Non-Contact Fringe Projection Method with Phase Stepping

摘要

When designing large, flexible engineering structures, mode shapes and natural frequencies are often of great interest In the space industry, for example, where structures tend to be relativity light, flexible and sometimes large in overall size, low frequency vibrations can achieve significant amplitudes. Testing such structures using conventional accelerometers may add sufficient mass to influence the structure's natural response and/or mar its surface. A non-contact method of studying natural frequencies and mode shapes is, therefore, desirable. Presently there are several non-contacting methods for the study of surface characteristics and vibrations. Holographic interferometry [1] and moire interferometry [2] are two techniques that have been used to measure both surface contours and vibrational characteristics. Because of the use of a coherent light source, these methods tend to be used for the study of relatively small objects. Shadow moire [3] is another non-contacting method that has been applied to vibration measurement. This technique requires placing a reference grid very close to the object being studied, which can cause problems due to coupling effects when the gap between the grid and object is filled with air. As a non-contact system for the measurement of surface characteristics, the fringe projection method is well documented [4]. Fringe projection is the practice of projecting a grid or fringe pattern onto a surface and viewing the distortion of the pattern on the surface from a different direction. Fringe patterns can be produced either interferometrically or by using conventional imaging systems. A review of various fringe projection and interferometry methods are given in [4] and [5]. feveral authors have worked on variations and problems associated with attaining phase information from projected fringe images obtained using both interferometric and conventional projection systems [6]-[10]. Recently Sutton et al. [11] proposed a method of using projected fringes to measure vibration of a surface. Their method obtains full-field phase information from a single fringe image. In this paper, we discuss a white-light fringe projection method for vibration analysis using multiple phase shifted fringe images and present preliminary experimental results obtained with such a system.