三轴视觉测量系统自动对焦技术

Auto focusing technology of three-axis vision measuring system

为了减少三轴视觉测量系统在对焦过程中的时间消耗和提高对焦的准确性,提出基于光学离焦模型的自动对焦算法。自动对焦算法评价函数采用Tenengrad梯度函数,搜索算法分两步:(1)将光学离焦模型分解成两个曲线函数,通过采集4张图像的清晰度值和x轴坐标求出两条曲线函数,最终得到两条曲线的交点位置,交点位置即为正焦位置粗定位位置;(2)在交点位置采集1张图像以及在交点左右两侧各采集2张图像,通过高斯函数拟合得到拟合高斯函数的均值,均值即为准确的正焦位置。为了验证本方法的有效性,首先进行10次重复性试验,验证算法粗定位的重复定位误差4.1μm。其次,在粗定位位置采集1张图像及其两边各采集2张图像,通过高斯拟合得到精确正焦位置,10次精确位置的重复定位误差为5.1μm。该算法只需采集9张图像,得到的合成标准不确定度为2.12μm。该方法提高了三轴视觉测量系统的对焦效率和精度。

To reduce the time consumption of the three-axis vision measurement system in the focusing process and improve the accuracy of focusing, an auto-focus algorithm based on the optical defocus model was proposed.The evaluation function of the auto-focus algorithm adopted the Tenengrad gradient function. The search algorithm was divided into two steps:(1) The optical defocus model was decomposed into two curve functions,the two curve functions were solved through collecting the sharpness values of 4 images and their x-axis coordinates and the intersection position of the two curves was obtained. The intersection position was the approximate position of the on-focus position;(2) One image was collected at the intersection position and two images were collected on the left and right sides of the intersection point. The mean value of the fitting Gaussian function was obtained by Gaussian function fitting, which was the exact on-focus position. To verify the method,firstly, 10 times repeatability experiment was processed, the error of repeatability was 4.1 μm. Secondly, an image at rough focus position was obtained and 2 images on the left of rough focus position and 2 images on the right were obtained. The precise focus position was given by Gaussian fitting. The 10 times repeatability error of precise focus position was 5.1 μm. The algorithm only needed to collect 9 images, and the synthetic standard uncertainty was 2.12 μm. The algorithm improves the focusing efficiency and accuracy of the three-axis vision measurement system.