Assembly Accuracy Prediction And Control For Precision Mechanical System

Based on the current research,the assembly accuracy and product performance of a precision mechanical system is highly related to the part manufacturing characteristics(mainly including the dimension error,form error,position error,surface morphological characteristics,and assembly error,et al).Therefore,a phenomenon always happens that the system accuracy and system performance of an assembly cannot satisfy the design requirements,even though all the machined parts satisfy the design requirements,leading to the low yield.The main reasons lie in the lack of the theory on how the part machining error influences the assembly error,in turn,the effective model relating the part manufacturing error to the assembly error cannot be established.To solve the problem,the author read numerous references on “assembly accuracy analysis and prediction”,and did deeply research on the related problems.Finally,starting from the study on the distribution law of part form error,the author did research on assembly accuracy prediction for the precision rigid mechanical system and the large flexible mechanical system by taking the machining error and the fixture position error as the sources of variation.The main contents are shown as follows:1.Form error modeling for a single part and form error distribution law analysis for multiple pars.Based on the plane form error,a set of innovative basis functionskernel functions of discrete cosine transformation is taken as basis functions.The form error of a single part can be modeled by linear superposition of the basis surfaces.Based on the form error characteristics,two indexes are proposed to evaluate the accuracy of the form error models for a single part.The method proposed in this thesis and the traditional method is used to model the form error of a batch of milling parts.It is proved that the model established with the method proposed in this thesis is 25% more accurate than that established with the traditional method by comparing the accuracy of the two models.For a batch of parts machined by the same process,the distribution characteristics of the form error are usually the same.Based on the form error model of a single part,the distribution law model of form error is statistically established using the principal component method.Moreover,the indexes for the model are proposed.The form error distribution law model of a batch of milling parts is established to illustrate how to use the method proposed in this thesis.The method of modeling form error lays the theoretical foundation for modeling the 3D solid model with form error.2.The effects of form error on the assembly accuracy of the precision rigid parts and assembly accuracy prediction.Usually,only a few points touch each other when two plane parts with form error are assembled togather.First of all,a method of determining the position of the contact points is proposed.And then,the validity of the method is verified by the ‘theoretical analysis method' and ‘finite element simulation method'.On this basis,a method to predict the assembly accuracy for precision rigid parts considering form error is proposed by using small displacement torsor to characterize part form error.The method can be used to predict the assembly accuracy,and offer support to assembly process optimization and assembly accuracy promotion.Based on the method of predicting the assembly accuracy,a method of calculating product yields is proposed by defining the functional requirement region.The importance of considering form error in the actual production is statistically proved by comparing the product yields in the case of considering the effects of form error and not.3.Assembly accuracy prediction for compliant composite assembly in single station assembly process.The composite material is widely used in the aerospace and ships due to its outstanding behavior,but the variation propogation theory of composite structures is not clear.This thesis identifies the sources of variation firstly,including the part machining error and fixture position error.Depended on the actual assembly process and the finite element method,the mathematic relationship model between the part machining error and assembly error is deviated.Considering the part machining error and fixture position error,a method of obtaining the part total error is developed by homogeneous coordinate transformation and finite element method.Combined the constructed mathematic model,a model of predicting assembly accuracy is proposed by considering part machining error and fixture position error.Taking the designed single station assembly process of two composite parts as example,the prediction procedures of the assembly accuracy are illustrated.The Monte Carlo Simulation is utilized to verify the validity and the accuracy of the assembly accuracy prediction method.The model can be used to predict the assembly accuracy of composite laminated plates in single station assembly process,and then lays the foundation for the prediction of assembly accuracy in multi-station assembly process.4.Assembly accuracy prediction for compliant composite assembly in multi-station assembly process.This research is implemented based on the study of assembly accuracy prediction for compliant composite assembly in single station assembly process.Different from single station assembly process,two assembly steps that the release of the sub-assembly from the fixtures and the relocation of the sub-assembly are involved in the multi-station assembly process.Therefore,in addition to the part machining error and fixture position error,the deformation of a sub-assembly caused by the release of the fixtures and the error induced by the relocation of a sub-assembly.This chapter uses the finite element method and the distance calculation in the geometry space to calculate the assembly error caused by the extra two assembly steps.Combined the prediction model of the assembly accuracy for the single station assembly process,a model of predicting assembly accuracy in multi-station assembly process is proposed based on the state space model.Eventually,taking the designed multi-station assembly process of three compliant composite parts as example,the prediction procedures of the assembly accuracy are illustrated.The Monte Carlo Simulation is utilized to verify the validity and accuracy of the assembly accuracy prediction method.The model can be used to predict the assembly accuracy of composite laminated plates in the multi-station assembly process,and lay the foundation for the sources of variation identification and assembly deviation reduction.5.Factor analysis and assembly accuracy control for precision mechanical system.On the premise that the machining accuracy and the fixture position accuracy are not promoted,the assembly accuracy is optimized by adjusting the values of the assembly process parameters.The assembly process parameters involved in this chapter includes: the combination form of parts,the assembly sequence,and the assembly position of the assembled parts.Different assemblies are obtained by changing the combination form of parts,adjusting the assembly sequence,and rotating the rotary parts.And then,the assembly accuracies of different assemblies are simulated.Based on the simulation results,the optimal values of the assembly process parameters are determined,which can help direct actual assembly.