| As one of the most important engineering equipments in modern construction,hydraulic excavator with many outstanding advantages,such as compact structure,high flexibility and controllability,high digging capability,high efficiency,multiple functions,etc.,has been widely used in many fields,such as agricultural engineering,transpotation,emergency rescue,exploration and mining,etc..However,in actual digging process,some phenomena such as insufficient digging capability,termination of digging process,high emission and structure failure,not only decrease the efficiency of construction,but also limit the development of excavator with high performance,low pollution and long life.One of the most important reasons for above phenomena is that the existing theories and methods for quantification of digging performance and the common criteria for design of the excavator working attachment and assessment of digging styles and digging trajectories cannot reflect the uncertain characteristics of digging resistance generated in actual digging process.In addition,unreasonable digging styles and digging trajectories can also cause above phenomena.Thus,it has high theoretical significance and industrial application value to conduct studies on theories and methods for digging performance quantification and its applications in excavator design and motion planning.In this dissertation,bucket-soil interaction mechanism was first investigated.Next,a complete dynamic model for digging process was established.Then,a comprehensive evaluation system for digging performance was established based on the characteristics of the digging resistance and a set of common criteria which can be used to guide the assessment of the digging performance of the whole machine,optimization design of the mechanism and structure of an excavator working attachment,assessment of digging styles and analysis and planning of digging trajectories were also proposed.Finally,the theories and methods aiming at the optimization design of high-performance working attachment and the planning of high-performance digging trajectories were investigated.The main contents and achievements are summarized as follows:1)Research on the dynamic model of digging process with the intergration of soil-bucket interaction.A lumped parameter model including all relative factors was established.The unified analytic expressions of the tangential digging resistive force,normal digging resistive force and digging resistive moment in bucket resistive force space were obtained to reveal the inner generation mechanism of digging resistive forces and moment.A complete dynamic model for digging process was established by integrating the analytic expressions of tangential and normal digging resistive forces and digging resistive moment into the multi-body dynamic model of the excavator working attachment.The proposed model has been verified by comparing the simulated results with the measured results for the level digging work of a 36-ton excavator.This research provides a foundation for the characteristic analysis of digging resistance,calculation of static and dynamic digging capability and motion planning for digging process.2)Establishment of comprehensive evaluation system for digging performance of a hydraulic backhoe excavator.The customary interval of digging resistance was obtained by analyzing its uncertainty in the bucket resistive force space.Digging capability calculation model based on the feasible zone construction method was established,with the consideration of the constraints of the hydraulic driving pressures,stability and slipping,and the customary intervals of the digging resistance.The customary digging capability polygon and customary digging capability polytope were defined,and comprehensive evaluation system for digging performance was established.This evaluation system can be used to quantify the digging capability,mechanical efficiency,matching degree between mechanism and hydraulic system,and the influence of each constraint on the digging capability.This evaluation system gives the performance incices for the digging styles and entire workspace,and thus provides the common criteria for performance analysis of the whole machine,optimization design of the working attachment,digging style assessment and digging trajectory analysis and planning.3)Research on the applications of comprehensive performance evaluation system in creative design of working attachment mechanism.Two optimization models were established to maximize the digging capability,efficiency of working hydraulic cylinders,mechanical efficiency and the matching degree between working attachment mechanism and hydraulic driving pressures.In the first model,the optimization criteria are defined based on the performance indices obtained by using digging capability polygon which can quantify digging performance in entire 2D bucket force space,thus this optimization model can be used to design the working attachment mechanism applicable to uncertain and unstructural environment and complex working conditions.In the second model,the optimization criteria were defined based on the performance indices obtained by using digging capability polytope which can quantify the digging performance in the 3D customary bucket force space,thus this optimization model can be mainly used to design the mechanism aiming at normal digging condition and improving performance in customary bucket force space.The optimization results of both these two models show that the optimal working attachment mechanisms have preferable comprehensive digging performance.4)Task-space based dynamic trajectory planning to improve digging performance has been studied.Based on the feasible zone construction method,quantification model and indices for dynamic digging performance have been proposed with the consideration of the customary interval constraints of the digging resistance.The B-spline curves were used to parameterize the digging trajectory of the task space,and the self-motion parameters were introduced into the inverse kinematics model from the task space to the joint space.In the process of trajectory planning,through the adjustment of B-spline control points and self-motion parameters,the task and joint space trajectories can be jointly adjusted to enhance the task-oriented function.Dynamic trajectory planning models for active operation and intelligent implementation of practical operation processes were established.Dynamic trajectory planning for maximizing theoretical digging efficiency and minimizing digging time,energy consumption and mechanical damage has been respectively realized,and the corresponding optimal digging trajectories have also been obtained.5)A method of high-strength and lightweight design for full parameterized work attachment structure based on the limiting theoretical digging capability model was proposed.Based on the customary digging capacity polytope,the limiting theoretical digging capacity calculation model was proposed.The optimization model and system for a full parameterized working attachment were established.The relations between the maximum stresses of the boom and stick structure of the working attachment and the digging force and digging moment(or the digging resistive force coefficient and digging resistive moment coefficient)in the customary digging capability polytope under the four typical working conditions specified in the national standard GBT 9141-88 were explored.The maximum stress evaluation method and the optimization method were used to determine the dangerous working conditions and design loads of the boom and stick structure respectively.The hybrid discrete variable genetic algorithm was used to optimize the structure of the working attachment,and a lightweight and high-strength working attachment was designed which can adapt to the uncertain and variable load conditions.This research solves the problem that the working attachment structure optimized based on the traditional design load is prone to genetate early failure due to insufficient strength. |
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