Modeling And Compensation Of Volumetric Error For Five-axis Gantry-type Machine Tools

The development of high-end CNC machine tools is one of the key development plans by "Made in China 2025".which is also an important prerequisite for improving the level of China's equipment manufacturing industry.In addition,the large-stroke,high-precision five-axis gantry-type machine tools are of great importance in processing large and heavy complex parts,and play a vital role in the aerospace,power generation equipment,automotive industry and other fields.However,the more complex machine tool structure,kinematic relationship and larger machine stroke make it difficult to guarantee the machining accuracy of the five-axis gantry-type machine tools.Volumetric error describes the geometric deviation of the tool point in the machining space,which is a comprehensive expression of the geometric errors of the feed components of the machine tool.Therefore,it is very suitable for describing the final machining accuracy of the CNC machine tool.This paper focuses on the volumetric error modeling,error element identification and error compensation in the five-axis gantry-type machine tools.The main contents are as follows:(1)Based on the multi-body system theory and homogeneous coordinate transformation method,the volumetric error model of the double-swivel five-axis machine tools is established.First,according to the multi-body theory,the topology of the machine tool and the low-order body array are established,and a general method for constructing the kinematic relationship between bodies under the multi-body system theory is given.Secondly,considering the diverse forms of geometric errors,an error motion matrix is constructed by means of homogeneous coordinate transformation.Then,a coordinate system for defining geometric error elements is constructed,and the actual motion transformation matrix of the adjacent coordinate system is calculated step by step according to the action forms of different geometric errors.Finally,the error motion of each axis is transmitted to the tool tip along the kinematic chain to establish the volumetric error model.The model introduces the machine tool size parameters,so that the geometric error components in the model can directly trace the geometric defects of the corresponding shaft guide,which has certain guiding value for machine tool debugging.(2)A measurement scheme is formulated for the geometric errors of the translation axis and the rotation axis,and the identification model is derived.First of all,with regard to the translation axis,the measurement process and identification principle of the nine-line method are systematically analyzed,from which the principle deviation of the traditional nine-line method is pointed out,and a new improved solution is proposed considerring the measurement coordinate system,solution model,and measurement point combination.Secondly,simulation analysis verifies that the improved identification model and optimized measuring point combination can significantly improve the stability of geometric error identification.Then,in terms of the rotation axis,different measurement schemes were developed for two axes,and the identification model of geometric error was derived.Subsequently,a correction method is proposed for the interference of the tool axis offset to the measurement results,and the MATLAB is used to decouple the corrected data.Finally,the identification results show that the radial offset of the B and C axes is significantly higher than the axial offset and the angular error of the other axis is small except for the angular positioning error of the B axis.(3)Several error compensation strategies are proposed for the geometric errors of translation axis and rotation axis.First,in terms of the translational axis,the pitch compensation module built into the CNC system is used to precompensate the positioning error.Secondly,for the two basic feed modes of linear and circular path,compensation algorithms for linear interpolation and circular interpolation are designed respectively.Experiments show that both algorithms can correct the offset of the feed path to a certain extent.Then,regarding the rotation axis,based on the analysis of the geometric error sensitivity of the rotation axis,the error compensation principle is determined,and the idea of point-by-point compensation for the rotation path is proposed.Subsequently.The error compensation experiments were carried out for the four typical path forms of the rotating shaft.The final results show that whether it is a single rotation of the rotating shaft or a three-axis or five-axis linkage,the point-by-point compensation method can significantly reduce the linear geometric deviation,which verifies the effectiveness of the compensation scheme.This research has certain theoretical and practical significance for creating a spatial error model in a complete working domain,quickly measuring and identifying geometric errors of translational and rotating axes,and improving the machining accuracy of five-axis gantry-type machine tools.