科创中国

从运力供给、行业利润、发展模式和不可抗因素等几个方面分析了快递爆仓的成因,在此基础上,从增加运力供给、制定合理价格、转变经营模式、构建应急预案等角度提出了针对性的措施,对于实现快递业和电子商务的协调发展具有现实意义.

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.

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.

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.

首先对物流配送及相关理论进行了综述,提出了共同配送模式在广西-东盟物流中的应用,同时,对广西现行的自营配送模式与共同配送模式进行了比较,并从成本、路线、经营主体等方面进行了定性、定量分析,由此,得出了自营配送模式存在成本高、路线不完善、经济主体单一等一系列问题,而共同配送模式更适合于广西物流发展的现状,并提出了共同配送的三种组织模式在广西物流应用中的优势和劣势.最后,提出了共同配送模式在广西物流配送应用中需要注意的问题及其对策.