| Free-form surfaces are widely used in aerospace,automotive and other fields.Surface inspection and reconstruction are of great significance for profile error evaluation and adaptive machining.Since free-form surfaces usually have complex geometric shapes and high measurement accuracy requirements,they are generally inspected by coordinate measurement machine(CMM)equipped with touch-trigger probe or on-machine measurement system(OMM).For touch-trigger measurement,the sampling strategy plays a decisive role in improving the measurement accuracy and efficiency,which is determined by the sample size and the distribution of sampling points.However,due to the complexity of free-form surface geometry,it is difficult to establish a mathematical model of sampling accuracy with respect to sampling strategy.The measurement path planning is the key to ensure the efficiency of free-form surface inspection.It should consider not only the inspection time,but also the stability and accessibility of probe motion.Surface reconstruction is an important step for adaptive machining.The smaller the deviation between the reconstructed surface and the actual surface,the higher the precision of adaptive machining.However,the measurement points of touch-trigger probe are usually few,and the limited number of measurement points will affect the quality of surface reconstruction.Therefore,a high-precision surface reconstruction method based on touch-trigger measurement should be studied.Large thin-walled curved surface parts are widely used in aerospace field.In order to reduce weight and ensure stiffness,there are usually a large number grid ribs on the curved surface.Because of the inevitable deformation of real parts,it is a difficult task to machining contours on thin-walled freeform surface parts.If the milling is carried out according to the tool path of the designed contour,it will lead to undercutting or overcutting,and affect the precision of the contour,so as to reduce the stiffness of the whole part.Therefore,it is necessary to measure the deformed surface,reconstruct the actual surface,and then recalculate the contour position and shape according to the reconstructed surface,and reduce the deviation of the adaptive contour from the designed contour to realize the contour adaptive machining.In this paper,in order to improve the performance of the free-form surface inspection methods,by the means of touch-trigger measurement,from two aspects of measurement precision and efficiency,considering sampling methods and inspection path planning as research perspectives,research on the free-form surface sampling point planning and interference-free inspection path planning is conducted to achieve the high efficiency and high precision measurement of free-form surfaces.Then the algorithm of free-form surface reconstruction is studied and the high precision surface reconstruction based on touch-trigger measurement is realized.Finally,based on the touch-trigger measurement and surface reconstruction,the contour adaptive machining method of deformed free-form surface is studied to reduce the deviation of the deformed contour compared with the design contour.The main research contents and innovative achievements of this paper are as follows:By applying triangular mesh simplification to sampling point planning,adaptive sampling under multiple constraints is realized.Because triangular mesh has outstanding advantages in expressing surface features,and its structure is stable and easy to modify,the free-form surface is transformed into dense triangular mesh.Under the constraint of discrete curvature,an improved quadric error metrics(QEM)is established to determine the order of edge contraction and the optimal target vertex,and the sampling point density is controlled by limiting the triangle edge length.On this basis,an adaptive sampling algorithm under multiple constraints is proposed,and the feasibility and effectiveness of the proposed method are verified by simulation and experiment.Compared with the uniform sampling and the existing adaptive sampling strategy,results show that the proposed method reduces the sampling error by 30.8% and 13.3% when the sample size is64,respectively.A path planning method for free-form surface inspection based on grouping measurement is proposed.Firstly,the accessibility cone is defined to describe the interference-free measurement direction of the measurement points.By transforming the accessibility problem into the visibility problem,the accessibility cone is calculated by GPU.The measurement points are divided into several groups by the proposed classification algorithm,and in each group,the measurement points can be inspected in the same probe direction.Finally,the measurement points in each group are sorted according to their spatial positions to shorten the total path length,and a global interference-free inspection path is generated according to certain rules.This method and the existing methods are applied to a impeller blade to conduct experimental research,and the probe posture variation and the total inspection time are analyzed.Compared with the existing method,the proposed method significantly reduces the motion of the probe rotation axis and reduces the total measurement time by 38.2%.The surface reconstruction method based on the designed surface modification is studied.The profile error of free-form surface was decomposed into surface location error and deformation error.The surface matching algorithm based on the point to surface distance function is used to adjust the position and direction of the surface.One of the advantages of this algorithm is that the probe size is taken into account,so there is no need to further compensate the probe radius.Then,the surface modification algorithm considering the thin plate energy functional is used to adjust the geometry of the designed surface,and the reconstructed surface is obtained.Experiments are conducted to verify the effectiveness of the proposed method.Results show that the proposed method reduces reconstruction error by 37.9% compared with the existing method.An adaptive machining method of free-form surface contour based on optimized parallel projection is proposed.The geometric data of free-form surface are obtained by OMM,and the measured data are used to reconstruct the actual surface.After matching the reconstructed surface with the design surface,the design contour is projected onto the deformed surface to obtain the locating contour.The iterative closest point(ICP)algorithm is used to adjust the design contour to match the locating contour.Finally,the projection direction is optimized with the goal of minimizing the contour deformation,then the adjusted contour is projected onto the deformed surface along the optimal direction to obtain the adaptive contours.The effectiveness of the proposed method is verified by machining and measurement experiments.In this paper,the method of high precision and high efficiency inspection and reconstruction of free-form surface based on touch-trigger measurement is systematically studied,and on this basis,the adaptive machining of contour on deformed surface is studied.This paper provides new methods for inspection of free-form surface and contour adaptive machining. |
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