Limited by the imaging dynamic range of the camera, the phenomenon of over-exposure and over-dark often occurs in the 3D measurement of strong reflective sheet metal parts, resulting in incomplete measurement result. One of existing methods such as multiple exposure can measure most of the visible area under a single viewpoint, but the visible area with too small or too large incidence angle still cannot be measured. To solve this problem, in this paper, a method of viewpoint planning for sheet metal parts with strong reflection is proposed. The method introduces the surface reflection model of reflective sheet metal parts into viewpoint planning to achieve the synchronous optimum of measurement efficiency and data integrity. Firstly, according to the measurable region of the surface structured light 3D measurement system and CAD model, the candidate viewpoint set is randomly generated in the sampling space, and the visibility matrix is constructed by analyzing whether each candidate viewpoint is visible to each patch of the model. Then, the surface reflection model of sheet metal parts with strong reflection is constructed, and the reflection coefficient of the visible patches under each viewpoint is calculated according to the reflection model. Based on this, the measurability of the visible patches of the viewpoint under multiple exposures is calculated, and the visibility matrix is updated. Lastly, through the viewpoint quality evaluation function constructed based on the data coverage increment and multipleexposure time, the viewpoint with the highest quality is selected heuristically until the coverage requirement is met. Experiments show that the algorithm can improve the measurement efficiency and ensure the integrity of the measurement data.
The geometric dimensions and tolerances of blades, which are critical parts of turbomachinery with complex features, must be strictly controlled to ensure the efficiency and safety of the engine. Optical-based inspection systems for blades are increasingly receiving attention because of their high efficiency and flexibility. However, as a key issue in blade inspection, the matching of the part coordinate system and machine coordinate system directly determines the measurement accuracy and automation. The blade surface is complex and has no obvious features, and accurate and rapid matching thus remains a challenging problem to solve. To overcome these problems and realize the accurate inspection of blade profiles, an automatic and high-accuracy matching method for a blade measurement system integrating fringe projection profilometry and conoscopic holography is proposed in this paper. First, automatic rough matching is realized making use of the ability of fringe projection profilometry to quickly obtain high-resolution cloud of points and improving the four-point congruent sets algorithm. The path of the conoscopic holography measurement based on the calibration and rough matching result is then planned, to sample high-precision and uniform cloud-of-points data on the blade surface. Finally, a fine matching optimization algorithm is implemented with the signal-to-noise ratio as the weight. The results of simulation experiments and inspection case studies demonstrate that the proposed matching method is efficient and accurate.
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