Study On The Posture Adjustment Of Workpiece Based On On-machine Measurement Data

Precision machining of complex parts usually involves a series of processes.There exist workpiece location error,distortion error,machine tool geometric error and other machining error in previous process.These factors may lead to overcut or undercut(non-uniform allowance distribution)of some surfaces in current process even if the shape of the workpiece meets the design requirements.Therefore,the allowance distribution is very significant for guaranteeing machining accuracy of workpiece.However,the workpiece pose may have a significant impact on its allowance distribution and precise position.In mechanical machining,due to multiple clamping and other reasons,the workpiece pose may be altered easily,which can result in inaccurate workpiece position and nonuniform allowance distribution.So it is necessary to find a method to ensure the accuracy of the workpiece pose during machining.In view of this situation,this paper conducts some relevant research,which combines on-machine measurement technology,for the methods of workpiece pose adjustment.We hope can ensure the uniform allowance distribution and precise position of workpiece through real-time pose adjustment and control.The main contents are as follows:(1)In this thesis,a workpiece registration and localization adjustment method with contact inspection under multi-tolerance conditions based on measured points and characteristic vector for workpiece pose adjustment is developed.Firstly,the measured point sets on actual surface and corresponding theoretical surface are obtained by on-machine measuring.Then,the actual and theoretical characteristic vectors of different surfaces of the workpiece are obtained by fitting the measured point sets(the characteristic vector of cylinder is its axial vector,the characteristic vector of plane is its normal vector,and the characteristic vector of the surface is the integrated vector).Subsequently,a total deviation vector is calculated by means of coupling the actual and theoretical characteristic vectors with standard reference vector.According to spatial error relation between total deviation vector and the standard reference vector,the best spatial rotation and translation transformation matrixes can be obtained by the iterative registration algorithm to achieve the best matching between the measured data and the surface model.Finally,the workpiece pose adjustment under multi-tolerance conditions is completed.(2)A method of workpiece pose adjustment based on measured point sets and self-adaptive Differential Evolution algorithm is proposed.According to the actual and theoretical measured point sets of workpiece surface,a stepwise matching algorithm based on point sets and self-adaptive Differential Evolution algorithm is developed.The method includes an initial matching and a fine matching.At first,the reference points on the theoretical and actual surfaces are selected to build the local coordinate systems,and then the initial matching is achieved by aligning the local coordinate systems.During the fine matching,the objective function based on the least square method is established by the point sets after the initial matching.The optimal transformation matrixes can be acquired by self-adaptive Differential Evolution algorithm.Then the ultimate transformation matrixes can be obtained.The method proposed has advantages of fast convergence,high matching precision,easy operation and programming.(3)The parameters calculation of workpiece pose adjustment.According to the best spatial transformation matrixes calculated by surfaces matching,the machine tool structure and its corresponding kinematic equation,the adjustment values of 5-axis CNC machine tool can be calculated.And then,5-axis CNC machine tool is adjusted to complete workpiece pose adjustment.(4)Develop adaptive control system of workpiece pose adjustment.Based on the above matching algorithm,the system of workpiece pose adjustment is developed through MATLAB and its GUI function.The system includes three modules that are data input,registration calculation,adjustment parameters output respectively.Finally,the pose adjustment and precise position of an S-shaped workpiece is achieved by using the workpiece pose adjustment method proposed in this thesis.The validity and practicability of the proposed method are verified.