Guiju Song

OCT for industrial applications

Optical coherence tomography (OCT), as an interferometric method, has been studied as a distance ranger. As a technology capable of producing high-resolution, depth-resolved images of biological tissue, OCT had been widely used for the application of ophthalmology and has been commercialized in the market today. Enlightened by the emerging research interest in biomedical domain, the applications of OCT in industrial inspection were rejuvenated by a few groups to explore its potential for characterizing new materials, imaging or inspecting industrial parts as a service solution[3]. Benefiting from novel photonics components and devices, the industrial application of the older concepts in OCT can be re-visited with respect to the unique performance and availability. Commercial OCT developers such as Michelson Diagnostics (MDL; Orpington, U.K.) and Thorlabs (Newton, NJ) are actively exploring the application of OCT to industrial applications and they have outlined meaningful path toward the metrology application in emerging industry[3]. In this chapter, we will introduce the fundamental concepts of OCT and discuss its current and potential industrial applications.

3D surface defect analysis and evaluation

A method is proposed for surface defect analysis and evaluation. Good 3D point clouds can now be obtained through a variety of surface profiling methods such as stylus tracers, structured light, or interferometry. In order to inspect a surface for defects, first a reference surface that represents the surface without any defects needs to be identified. This reference surface can then be fit to the point cloud. The algorithm we present finds the least square solution for the overdetermined equation set to obtain the parameters of the reference surface mathematical description. The distance between each point within the point cloud and the reference surface is then calculated using to the derived reference surface equation. For analysis of the data, the user can preset a threshold distance value. If the calculated distance is bigger than the threshold value, the corresponding point is marked as a defect point. The software then generates a color-coded map of the measured surface. Defect points that are connected together are formed into a defect-clustering domain. Each defect-clustering domain is treated as one defect area. We then use a clustering domain searching algorithm to auto-search all the defect areas in the point cloud. The different critical parameters used for evaluating the defect status of a point cloud that can be calculated are described as: P-Depth,a peak depth of all defects; Defect Number, the number of surface defects; Defects/Area, the defect number in unit area; and Defect Coverage Ratio which is a ratio of the defect area to the region of interest.

3D tracking for borescope inspections

Service providers perform regular borescope inspection of field products to ensure system performance and prevent accidents. A full borescope inspection for large equipment usually takes several days to cover every region of interest in the turbine. One of the challenges that causes this long inspection time is the difficulty in navigating the borescope tip to some position of interest and aiming the view of the borescope in given direction. The image from the borescope tip is the only information available to the operator to judge the position of tip. In some cases, the operator can get lost due to the limited field-of-view and illumination provided by the borescope. It is very hard to tell the borescope tip position from one borescope image. This increases the difficulty of correlating the inspection results obtained at different times that might be used to predict potential machinery failure. This paper discusses various methods that have been investigated for 3D borescope tracking and presents a new approach using a shape sensor integrated in the borescope tube used with image based location determination to determine the position of the borescope tip during inspections. The shape sensor provides a real time estimate of the borescope tip position and orientation then the image based analysis uses the part CAD model to fine tune this position information. The tracking result can provide better information of the tip position for the operator. This enhanced position information can be used to better monitor defect changes over time by comparing the inspection result at different times in the parts lifetime.

Development of feature extraction analysis for a multi-functional optical profiling device applied to field engineering applications

In the real application environment of field engineering, a large variety of metrology tools are required by the technician to inspect part profile features. However, some of these tools are burdensome and only address a sole application or measurement. In other cases, standard tools lack the capability of accessing irregular profile features. Customers of field engineering want the next generation metrology devices to have the ability to replace the many current tools with one single device. This paper will describe a method based on the ring optical gage concept to the measurement of numerous kinds of profile features useful for the field technician. The ring optical system is composed of a collimated laser, a conical mirror and a CCD camera. To be useful for a wide range of applications, the ring optical system requires profile feature extraction algorithms and data manipulation directed toward real world applications in field operation. The paper will discuss such practical applications as measuring the non-ideal round hole with both off-centered and oblique axes. The algorithms needed to analyze other features such as measuring the width of gaps, radius of transition fillets, fall of step surfaces, and surface parallelism will also be discussed in this paper. With the assistance of image processing and geometric algorithms, these features can be extracted with a reasonable performance. Tailoring the feature extraction analysis to this specific gage offers the potential for a wider application base beyond simple inner diameter measurements. The paper will present experimental results that are compared with standard gages to prove the performance and feasibility of the analysis in real world field engineering. Potential accuracy improvement methods, a new dual ring design and future work will be discussed at the end of this paper.

Blade counting tool with a 3D borescope for turbine applications

Video borescopes are widely used for turbine and aviation engine inspection to guarantee the health of blades and prevent blade failure during running. When the moving components of a turbine engine are inspected with a video borescope, the operator must view every blade in a given stage. The blade counting tool is video interpretation software that runs simultaneously in the background during inspection. It identifies moving turbine blades in a video stream, tracks and counts the blades as they move across the screen. This approach includes blade detection to identify blades in different inspection scenarios and blade tracking to perceive blade movement even in hand-turning engine inspections. The software is able to label each blade by comparing counting results to a known blade count for the engine type and stage. On-screen indications show the borescope user labels for each blade and how many blades have been viewed as the turbine is rotated.

Effect of varied fringe width on measured profile in structured line projection method

In industry, there are needs to accurately measure the 3-D profile of edges parts in order to evaluate edge condition. Optical methods are increasingly used for this purpose due to its advantages such as being non-contact, fast, accurate, and easy to integrate with software for data acquisition and feature analysis. We utilized structured line projection technology to measure edges profile. In this method a structured light distribution, created by the transmission of a sinusoidal grating, was projected onto the inspected parts at a certain incidence angle. The projected light lines were deformed due to the depth change on the edge surface. A CCD camera sitting at a different angle was used to record the deformed fringes. From the deformed fringes the 3D surface profile was extracted based on triangulation principal. Because the projected grating pattern can be interpreted as an interference pattern, we used the spatial carrier phase shifting and phase unwrapping method as in classic interferometry to extract the phase information from the intensity distribution of fringes. Due to the limited depth-of-range of the fringe image and depth-of-focus of the imaging lens on the CCD camera, the observed deformed fringes have different widths and frequencies at different depth. According to the definition of depth-of-focus, the fringe width out of focus can be on the order of the square root of 2 wider than that which is in focus. The width change can also be due to a tilt across the object. This width change affects the accuracy of the spatial carrier phase shifting and subsequently the accuracy of extracted profile. In this paper, we proposed to monitor the change of the fringe width with imaging depth. According to the width of the fringes, we defined a parameter called compressing rate, to use in computing the edge profile. For different edge types, the compressing rate was optimized in order to get the profile that can match the results from traditional the methods. By using this method, the system repeatability can be improved significantly.