Study On Key Techniques Of High-speed Laser Scanning And Stroboscopic Imaging For 3D Size Precision Measurement

In the three-dimensional(3D) size measurement, the commonly used Digital Light Processing(DLP) technology is difficult to project high spatial resolution, continuous sinusoidal fringes and the refresh rate is limited. The contrast ratio and precision of laser interference fringe are not ideal, and the equipments are somewhat complicated and with poor adaptability. Therefore, these devices are limitedly used in 3D online precision measurement, especially for the micro or small size metal devices with complex reflectivity surface(semiconductor, microelectronics devices, etc.). To solve the problems among the measuring speed, accuracy, and adaptability in 3D precision measurement, a high-speed laser scanning and stroboscopic imaging fringe pattern projector is designed, and the principle of stroboscopic imaging for 3D measurement is studied. The main contents are as follows:1. A high-speed laser scanning and stroboscopic imaging fringe pattern projection method is proposed, which combines the high measurement speed of the overall projected grating with the high accuracy of the line-structured laser light source. The principle of stroboscopic scanning projection is presented. The optical and mechanical system, photoelectric detection circuit and FPGA real-time control circuit are designed.2. The characteristics of the fringe patterns projected by the proposed laser scanning stroboscopic imaging projector are analyzed. The projected high brightness fringe patterns have high precision, and high refresh rate. In addition, the intensity, frequency, pulse width and phase shift can be controlled in real time by dynamic programming.The characteristics of line width, density, sinusoidal distribution and the stability of this system are verified by theory and experiment analysis. The result shows that the projected fringe patterns can not only be used as high density quasi sinusoidal gratings, but also a high resolution multiline structured laser light source, which can be applied for 3D shape precision measurement.3. A 3D precision measurement system is desgned by using the proposed fringepattern projector, especially for measuring the micro or small size metal devices with complex reflectivity surface. The 3D measurement principles are studied. The space angle constraint model(SACM) is proposed, which can convert to two kinds of easy to implement models according to pulse width modulation properties of the fringe patterns. The two instance models are spatial phase mapping model and multiple structured light plane constraint model. The two models are unified in mathematical and physical sense.4. The fringe analysis algorithms, and the calibration methods are also studied. The measurement models do not need additional reference plane constraint so that they are easy to realize. The calibration process is simple, the calibration results and accuracy are reliable, which shows that it is suitable for field implementation. The root mean square error(RMSE) of gauge block measurement for accuracy verification is 6.7μm. And the printed circuit board(PCB) solder paste measurement shows the practicality of the proposed system in fringe pattern 3D size precision measurement.5. The phase errors in fringe analysis and the precision ascension method are studied. Considering the non-sinusoidal modulation phase errors in the actual measurement experiment of PCB solder paste, some new phase detection algorithms based on the typical phase shift method or Fourier transform method are proposed. The phase shift method based on spatial smoothing constraint, the time-frequency analysis method based on wavelet transform(WT) and the adaptive fringe analysis method based on empirical mode decomposition(EMD) are introduced in detail. These methods can improve the reliability and applicability of the proposed system.