Qingying Hu

Shiny parts measurement using color separation

In manufacturing, inspection and measurement systems have long been desired to be able to measure as many kinds of parts as possible without treating the surface. Specifically, measuring shiny parts has been a big challenge for optical metrology because of double-bounced light -- a phenomenon that light can be reflected from an area to another on the surface. The unwanted light will result in higher noise and can even make the measured results unacceptable. Traditionally, a polarizer is placed in front of both the light sources and the camera. After properly adjusting the polarizer in front of the camera, the double bounced reflected light can be blocked to some degree while the normally reflected light can go through. By this way, the extra reflections can be reduced but not totally eliminated. This paper presents a new method to totally eliminate double bounced light. Here, color light sources are used to illuminate the part and multiple cameras are used to measure different areas. Each camera views through an appropriate color filter so that only a certain color light is seen. The measured results from all cameras are then merged together to create the complete image. This method is more efficient than the traditional solution that uses polarizers. Both measurement principle and some results are given.

Multiple views merging from different cameras in fringe-projection based phase-shifting method

This paper discusses issues related to accurate measurement using multiple cameras with phase-shifting techniques. Phase-shifting methods have been widely used in industrial inspections due to high accuracy and excellent tolerance to surface finish. But so far, most such systems use only one camera. In our applications to inspect manufactured part with complex shapes, one camera cannot capture the whole surface because of occlusions, double bounced light, and the limited dynamic range of cameras. Multiple cameras have to be used and the data from different cameras must be merged together. Because different cameras have individual error sources when a part is to be measured, it is a challenge to obtain the same shape, in the same 3D coordinates system from all cameras without data manipulation such as iterative registration. This paper addresses this challenge of data registration. The error sources are analyzed and demonstrated and several paths for error reduction are presented. Experiment results show the significant improvement obtained.

Image bias correction in structured light sensor

Structured light techniques have been used in a lot of applications. As a two-dimensional optical measurement method, structured light sensors are faster than one-dimensional point triangulation sensor while easier to calibrate and move than full-field three-dimensional sensors. The accuracy of structured light sensors mainly depends on the accuracy of both calibration and beam center extraction. In some applications with complicated surface shapes, the extracted center may not be the actual “true” center, which results in image bias. This paper presents a method to compensate the image bias and improve the measurement accuracy of structured light sensors. The basic concepts of image bias correction are given and some initial results are provided.

Smooth 3D edge detection in scarfed composite surfaces

We describe a new algorithm for 3D edge detection on composite part surfaces based upon phase shift analysis. Current phase shift based algorithms generate 3D surface profiles, they do not directly compute 3D edge information. The proposed algorithm has been developed in this context for 3D edge detection. One advantage of this method is its ability to measure smooth 3D edges that cannot be accurately measured using traditional contact techniques. A dense 3D point cloud representing part edges are computed, all such edges in view may be computed simultaneously. The inherent accuracy available with phase shift analysis is leveraged for detecting the smooth edges with minimal error. Experimental results with some test parts are presented.

Edge measurement using stereovision and phase-shifting methods

Three-dimensional edge measurement is critical in many applications such as blade manufacturing due to the stringent requirements on aerodynamic performance of blades. Optical metrology techniques provide very good tools in terms of speed and accuracy, but face some challenges as well, such as discontinuity, irregularity, and size variation in the edge shape. This paper presents several methods that include stereovision techniques to capture a sharp edge and phase-shifting to capture an edge profile of a blunt edge. For sharp edge measurement with stereovision, multiple cameras are used to view the edge from different directions. Then the captured images are processed to obtain the point cloud of the edge. For blunt edge measurement, a phase-shifting method is applied. After the edge profile is obtained, all edge information can be extracted. In this paper, different illumination methods are discussed and different edges are measured. Experimental data shows that these methods are practical in obtaining accurate edge or edge profile.

Multiresolution 3D measurement using a hybrid fringe projection and moire approach

Projected fringe methods have lead to a wide selection of commercial sensors for 3D measurement applications. The basis of these systems is a projector such as an LCD presentation projector that is used to generate a coarse pattern that is shifted across the part and viewed by a camera. Three or more images with a small pattern shift between each are sufficient to obtain a detailed 3D map using phase shift analysis methods. The limitation of these systems has been that to obtain high resolution the system is limited to viewing only a small field-of-view. Moire methods are a way to leverage this resolution, particularly on flat or only slightly contoured areas. The approach described here takes advantage of moire methods used in connection with the fringe projection method to provide high resolution over key reference areas, while still provided a less precision measurement over a larger region.