The Kinematic Parameter Identification Of Articulated Arm Coordinate Measuring Machines

The articulated arm coordinate measuring machine (AACMM) is a kind of coordinate measuring equipment based on articular robotic structure. Compared with traditional coordinate measuring machines (CMMs), AACMM has the characteristics of small volume, light weight, easy to carry, simple structure, easy to use in industrial field, which is widely used in product reverse design, assembly of large precision parts, art reproduction and other fields. And it has broad application prospects. However, its measurement accuracy is still much lower than traditional CMMs. Structure parameter identification is one of the most important methods to improve the accuracy of AACMM.The structural characteristics of the AACMM are studied in chapter 2. To study the influence of kinematic model on kinematic parameter identification and improve the accuracy of AACMM, the kinematic model of the AACMM is established by using the traditional DH (Denavit-Hartenberg) method, firstly. Then, another kinematic model based on MCPC (Modified Complete and Parametrically Continuous) method is established to solve the problem of continuity and singularity of the kinematic parameters of the DH method. And the established kinematic model is verified by simulation and experiment. The main error sources affecting the measuring accuracy of the AACMM are analyzed. Therefore, the error models based on the DH method and the MCPC method are established.The kinematic parameter identification methods of the AACMM are studied in chapter 3. By analyzing the three common methods of single point method, two point distance method and plane method, it is found that there are some problems such as the high degree of redundancy of the data acquisition, the need of precision instrument and the large amount of identification calculation. Therefore, a kinematic parameter identification method based on straight-line distance is proposed, which uses the grating sensor to calculate the line distance of the probe and has the advantages of high precision and less identification calculation. To increase the flexibility of the device, a new method of parameter identification for industrial field is presented. The method uses angle sensors to realize the calculation of the space distance of the probe, which has the advantages of large movement space, flexible movement, wide collection of data and small amount of calculation. The kinematic parameter identification devices of the two methods are designed, and the invention patent of which are applied.The simulation of kinematic parameter identification based on two kinds of models of AACMM is studied respectively by using the least square method. Firstly, the least amount of data needed for kinematic parameter identification of two kinds of models is determined. Secondly, the linear correlation of the kinematic parameters of the DH method is analyzed. The parameters of linear correlation are obtained. Then, the influence of the parameters of linear correlation on the kinematic parameters identification is analyzed. Furthermore, the influence of the data on the identification results is studied and the identification data is determined. Finally, through the simulation results of the two models, we can find that the accuracy of kinematic parameters identification based on MCPC model is higher than that based on the DH model, which shows the superiority of the MCPC model in parameter identifications.Finally, experiments are carried out. Firstly, the joint angle value and the value of the grating sensor are acquired to verify the eccentric error of the joint rotation angles. Secondly, the joint rotation angle and the coordinate values are acquired for identifying the kinematic parameters of the two models by the least square method. The experimental results are analyzed, which shows that the identification accuracy based on the MCPC model is higher than the accuracy based on the DH model without the identification of the joint angle. But the accuracy of AACMM is not improved significantly after the identification. Compared with MCPC model DH model can be used to identify the joint angle deviation at the zero position first, and to identify the other kinematic parameters. Then, the accuracy of the AACMM can be improved greatly, which also indicates that the joint angle deviations at the zero position have a great influence on the accuracy of AACMM although they are relatively small. Therefore, if MCPC model is used to identify the kinematic parameters, the installation accuracy of the angle sensors should be high enough to ensure the accuracy of joint angles.