Calibration method of liquid lens focusing system for machine vision measurement

High performance optical imaging device is an important basis for machine vision measurement, as a compact and fast electronic controlled focusing device, liquid lens has a promising application in the field of machine vision measurement. In practical, the performance of liquid lens focusing device is easily interfered by the environment, and the intrinsic parameters are difficult to be calibrated accurately, which limits their application in machine vision measurement. The problems described above were researched, a machine vision measurement system was built based on Optotune's liquid lens. By analyzing the electronically controlled focusing mechanism of liquid lens, the model between current and focal length was proposed. Then the interaction mechanism between system parameters and temperature factor and gravity factors was studied, followed by the proposition of temperature-focal length model and gravity-main point vertical position model. By combining the system models with pin-hole imaging model, a functional expression of system intrinsic parameters was proposed, and the corresponding calibration device and calibration process to acquire all the coefficients of the expression were designed and verified through experiment. Finally, to evaluate parameter calibration accuracy, measurement test was performed using a checkerboard with sides of 30 mm. Test results show that the proposed calibration method can get more accurate intrinsic parameters, the average positional error of image corner points' world coordinate mapping result is 0.10 mm, meanwhile the maximum relative error of the length of the side of checkerboard is 0.68%, the errors are reduced by 27.2% and 54.4% respectively compared with test results using intrinsic parameters calibrated by mostly used interpolation method.