. Each combustor consists of a combustion space and a pair of counter-current channels for heat recirculation. The channel gap was less than the ordinary quenching distance of a stoichiometric methane-air premixed flame. Depending on the flame locations and structures, flame stabilization was classified into four modes: an ordinary mode, a channel mode, a radiation mode, and a well-stirred reaction mode. Base-scale combustors of stainless steel were initially examined. Additional half-scale combustors of stainless steel and quartz were fabricated and their flame stabilization conditions were compared. Consequently, a change of the material of the combustor significantly affected the flame stabilization compared to the effects of a scale-down design. A half-scale quartz combustor had a wide range of flame stabilization conditions. Surface temperatures and the composition of the emission gas were measured. At a higher flow rate, the combustor temperature increases and the light emission from the middle wall is enhanced to extend the flame stabilization conditions. The combustion efficiency and the composition of emitted gas were feasible. These results provide useful information for the design of small-scale combustors.     

Atomistic simulations on multilayer graphene reinforced epoxy composites

We use molecular dynamics simulations to characterize multilayer graphene reinforced epoxy composites. We focus on two configurations, one where the graphene layers are parallel to polymer/graphene interface and a perpendicular case, and characterize the in situ curing process of the resin and the thermo-mechanical response of the composites. The yield stress of the composites under uniaxial loading normal to the interface is in all cases larger than that of the bulk polymer even after the constraint of the reinforcement to transverse relaxation is taken into account. While both the parallel and normal configurations have very similar strengths, the parallel case exhibits cohesive yield with strain localization and nano-void formation within the bulk polymer while the case with graphene sheets oriented normal to the interface exhibit interfacial debonding. These two mechanisms lead to different post yield behavior and provide key insight for the development of predictive models of carbon fiber polymer composites.     

Mechanical properties of graphene nanoplatelet/carbon fiber/epoxy hybrid composites: Multiscale modeling and experiments

Because of the relatively high specific mechanical properties of carbon fiber/epoxy composite materials, they are often used as structural components in aerospace applications. Graphene nanoplatelets (GNPs) can be added to the epoxy matrix to improve the overall mechanical properties of the composite. The resulting GNP/carbon fiber/epoxy hybrid composites have been studied using multiscale modeling to determine the influence of GNP volume fraction, epoxy crosslink density, and GNP dispersion on the mechanical performance. The hierarchical multiscale modeling approach developed herein includes Molecular Dynamics (MD) and micromechanical modeling, and it is validated with experimental testing of the same hybrid composite material system. The results indicate that the multiscale modeling approach is accurate and provides physical insight into the composite mechanical behavior. Also, the results quantify the substantial impact of GNP volume fraction and dispersion on the transverse mechanical properties of the hybrid composite while the effect on the axial properties is shown to be insignificant.