Research On 3D Surface Measurement Of Highly Reflective Objects Based On Raster Projection

Compared with the traditional contact 3D measurement method,the grating projection 3D measurement technology is widely used because of its advantages of wide applicability,non-contact and high precision.However,for the measured object with a strongly reflective surface,the image acquired by this method may be locally over-bright or over-dark,resulting in phase error during phase resolution,resulting in the absence of the calculated point cloud,and greatly reducing the measurement accuracy of the object.Therefore,in order to improve the measurement accuracy of raster projection three-dimensional measurement technology for strongly reflective objects,the following work is done in this paper:Firstly,based on a flexible measurement model,the standard four-step phase-shifting method is used to extract the phase principal value of the measured object,and then it needs to be unrolled to get a complete and continuous absolute phase.In this paper,the dual-frequency heterodyne method is used to carry out phase unwrapping.Finally,the unrolled absolute phase is combined with the parameters obtained from the system calibration to solve the three-dimensional point cloud data.At the same time,an improved phase unwrapping method based on three-frequency heterodyne is proposed to solve the phase hopping caused by the error.Firstly,the phase principal values of three frequencies are obtained by the standard four-step phase shift method,and two different wrapping phases with lower frequencies are obtained by the double-frequency heterodyne phase unwrapping method.Then,the theoretical value of the fractional part of fringe series and the actual value of the fractional part of fringe series were calculated by the principle of two-frequency heterodyne phase solution,and the absolute value of the difference between the two was used to set the threshold,which was related to the grating fringe pitch.The threshold was compared with the above difference,so as to adjust the number of cycles.The number of jumps is reduced to correct the absolute phase.The experimental results show that compared with the traditional two-frequency heterodyne phase solution principle,the proposed method can eliminate the jump point more than 96.8%,and has strong anti-interference to noise.Secondly,in order to solve the problem that the grating fringe cannot be clearly projected on the surface of the measured object due to obvious reflection and scattering when measuring the surface of the strongly reflected object,a three-dimensional measurement method combining the projection grid phase method and the improved multi-exposure image fusion technology is proposed.Firstly,the image of the object under different exposure time is collected under the white light projected by the projector,and the mask image is made by removing the overexposed area in the image by setting the threshold.Secondly,the grating fringe image acquired at the corresponding exposure time is multiplied with the mask image at the same exposure time,and the fringe image is obtained with the highlight area removed.Then,appropriate coefficients are selected to superimpose fringe images after linear transformation in order to prevent new highlight areas from being generated after superimposition.After that,the gray value of the superimposed fringe image is normalized,and finally the gamma transform and gray value are restored.At this time,the fringe image can remove the areas that were originally too bright or too dark in the image,and the fringe is clear and uniform.The three-dimensional point cloud data of the measured object can be solved by using the phase resolution principle and calibration parameters.In the experiment,three samples with smooth,hollow,concave and convex characteristics are selected for measurement and analysis.The method in this paper can not only remove the overexposed area in the image and no new overexposed area will appear in the fusion,but also improve the area with low gray value of the image,so as to ensure the quality of the fringe image and enable the phase information to be extracted in the subsequent steps.At the same time,the feasibility and applicability of the proposed method are verified.Finally,based on the flexible system model,a 3D measurement platform of structured light was built to reconstruct the 3D morphology of different metal objects.The method in this paper and other existing methods are used to compare and analyze the point cloud images obtained from the measurement of metal objects.The experiment proves that the method in this paper can detect more than 99.3%three-dimensional point clouds.Meanwhile,the phase error of the selected exposure area is analyzed.It shows that the method proposed in this paper does not introduce large errors,which guarantees the measurement accuracy and verifies the universality and effectiveness of the method proposed in this paper.Figure [41] table [10] reference [62]...