Precision Inspection And Error Compensation Technique For Parts With Complex Surfaces

With the development of manufacturing technology and equipment, the demand for parts with precise complex surfaces is increasing. The corresponding inspection technology is needed to inspect and control the machining accuracy. Current the inspection technology based on the coordinate measuring machine(CMM) is widely used for the form and position accuracy inspection of precision parts. However, there are still some problems, such as the positioning errors caused by relocation which are difficult to eliminate, larger parts can not be inspected, etc. Recently, more attention has been given to direct inspection on numerical control machines and development of a closed-loop processing system of machining-inspection-compensation. To solve the problems of on-line inspection of complex surfaces, some important technologies, such as planning the measured points using a standard computer-aided design (CAD) model of StereoLithographic (STL) format, error compensation of obtained data, decomposition and compensation of machining errors, are illustrated in this thesis. The aim is to improve the machining accuracy of complex surfaces by on-line inspection and in situ error compensation. The research contents and main conclusions of this thesis are as follows:(1) The precision inspection and error compensation technology of complex surfaces are reviewed. The existing on-line inspection systems, inspection methods and error compensation technology are investigated. Some relevant problems in this field are discussed.(2) The measured point planning method based on the CAD model with STL format is introduced. To overcome the redundancy and independency of triangular meshes, the method of topological reconstruction of a mesh model is put forward by building the topological relations. For planning the measured point using the section line method, two methods are presented to approximate the original CAD model, i.e. direct intersection method and midpoint offset parabolic intersection method. The reconstruction technology of non-uniform B-spline curves is employed. The method of chord deviation is presented to obtain measured points with adaptive distribution.(3) Factors influencing the inspection accuracy of an on-line inspection system are discussed and error compensation is studied in detail. The main errors of the inspection system, i.e. geometric errors of machine tools, pretravel errors and probe radius errors, are analyzed. The geometric error model of a three-axis numerical control machine is established and the individual error is identified with a laser interferometer. For the pretravel errors, the prediction and compensation method is presented based on a radial basis function algorithm. The distribution of pretravel errors is obtained and the real-time compensation of these errors is conducted. Estimation of the normal vector of triangular mesh vertex is also improved. The probe radius error compensation is conducted according to the accurate normal vectors of measured points.(4) According to the measurement of machining accuracy of complex surfaces, an in situ error compensation machining method is presented to improve the machining accuracy using only one positioning. For this, a decomposition and compensation methodology of machining errors is presented based on the spatial statistical analysis. A deterministic surface, which is defined by superimposing the systematic errors on the nominal surface, is established based on bicubic B-spline surfaces. Through the spatial statistical analysis of the residual errors of a regression model, the machining errors are decomposed into systematic errors and random errors. According to the decomposed systematic errors, the numeric control codes are modified and the in situ machining errors compensation is conducted. After the compensation, the machining accuracy is greatly improved.(5) To improve the efficiency of the compensation machining of surfaces, the method of local machining error adaptive compensation is presented according to the different values and the distributions of machining error. The main steps of local machining error compensation include the determination of local compensation areas, judgment of the relative position of cutter locations and local compensation areas, and generation of local comensation tool paths.(6) Based on the investigations mentioned above, using Visual C++ 6.0 and OpenGL as development tools, an on-line inspection and in situ error compensation integrated system for complex surfaces is developed. Experiments of numerical control machining, inspection and in situ compensation machining of several surfaces are conducted to validate the effectiveness of the proposed methodology. Some standard parts are used to evaluate the inspection accuracy of the developed system. The inspection accuracy of the system is compared with that of a CMM to validate the practicability of the system.Finally, the conclusions of this thesis are summarized and future research directions are presented in the field of on-line inspection and error compensation.