Research On The Technology Of The Measurement Planning And Localization For The Complex Curved Surfaces

Workpiece with complex curved surfaces becomes an important target in digital manufacturing in recent years. Aiming at the main difficulties such as the automatic inspection path programming, the precise localization of the workpiece, the accurate measurement of the workpiece clamped by the aided equipment and the accuracy of the data evaluation, this dissertation provides an extensive and systematical research on the subject of measurement, localization and precision assessment of the workpiece with complex curved surfaces under the background of practical engineering. The main contents and creative ideas are as follows.1.The general mathematical measurement model based on the characterized curves of complex curved surfaces is systematically researched and a method to establish the profile model of the globoidal indexing cam is put forward according to the required motion, geometry of the follower and the theory of envelopes for a one-parameter family of surfaces. According to the designed parameters of the globoidal indexing cam, the mathematic points of the profile and the characterized curves on the globoidal indexing cam can be obtained by the mothed, both the efficiency and the accuracy of the measurement are improved.2. The critical issues of the automatic inspection path programming and data processing of the workpiece clamped by the scale division device are researched,an effective method of unified coordinate system for compensating the eccentric error of the axis between the scale division device and the integral impeller is presented. The precision of the measurement is improved as a result of the modification of the inspection path planning and the processing of the measurement data on the basis of this method. The methods proposed above are verified by the measurement of the integral impeller.3. The measurement and localization of complex curved surfaces is researched and a stepwise refinement approach to localize the integral impeller for CMM inspection based on the freedom constraint and classification of the characteristics precision is developed. First step of the method is to catalog the surfaces on the integral impeller by both the influence on the orientation constraint of the model and the forming precision of each part of the workpiece. Then a localization datum selection scheme is produced based on the surface classification, which helps coarsely positioning the workpiece on the CMM. The localization refinement process is carried out by a data registration algorithm after the data points are captured. In the algorithm, registration transformation matrix is decomposed and constraints are applied on some degrees of freedom and a more accurate result is achieved. Experiments on localizing an axial-flow impeller are conducted and show that the average error of the blade profile is reduced.4.A new shape registration technique considering the regional difference in precision is proposed. The shape of a mechanical part is composed of a number of specific feature regions, each with their own precision level. If the shape registration between the scanned data and the CAD model is performed when ignoring the precision difference, an inaccurate alignment result will be produced due to the deviation of low fidelity regions.A new registration method taking into consideration of the region precision is proposed. The procedure is divided into several region-to-region matching steps. Firstly, the regions with high precision are matched. This matching transformation brings a set of standard orthogonal bases for the transformation space, and at the same time it gives registration constraints on some principal directions. Then the low precision regions are used to do a new matching. By projecting the new transformation on the previous standard orthogonal bases and setting some terms to zero, the model transformation is restricted and has no movement on the principal directions determined by the high precision regions. The method produces high matching accuracy on high precision regions and low matching accuracy on low precision regions, which is suitable for practical utility in engineering. The method is verified by experiments on a real impeller.