科创中国

分析了我国农产品流通中存在的问题,介绍了国外发达国家成功的农产品流通模式,最后,提出发展我国农产品流通的相关对策建议.

As a redundant drive mechanism,twin ball screw feed system has the advantage of high stiffness and little yaw vibration in the feeding process,while leads to increased difficulty with vibration characteristics analysis and structure optimization.Only low-dimensional structure and dynamics parameters are considered in the existing research,the complete and effective model for predicting the table's vibrations is lacked.A three-dimensional(3D)mechanical model of twin ball screw driving table is proposed.In order to predict the vibration modes of the table quantitatively,an analytical formulation following a comprehensive approach is developed,where the drive system is modeled as a lumped mass-spring system,and the Lagrangian method is used to obtain the table's independent and coupled axial,yaw,and pitch vibration modes.The frequency variation of each mode is studied for different heights of the center of gravity,nut positions and table masses by numerical simulations.Modal experiment is carried out on the Z-axis feed table of the horizontal machining center MCH63.The results show that for each mode,the error between the estimated and the measured frequencies is less than 13%.The independent and coupled vibration modes are in accordance with the experimental results,respectively.The proposed work can serve a better understanding of the table's dynamics and be beneficial for optimizing the structure parameters of twin ball screw drive system in the design stage.

鉴于南水北调中线工程线路选择的位置对工程安全、水质保护、投资等方面影响较大,虽然南水北调中线工程总干渠线路的选择经过了几十年的论证,在总体线路方案已经确定的情况下,仍需开展局部线路论证工作并对其进行调整与优化.现以十里河段为例对局部线路的调整与优化进行了分析与研究.在开展十里河段线路优化工作阶段,经过现场查勘并考虑地形、地质条件及村镇分布等情况,初步选择了深挖方的北线方案及高填方的南线方案,并对北线、南线两个方案开展了有关工程安全、投资、技术等方丽的分析与研究工作,最终提出了技术可行、经济合理的输水线路布置方案.

在中国农村物流发展现状及特点分析的基础上,对中国邮政发展农村物流分销模式进行了研究和探讨,总结出邮政农村分销的四大模式,并对分销模式的进一步推广实施提出了相关建议.

Friction compensation is particularly important for motion trajectory tracking control of pneumatic cylinders at low speed movement. However, most of the existing model-based friction compensation schemes use simple classical models, which are not enough to address applications with high-accuracy position requirements. Furthermore, the friction force in the cylinder is time-varying, and there exist rather severe unmodelled dynamics and unknown disturbances in the pneumatic system. To deal with these problems effectively, an adaptive robust controller with LuGre model-based dynamic friction compensation is constructed. The proposed controller employs on-line recursive least squares estimation(RLSE) to reduce the extent of parametric uncertainties, and utilizes the sliding mode control method to attenuate the effects of parameter estimation errors, unmodelled dynamics and disturbances. In addition, in order to realize LuGre model-based friction compensation, the modified dual-observer structure for estimating immeasurable friction internal state is developed. Therefore, a prescribed motion tracking transient performance and final tracking accuracy can be guaranteed. Since the system model uncertainties are unmatched, the recursive backstepping design technology is applied. In order to solve the conflicts between the sliding mode control design and the adaptive control design, the projection mapping is used to condition the RLSE algorithm so that the parameter estimates are kept within a known bounded convex set. Finally, the proposed controller is tested for tracking sinusoidal trajectories and smooth square trajectory under different loads and sudden disturbance. The testing results demonstrate that the achievable performance of the proposed controller is excellent and is much better than most other studies in literature. Especially when a 0.5 Hz sinusoidal trajectory is tracked, the maximum tracking error is 0.96 mm and the average tracking error is 0.45 mm. This paper constructs an adaptive robust controller which can compensate the friction force in the cylinder.

For the Cooperative Adaptive Cruise Control(CACC)Algorithm,existing research studies mainly focus on how inter-vehicle communication can be used to develop CACC controller,the influence of the communication delays and lags of the actuators to the string stability.However,whether the string stability can be guaranteed when inter-vehicle communication is invalid partially has hardly been considered.This paper presents an improved CACC algorithm based on the sliding mode control theory and analyses the range of CACC controller parameters to maintain string stability.A dynamic model of vehicle spacing deviation in a platoon is then established,and the string stability conditions under improved CACC are analyzed.Unlike the traditional CACC algorithms,the proposed algorithm can ensure the functionality of the CACC system even if inter-vehicle communication is partially invalid.Finally,this paper establishes a platoon of five vehicles to simulate the improved CACC algorithm in MATLAB/Simulink,and the simulation results demonstrate that the improved CACC algorithm can maintain the string stability of a CACC platoon through adjusting the controller parameters and enlarging the spacing to prevent accidents.With guaranteed string stability,the proposed CACC algorithm can prevent oscillation of vehicle spacing and reduce chain collision accidents under real-world circumstances.This research proposes an improved CACC algorithm,which can guarantee the string stability when inter-vehicle communication is invalid.

以南水北调中线SG09标石方开挖爆破施工为例,采用理论计算、数值分析和本工程实际施工相结合的方法,详细分析了爆破对周围环境的影响范围及安全控制措施,为类似工程的施工提供了参考.

结合南水北调天津干线工程建设管理具体工作实践,对南水北调中线天津干线施工控制体系构建、主要做法和成功经验进行了分析梳理总结,结果表明南水北调天津干线工程施工中的土方工程、混凝土工程,施工受季节影响以及施工工期受资源配置、施工方案的选择影响较大,在建设实施过程中通过合理安排施工,配置资源合理,优化施工方案,充分发挥整体钢模台车的整体优势,对加强工程实体质量控制等方面的实践是成功的,对进一步提高南水北调天津干线工程以及其它大型水利工程建设水平具有一定的借鉴意义.

为了推进河北省经济建设,响应国家发展纲领,河北省在"十二五"规划中提出了"四个一"战略,BOT作为一种融资模式,在很多国家和地区的基础实施建设中得以应用,在详细分析"四个一"战略的具体内容和BOT模式含义的基础上,针对如何引导BOT融资模式参与河北省"四个一"战略的实施进行了探讨.

运用瑞典圆弧滑动法对围堰布置及结构形式进行了验算,阐释了施工方法和施工过程.通过实践表明工程实施中采用木桩拦栅围挡、就近水中取土筑施工围堰,能有效地解决了土源问题,节约工程造价,缩短工程工期.木桩拦栅筑堤土围堰技术方案在浅滩、湖泊、内河等水面宽阔、水深在2~4m的地域推广运用,能取得明显的社会和经济效益.