Trace and profile measurements for dynamic objects

Abstract

ABSTRACT A fringe projection technique for finding the absolute shape at a sequence of time for a dynamic object is proposed. This method makes it possible to simultaneously identify the trace and the speed of the dynamic object. Keywords: fringe projection, projected fringe profilometry, sinusoidal grating 1. INTRODUCTION Trace detection and speed sensing in 3D co ordinates have become an interesting topic in computer vision and automatic optical inspection [1-3]. However, it is a challenge to analyze the dynamic 3D problems just from a set of 2D images. Depth positions and moving vectors in the depth axis cannot be described directly by the 2D images. One solution is to describe the 3D profile of the dynamic object in absolute coordinates at a sequence of time. Trace and moving speed is then defined by the change in positions w ith reference to the associated points. To achieve the 3D profile measurement and simultaneously to trace the motion in the world coordinates, the Fourier transform profilometry [4] combined with a video camera is a desirable approach. A sinusoidal fringe pattern is projected onto the dynamic surface. Fringes are obser ved by the video camera at a different viewpoint. With the Fourier transform method, phase of the fringes in each image frame can be extracted to retrieve the 3D shape. The retrieved 3D shapes obtained at a sequence of time are therefore analyzable to describe the trace or speed of the object. When the inspected object moves with a higher speed, or ro tates with a higher frequency, the image acquisition time is required to be short enough that the observed image is not blurred by motion. As a result, the image intensities might not be high enough to extract the phases. An alternated approach is using a high power strobe lamp which generates periodic pulses as the light source. Fringes are illuminated periodically by the high power strobe lamp and then projected onto the dynamic object in a sequence of time. Th e duration of each pulse is so short that the dynamic object seems static in each image frame. Fringes frozen in each image frame by the pulsed illumination are therefore available to retrieve the 3D shape. Unfortunately, this approach still fails for ultra-fast moving objects, because the displacement of the object between two sequent measurements is generally much larger than the period of the projected fringes. The discrete measurements in time domain make it difficult to discern the correspondence of the fringe orders between two sequent image frames. It becomes a problem to un wrap [5] the phase without ambiguity. Phase unwrapping for fringe projection techniques has been extensively studied [6-12]. However, most of these techniques were developed to identify the fringe orders for objects with discontinuities, not for spatially isolated and dynamic objects. Among these techniques, the color-encoded fringe projection technique is a promising approach [13]. The sinusoidal fringes are spatially encoded with colors. Fringes are discerned with reference to the arrangement of colors. It takes only one shot measurement. To identify the fringe order in each image frame is desirable. However, this approach is sensitive to colors. Errors might occur when the inspected object is colorful. In this paper, a binary-encoded projection technique is proposed to identify the fringe orders. This encoding scheme provides a more reliable performance since it is not sensitive to the observed colors. Surface profile can be correctly described in absolute coordinates. Using a strobe source to inspect an ultra-fast moving object becomes desirable. Trace and speed of the dynamic object can be id entified from the sequent measurements.