科创中国

Parallel kinematic machines have drawn considerable attention and have been widely used in some special fields.However,high precision is still one of the challenges when they are used for advanced machine tools.One of the main reasons is that the kinematic chains of parallel kinematic machines are composed of elongated links that can easily suffer deformations,especially at high speeds and under heavy loads.A 3-RRR parallel kinematic machine is taken as a study object for investigating its accuracy with the consideration of the deformations of its links during the motion process.Based on the dynamic model constructed by the Newton-Euler method,all the inertia loads and constraint forces of the links are computed and their deformations are derived.Then the kinematic errors of the machine are derived with the consideration of the deformations of the links.Through further derivation,the accuracy of the machine is given in a simple explicit expression,which will be helpful to increase the calculating speed.The accuracy of this machine when following a selected circle path is simulated.The influences of magnitude of the maximum acceleration and external loads on the running accuracy of the machine are investigated.The results show that the external loads will deteriorate the accuracy of the machine tremendously when their direction coincides with the direction of the worst stiffness of the machine.The proposed method provides a solution for predicting the running accuracy of the parallel kinematic machines and can also be used in their design optimization as well as selection of suitable running parameters.

Influence of geometric and cutting parameters of cemented carbide cutting tool on reliability of cutting tool has become more and more mature,yet influence of its physical and material parameters on reliability is still blank.In view of this,cutting test and fatigue crack growth test of YT05 cemented carbide cutting tool are conducted to measure such data as the original crack size,growth size,times of impact loading,number and time of cutting tool in failure,and stress distribution of cutting tool is also obtained by simulating cutting process of tools.Mathematical models on dynamic reliability and dynamic reliability sensitivity of cutting tool are derived respectively by taking machining time and times of impact loading into account,thus change rules of dynamic reliability sensitivity to physical and material parameters can be obtained.Theoretical and experimental results show that sensitive degree on each parameter of tools increases gradually with the increase of machining time and times of impact loading,especially for parameters such as fracture toughness,shape parameter,and cutting stress.This proposed model solves such problems as how to determine the most sensitive parameter and influence degree of physical parameters and material parameters to reliability,which is sensitivity,and can provide theoretical foundation for improving reliability of cutting tool system.

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.

Published studies in regard to coupler systems have been mainly focused on the manufacturing process or coupler strength issues. With the ever increasing of tonnage and length of heavy haul trains, lateral in-train forces generated by longitudinal in-train forces and coupler rotations have become a more and more significant safety issue for heavy haul train operations. Derailments caused by excessive lateral in-train forces are frequently reported. This article studies two typical coupler systems used on heavy haul locomotives. Their structures and stabilizing mechanism are analyzed before the corresponding models are developed. Coupler systems models are featured by two distinct stabilizing mechanism models and draft gear models with hysteresis considered. A model set which consists of four locomotives and three coupler systems is developed to study the rotational behavior of different coupler systems and their implications for locomotive dynamics. Simulated results indicate that when the locomotives are equipped with the type B coupler system, locomotives can meet the dynamics standard on tangent tracks; while the dynamics performance on curved tracks is very poor. The maximum longitudinal in-train force for locomotives equipped with the type B coupler system is 2000 kN. Simulations revealed a distinct trend for the type A coupler system. Locomotive dynamics are poorer for the type A case when locomotives are running on tangent tracks, while the dynamics are better for the type A case when locomotives are running on curved tracks. Theoretical studies and simulations carried out in this article suggest that a combination of the two types of stabilizing mechanism can result in a good design which can significantly decrease the relevant derailments.

The methods of improving the dynamic performance of high speed on/off solenoid valve include increasing the magnetic force of armature and the slew rate of coil current, decreasing the mass and stroke of moving parts. The increase of magnetic force usually leads to the decrease of current slew rate, which could increase the delay time of the dynamic response of solenoid valve. Using a high voltage to drive coil can solve this contradiction, but a high driving voltage can also lead to more cost and a decrease of safety and reliability. In this paper, a new scheme of parallel coils is investigated, in which the single coil of solenoid is replaced by parallel coils with same ampere turns. Based on the mathematic model of high speed solenoid valve, the theoretical formula for the delay time of solenoid valve is deduced. Both the theoretical analysis and the dynamic simulation show that the effect of dividing a single coil into N parallel sub-coils is close to that of driving the single coil with N times of the original driving voltage as far as the delay time of solenoid valve is concerned. A specific test bench is designed to measure the dynamic performance of high speed on/off solenoid valve. The experimental results also prove that both the delay time and switching time of the solenoid valves can be decreased greatly by adopting the parallel coil scheme. This research presents a simple and practical method to improve the dynamic performance of high speed on/off solenoid valve.

Quadruped robots consume a lot of energy,which is one of the factors restricting their application.Energy efficiency is one of the key evaluating indicators for walking robots.The relationship between energy and elastic elements of walking robots have been studied,but different walking gait patterns and contact status have important influences on locomotion energy efficiency,and the energy efficiency considering the foot-end trajectory has not been reported.Therefore,the energy consumption and energy efficiency of quadruped robot with trot gait and combined cycloid foot trajectory are studied.The forward and inverse kinematics of quadruped robot is derived.The combined cycloid function is proposed to generate horizontal and vertical foot trajectory respectively,which can ensure the acceleration curve of the foot-end smoother and more successive,and reduce the contact force between feet and environment.Because of the variable topology mechanism characteristic of quadruped robot,the leg state is divided into three different phases which are swing phase,transition phase and stance phase during one trot gait cycle.The non-continuous variable constraint between feet and environment of quadruped robot is studied.The dynamic model of quadruped robot is derived considering the variable topology mechanism characteristic,the periodic contact and elastic elements of the robot.The total energy consumption of walking robot during one gait cycle is analyzed based on the dynamic model.The specific resistance is used to evaluate energy efficiency of quadruped robot.The calculation results show the relationships between specific resistance and gait parameters,which can be used to determine the reasonable gait parameters.

In working state, the dynamic performance of dry gas seal, generated by the rotating end face with spiral grooves, is determined by the open force of gas film and leakage flow rate. Generally, the open force and the leakage flow rate can be obtained by finite element method, computational fluid dynamics method and experimental measurement method. However, it will take much time to carry out the above measurements and calculations. In this paper, the approximate model of parallel grooves based on the narrow groove theory is used to establish the dynamic equations of the gas film for the purpose of obtaining the dynamic parameters of gas film. The nonlinear differential equations of gas film model are solved by Runge-Kutta method and shooting method. The numerical values of the pressure profiles, leakage flux and opening force on the seal surface are integrated, and then compared to experimental data for the reliability of the numerical simulation. The results show that the numerical simulation curves are in good agreement with experimental values. Furthermore, the opening force and the leakage flux are proved to be strongly correlated with the operating parameters. Then, the function-coupling method is introduced to analyze the numerical results to obtain the correlation formulae of the opening force and leakage flux respectively with the operating parameters, i.e., the inlet pressure and the rotating speed. This study intends to provide an effective way to predict the aerodynamic performance for designing and optimizing the groove styles in dry gas seal rapidly and accurately.

Extensive studies on rotor systems with single or coupled multiple faults have been carried out. However these studies are limited to single-span rotor systems. A finite element model for a complex rotor-bearing system with coupled faults is presented. The dynamic responses of the rotor-bearing system are obtained by using the rotor dynamics theory and the modern nonlinear dynamics theory in connection with the continuation-shooting algorithm(commonly used for obtaining a periodic solution for a nonlinear system) for a range of rub-impact clearances and crack depths. The stability and Hopf instability of the periodic motion of the rotor-bearing system with coupled faults are analyzed by using the procedure described. The results indicate that the finite element method is an effective way for determining the dynamic responses of such complex rotor-bearing systems. Further for a rotor system with rub-impact and crack faults, the influences of the clearances are significantly different for different rub-impact stiffness. On the contrary, the influence of crack depths is rather small. The instability speeds of the rotor-bearing system increase due to the presence of the crack fault. The results obtained using the new finite element model, presented for computation and analysis of dynamic responses of the rotor-bearing systems with coupled faults, are in accordance with measurements in experiment. The formulations given can be used for diagnosis of faults, vibration control, and safe and stable operations of real rotor-bearing systems.

To improve the performance of the positive displacement blower,it is imperative to understand the detailed internal flow characteristics or enable a visualization of flow status.However,the existing two-dimensional unsteady,three-dimensional steady or quasi-unsteady numerical simulation and theoretical analysis cannot provide the detailed flow information,which is unfavorable to improve the performance of positive displacement blower.Therefore,the unsteady flow characteristics in a three-lobe positive displacement blower are numerically investigated by solving the three-dimensional,unsteady,compressible Navier-Stokes equations coupled with RNG k-εturbulent model.In the numerical simulation,the dynamic mesh technique and overset mesh updating method are adopted.Due to the air being compressed in the process of the rotors rotating,the variation of the temperature field in the positive displacement blower is considered.By comparing the experimental measurements and the numerical results on the variation of flow rate with the outlet pressure,the maximum relative error of the flow rate is less than 2.15%even at the maximum outlet pressure condition,which means that the calculation model and numerical computational method used are effective.The numerical results show that in the intake region,the fluctuations of the inlet flow are greatly affected by the direction of the velocity vectors.In the exhaust region,the temperature changes significantly,which leads to the increase of the airflow pulsation.Through analysis on the velocity,pressure and temperature fields obtained from the numerical simulations,three-dimensional unsteady flow characteristics in the positive displacement blower are revealed.The studied results will provide useful reference for improving the performance and empirical correction in the design of the positive displacement blower.