| Product assembly is the main activity in manufacturing industry,which consumes a lot of labour and material resources.It has serious influences on development cycle,product quality and production cost.Virtual assembly technology can be used to develop a realistic assembly environment,in which the user can simulate the actual assembly activities.Based on the realistic assembly simulation,the product's assemblability,the rationality of assembly process and the comfort level of assembly operation can be analyzed and verified,so as to discover the problems and defects in product design and assembly process planning at the early stage of product development.Thus,it will reduce design changes,product development cycle and improve the efficiency and quality of assembly production.At present,most of the virtual assembly systems do not have integrated virtual human model,so it is difficult for these systems to effectively support the increasingly complex ergonomic analysis and evaluation of the assembly process.In addition,the parts are assembled based on the geometric constraint information in the current virtual assembly system,which causes the assembly process simulation to have some limitations in the fidelity of the motion behavior of parts.Aiming at the above problems,this paper explores a new idea of integrating a virtual human model in the virtual assembly system in a "full virtual" way,and makes an in-depth study on the key technologies related to the virtual human based ergonomic assessment and virtual assembly.The main research work is summarized as follows:1.The construction strategies of the virtual human based ergonomic assessment and virtual assembly system are analyzed,and the functional requirements of the system are discussed.The system structure is divided into data layer,support layer,function layer and interaction layer to build a complete system architecture.The workflow of the system is given,including the stage of building assembly scene,the stage of ergonomic simulation and the stage of part assembly process simulation.2.A multi-objective optimization model for the prediction and evaluation of virtual human's working posture is proposed.The basic idea of virtual human's posture prediction based on multi-objective game is discussed.The influence of working posture on ergonomic factors(e.g.balance,joint load,joint angle and accessibility of working target)is analyzed.The corresponding ergonomic factor fitness function expression is established.Based on it,the optimization model for virtual human working posture prediction is constructed.In view of the complexity and nonlinearity of the optimization model,the multi-objective genetic algorithm is introduced to solve it,so as to obtain the Pareto optimal posture solution set of the posture optimization model.Based on the variable weight theory,the selection priority of each posture solution is calculated,and then the posture solution set is synthetically ordered to achieve the optimal decision of the final posture parameter scheme.The value of the ergonomic factors related to the posture comfort are extracted and integrated to calculate the overall working posture comfort level.3.An automatic evaluation method for the visibility of observed object in the virtual prototype environment is proposed.The surface of observed object is divided into a series of grid elements by using grid division technology.The determination method of mesh size and the requirement of mesh shape are given.This paper analyzes and summarizes the visual types of grid elements,and establishes an automatic recognition algorithm of visual types of grid elements,which solves the problem of visual grid classification and extraction in virtual human visual field.The characteristics of visibility factors,such as view-region factor,object-pose factor and visual-angle factor,are summarized and analyzed,and then the corresponding evaluation models of these visibility factors are established based on the ergonomics theory and experimental statistical methods,so as to realize the separate evaluation of these visibility factors.The variable weight theory is introduced to integrate these visibility factors for the calculation of the overall visibility of the observed object.4.An assembly simulation method considering ergonomic factors and physical properties of parts is proposed.Taking the most typical shaft-hole assembly as the research object,the assembly process is divided into four stages,including roaming stage of assembly space,hole finding stage,half in hole stage and in hole stage,based on the characteristics of assembly operation.The description of assembly behavior of parts in each stage is given.Based on the requirement analysis of parts' physical attributes involved in assembly simulation,the physical attribute model of parts are constructed.Based on the cognitive model of the assembler,a method of describing and controlling the object's pose for assembly simulation is proposed.The probability method is used to simulate the influence of ergonomic factors on assembly force / torque,and the assembly intention and operation characteristics of assembler in each assembly stage are analyzed.Based on it,the calculation model of assembly force / torque in each assembly stage is constructed.The calculation method of contact force is given to avoid the phenomenon of mutual embedment after parts collision.The dynamic and kinematic equations of the parts are established,and the motion guidance and assembly positioning of the parts are realized by dynamic simulation,which improves the reality and reliability of the assembly simulation.5.Based on the above research,a virtual human based ergonomic assessment & virtual assembly system—VEAVAS is developed.The development and running environment of the system are introduced.The function model of the system and the data flow direction between each function module are discussed.The theory and method proposed in this paper are verified through an assembly task case. |
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