Geometric Measurements With Grating Projection

The method of grating projection can provide high-quality images with a low demandfor environment and obtain a high degree of accuracy with a high speed. In recent years, withthe development of micro-electronics technique, modern optics, information theory and theappearance of computer, high capability devices, strong theories and material foundation areoffered. The method is used widely in the area of photomechanics testing such as objectshape measurement, displacement measurement for large-sized object, rotary shapemeasurement and object flatness measurement, etc. Based on the principle of sinusoidalgrating projection, geometric measurements are introduced in this paper which includesobject distance measurement, object shape measurement and object flatness measurement.In chapter one, the development of photomechanics testing and the advantage ofSinusoidal grating projection is introduced firstly. Then main research contents are included.In chapter two, the development of distance measurement and the reason why use themethod of grating projection to realize distance measurement are introduced firstly. Thenthree methods of distance measurement are presented. There are distance measuring whenCCD is put at a certain distance, triangulation based on grating projection and frequencyspectrum scanning method. Experimental principle and process are introduced in this paper.Experiment results indicate that these methods can measure the distance of the object fast andautomatically. But there are some shortcomings in these methods. Finally, comparingthe three methods，we can conclude that the method combine frequency spectrum scanningwith triangulation based on grating projection can measure the distance of non-planar objects.And it is simple in experimental operation. It also has a high degree of accuracy and speedand a low demand for environment.In chapter three, the development of shape measurement and the analysis of variousmethods of shape measurement are introduced firstly. Then by compared Phase-shift methodand Fourier transform method of grating projection, a new shape measurement withfrequency spectrum scanning is presented. This method has a lower demand for imageinformation than Phase-shift method, and a fewer amount of computation than Fourier transform method. No need to compute the phase distribution and phase unwrappingalgorithm, shape measurement is realized only by one deformed fringe pattern. In addition tothis，it can also realize the relative deformation measurement. Experimental principle andprocess are introduced in this paper. Experiment results indicate that this method can measurethe shape of the object fast and automatically and has a lower demand for environment. Therewill be a good future in photomechanics testing.In chapter four, the development of flatness measurement and main methods areintroduced firstly. Based on the research in chapter two and three, two new flatnessmeasuring methods are presented. Linear fit method when CCD is put at a certain distance isbased on the method of distance measuring when CCD is put at a certain distance, andfrequency spectrum scanning method is based on shape measurement with frequencyspectrum scanning. Experimental principle and process are introduced in this paper. Only byone deformed fringe pattern, can the flatness be measured by these two methods.In chapter five, the main research contents in this paper are included. The mainInnovative points are summarized. Based on the imaging characteristics of sinusoidal gratingprojection and its frequency spectrum, three distance measuring method, a shape measuringmethod and two flatness measuring methods are introduced in this paper. But there are someshortcomings in accuracy，scope and experimental conditions in these methods. It isnecessary to make some changes.Based on the system of sinusoidal grating projection, a new optical system is introduced.And a new method is present which can realize distance measurement, flatness measurementand shape measurement at the same time. Geometric measurements are obtained only by onesystem. It is characterized by high efficiency, speed and adjustability. It will be used widely inreverse engineering, CAD, numerical control machining, fast instrument shape casting,product quality inspection and medical science etc. But there are some shortcomings in theresearch, and it is necessary to do more research.