Study On Nonlinear Phase Error Compensation Methods In Fringe Projection Profilometry

Recently, fringe projection profilometry (FPP) develops very rapidly and it is typical optical profile measurement of objects with advantages of full-field, real-time, fast, non-contact and non-destruction and so on. It not only aplly to measure objects with continuous surface, but also can measure objects with non-continuous surface. With the development of science and technology, the precision demand on three-dimensional topography of the projected fringe method gets higher and higher, which not only depends on the measuring precision of equipment, but also the error cannot be ignored in the measurement process. Error source of projection fringe is mainly due to the nonlinear phase error caused by the projector. After the further study on the nonlinear phase error, an improved look-up-table method (LUT) and a reverse phase method (IPM) for the nonlinear phase error compensation are presented respectively. The main work of this thesis focuses on the following aspects.Firstly, the background and the development trend the basic principles of FPP are given, including the nonlinear phase error compensation method, single-frequency and multi-frequency fringe unwrapping (MPU) techniques, and the relationship between the height and phase. Through the simulation analysis of residual parameters using a polynomial model of the nonlinear error, it finds that the frequency of nonlinear phase error is4times of the fringe frequency for the four-step phase-shifting method. When the ratio of second-order and third-order residuals is closer to1under samefringe peroid, the nonlinear phase error gets small. If the residual ratio is a constant, the amplitude of nonlinear phase error does not vary with the fringe peroid.Then, an LUT method is improved and verified by single frequency and multiple frequency FPP experiments of a calibration block. The experimental results show that the nonlinear phase error is obviously compensated using the improved LUT. It proves the reliability of the improved LUT method.Finally, an IPM&MPU method is proposed for the nonlinear phase error compensation. The nonlinear phase changes its sign by introducing a phase-shift of π/4to the phase-shifitng method so that the nonlinear phase can be compensated. A plane, a spherical cap and a stage are simulated to analyze the nonlinear phase error compensation of the IPM&MPU method, respectively. For the four-stage phase-shifting method, the nonlinear phase has four times of the projected fringe frequence. The single-frequency and multiple-frequency of fringe projection experiments are performed to prove the effectness and feasibility of the proposed method for nonlinear phase error compensation. It improves the accuracy of the measured object.