Research On The Identification,Modeling And Compensation Method For CNC Machine Tool Key Error Sources

The machining precision of the CNC machine tool is seriously affected by the integrated geometric and thermal errors,and the effective identification and compensation for these error sources has attracted the widespread attention from the global researchers.This research is funded by the State Key Science and Technology Project,the State Scientific Instruments Development Project,and the State Natural Science Foundation.Many research experiments were conducted on a large gantry-type machine tool and a vertical machining center,so as to identify the key error sources for error modeling and compensation.Furthermore,a virtual compensation system is developed based the Labview software and an embedded computer,which can be used to compensate the integrated geometric and thermal errors of machine tools in real-time,and then the machining precision of the machine tool could be improved.The main research contents are listed as follows.(1)Establishing an unified comprehensive volumetric error model for multiple-type machine tools.According to the structure characteristic analysis of different types of machine tools,as well as the error distribution characteristic on these machine tools,an unified comprehensive volumetric error model is established for multiple-type machine tools based on multi-body kinematics theory and rigid-body assumption,which can be used for the vertical machining center,horizontal machining center,large gantry-type tool and horizontal boring machine.In addition,through the effective identification of the Abbe error for translational axis,which is induced by the angular error,the comprehensive error model achieves better applicability,which can be used for both conventional-sized machine tools and large structure machine tools.(2)Identification of the key error sources for multiple-type machine tools.Firstly,the influence mechanism of the error component(including positioning error,straightness error,squareness error,angular error)on the volumetric position accuracy and circular accuracy is analyzed for different types of machine tools,and the machine tool structure and working range are considered as important factors.Then,an effective modeling method is proposed to identify the key error sources for multiple-type machine tools based on the sensitivity analysis,so as to improve the compensation efficiency for machine tools.(3)Key error parameter measurement and modeling for the vertical machining center.Firstly,the positioning errors of the machining center were measured under different thermal states.Then,an approximation approach is proposed for the geometric error components with the Chebyshev polynomial based orthogonal least-square regression,which could confirm the best fitting algorithm for the error curves automatically with high accuracy.And for the thermally induced positioning error,a robust thermal error model is proposed based on the mechanism analysis of the machine tool thermal behaviors.It can be used to confirm the thermal deformation mechanism of the ball-screw system,as well as its variation pattern with the temperature field.And then the robust of the thermal error model could be improved,and keep high prediction accuracy under different working conditions.(4)Key error parameter measurement and modeling for the large gantry-type machine tool.Firstly,a straightness error measurement method is introduced based on the angular measurement method on a machined work-piece.Thereafter,to improve the modeling accuracy for the irregular and complex geometric error curves for the large gantry-type machine tool,an automatic modeling algorithm is proposed based on moving least squares in combination with Chebyshev polynomials,which has combined the advantages of the adaptive segmentation and weighted least square modeling method,so it could achieve better modeling accuracy for complex error curves.In addition,with the identification and modeling of the angular error induced Abbe error for the translational axis,the volumetric positioning accuracy of the gantry-type machine tool is greatly improved.(5)Development of the virtual error compensation system.A real-time compensation system is developed based on the Labview software and an embedded computer,which can be used to compensate the integrated geometric and thermal errors for machine tools in real-time.Some MATLAB procedures are embedded in the virtual compensation system,which can be used to analyze and extract the error components and temperature data automatically from the original measurement files,and then the online automatic modeling could module confirm the best fitting model for the geometric and thermal errors.In addition,in order to improve the applicability of the compensation technique,two different compensation methods are proposed for different types of CNC systems,which could interact with the CNC system through the embedded Ethernet interface and the machine tool I/O module,respectively.(6)Evaluation of the compensation effectiveness of the real-time compensation system in the actual application.To evaluate the effectiveness of the real-time compensation system,a series of compensation tests were conducted on the vertical machining center and the large gantry-type machine tool,respectively.And then a laser interferometer and a double ball-bar were used to identify the volumetric position accuracy and circular accuracy before and after compensation.Thereafter,some machining tests were also designed to evaluate the compensation effect during the actual machining process.Finally,the compensation validations on the machine tools showed that both the machine tool position accuracy and machining precision could be improved greatly after compensation,and the compensation technique could be promoted for actual application.