Research On Error Correction Method For Parallel Dual-joint Coordinate Measuring Machines

Parallel dual-joint coordinate measuring machine(PDCMM)which is consisted of one optical linear encoder and two rotary encoders can measure the three-dimensional coordinates of the surface of a workpiece.PDCMM solves the disadvantages of traditional orthogonal coordinate measuring machines and it is suitable for onsite measurement.The thesis is founded by Nation Science Foundation of China“Application Research on Abbe Error Compensations in Three Dimension on Articulated Coordinate Measuring Machines”(No.51875165).The main work which includes analysis and correction of the error motions of PDCMM,the establishment of measurement model with error motions,the angle measurement error correction techniques of rotary encoder under conditions of steady temperature and varying temperature separately is systematically researched.The main research work of this thesis is listed as follows.(1)The six error motions(measurement error of optical linear encoder,straightness error motion in the vertical direction,straightness error motion in the horizontal direction,yaw,pitch and roll error motion)of slide carriage of positioning system in Z-axis are analyzed and measured,and the polynomial fitting method is used to fit the function of error motions respectively.Based on the modified D-H modeling theory with four parameters,the measurement model of PDCMM including the error motions mentioned above is established.For this measurement model,a calibration method of structure parameters is developed.(2)The Abbe principle and its extension are reviewed and extended to angle measurement area.The concept of the Abbe error used in angle measurement field is defined and the correction method of angle measurement Abbe error is proposed.The main part of the correction method is detecting the Abbe angle caused by the tilt error motion of the rotating shaft with an autocollimator and a plane mirror.On the basis of correcting the angle measurement Abbe error and further correcting the residual error,the angle measurement accuracy of rotary encoders is significantly improved.(3)A new method of angle measurement error analysis of rotary encoders based on analysis-measurement-modeling of errors is proposed.The model of angle measurement error caused by the installation eccentricity of the grating disk and radial error motion of the rotating shaft is built.An image processing technology is developed to detect the amplitude and phase of the installation eccentricity.The proposed method is suitable for predicting the angle measurement error during the assembly process of precision rotating shaft and installation process of grating disk.To some extent,this theory can be applied to the design process of precision rotating shaft.(4)A method based on Fourier expansion-BP neural network optimized by genetic algorithm(FE-GABPNN)is proposed to correct the angle measurement error including ambient temperature factor.The angle measurement accuracy is significantly improved after correction.The FE-GABPNN method has the potential of being applied to other onsite precision instruments which contain rotation joints.The hardware circuit system with field-programmable gate array(FPGA)and single chip microcomputer(SCM)as the core processors is developed,and the supporting lower processor codes are programmed.Combined with the corresponding mechanical structure and computer program,a PDCMM is developed.Based on the above error modeling and correction techniques,the measurement accuracy of the newly developed PDCMM is 8.6?m.