Key Techniques For 3D Profilometry Based On Sinusoidal Phase Modulation And Fiber Optic Interferometric Fringe Projection

This paper is supported by the National Nature Science Foundation of China(Dual-wavelength fiber-optic interference fringe projection three-dimensional shape measurement method, NO. 51275349) and State Key Laboratory of Precision Measurement Technology and Instrument Explore Fund, focusing on the surface profile of small-size object with high precision. A surface profile measuring system with sinusoidal phase modulation and fringe projection has been developed based on the phase profilometry. The key technologies are mainly as follows: Combining Mach-Zehnder structure and Young's interference principle, the high-density interference fringes can be projected on the surface. A closed-loop phase compensation system is built to stabilize the phase of fringe images. Four consecutive fringe images are captured over one modulation period. Fringe images are pre-processed and then the phase of fringe images can be extracted. The main research content are as follows:1. An all-fiber interferometer system with sinusoidal phase modulation was established, and SPM can be achieved by change the length of the fiber arms. The high-density interference fringe projection of cosine distribution has been achieved on the surface using Mach-Zehnder interferometer structure and Young's double pinhole principle. A fringe projection mathematical model is built for this fiber interferometer, and influence superposed upon the projected fringes by the system structure parameters.2. Aiming at eliminating the effect of the environmental factors(especially the temperature fluctuation and mechanical vibration) for 3D profilometry measurement, a closed-loop phase compensation system is built. The Fresnel reflection on the exit faces of two interference arms is used to generate a Michelson interference signal, which is detected by the photoelectric detector and send into phase compensation sub-system. Phase difference between interference arms can be abstracted using the Phase Generated Carrier(PGC) method, and calculated by using utilizing the Coordinated Rotation Digital Computer(CORDIC). Then the fringe phase can be achieved with a good stabilization performance. The experimental results show that phase shift can be reduced to 5.25 mrad.3. A sinusoidal phase modulation integrating-bucket phase extraction method based on fringe projection has been developed. The effection of additional noise has been analyzed and the optimal modulation coefficient and initial phase have been achieved. Four consecutive fringe images over the four quarters of one modulation period can be captured under the optimum sinusoidal phase modulation condition. The wrapped phase distribution can be achieved using four-step SPM integrating-bucket method.4. The fringe images can be affected by the system noise and random noise. Image enhancement and filtering processing are essential to improve the contrast of fringe images without image distortion. A high quality of fringe images can be achieved by suppressing the influence of random noise. The phase distribution of fringe images can be extracted with sinusoidal phase integrating-bucket modulation.