Research On Key Techniques Of Line Light In Specular Surface Shape Measurement

Surface shape measurement is the crucial task for quality control, which directly determines the appearance characteristics and performance of products. Shape measurement techniques of Lambert surfaces are difficult to be used for measuring specular surfaces, because of their disadvantages such as slow speed, harsh environmental requirements, small measuring range, large computational quantity, mapping ambiguity and prone to blind areas, etc. To solve the abovementioned problems, the methods of specular surface shape measurement based on line light are studied, including laser triangulation, lateral shearing interferometry and synchronous polarization phase shift. The main research results are as follows:(1) Traditional inclined laser triangulation is introduced to specular surface shape measurement, and a method for sub-pixel center extraction of laser stripe in high noise background is proposed. Firstly, line light is projected to specular surface and modulated by the shape, then received by screen and CCD camera. Secondly, center offsets between laser stripe and reference plane stripe are calculated after obtaining the laser stripe image. Finally, the surface shape information contained by deformed light field is detected according to the triangular geometry relation among the light source, specular surface and devices. Experimental results show that extraction accuracy of the stripe center directly affects the precision of specular surface shape measurement, and the measurement error based on line laser triangulation is about ±0.25 mm.(2) To improve the measurement accuracy of line laser triangulation, two schemes for specular surface shape measurement are designed to eliminate the astigmatism. The schemes are based on the lateral shearing interferometry profilemeter proposed by María Frade, and one is without the cylindrical lens and the lens, the other is with the lens only. Firstly, shearing interferometry stripe image is obtained by using Savart birefringence crystal to later al shear the expanded line light. Secondly, the distance between surface and sensor is calculated after obtaining the frequency of stripe by Fourier transform. Finally, the shape of a whole surface profile can be measured with only one-frame image. Experimental results show that the anti-vibration and anti-disturbance ability of specular surface shape measurement can be enhanced by the self-comparison property and full common- path collinear setup of shearing interferometry, and the measurement accuracy is raised to better than 30 ?m.(3) By using the synchronous polarization phase shift technique, the problems in Fourier transform, such as the dispersion of interference signal and selection of parameters, are solved. Firstly, the polarization phase shift device composed of quarter-wave plate and polarizer is added behind the Savart crystal. Secondly, the shearing interferometry stripe images are grabbed by CCD cameras with different spatial position at the same time. Finally, the phase slope is obtained by four step phase shift and phase unwrapping, and the shape of the specular surface is measured. Experimental results show that results and the variation trend of the measurement are closer to true values, and the measurement accuracy is improved to better than 20 ?m, due to the advantages such as the anti-disturbance ability of shearing interferometry, noise canceling of phase shift, and time-sharing error eliminated by synchronous acquisition, etc.