Abstract:
A method for filtering undesired light reflections in a structured light measurement system during the inspection of shiny metal prismatic objects having uncoated prismatic surfaces, such as turbine blades, using polarized light.

Description:
BACKGROUND OF INVENTION 
     The present invention provides a method for reducing noise in a non-contact gauge measurement system utilizing structured light. In particular, the present invention provides a method to facilitate inspection of prismatic objects having uncoated surfaces, such as turbine or compressor blades, using a combination of object orientation and polarized light in an optical three-dimensional structured light gauge measurement system. 
     Traditionally, gauge measurement of a manufactured object having a complex three dimensional surface such as an airfoil (e.g. compressor blade) is a tedious and time consuming process. Airfoils, including forged blades such as those used on aircraft engines, electrical power generators, and the like, are inspected for deformations which may include, but are not limited to, skew, twist, scaling, and translation. More specifically, airfoils are inspected for deformation parameters such as platform orientation, contour cross-section, bow and twist along a stacking axis, thickness, and chord length at given cross-sections. 
     One method of obtaining dense and accurate digital data representing these parameters for an individual airfoil is through use of a coordinate measuring machine (commonly known as a “CMM”). CMM&#39;s translate and rotate a sensor probe into contact with the surface of an object undergoing testing to sample the position of various points on the object&#39;s surface. Before a sensor probe may be brought into contact with an object, the object must be secured in a known physical position and orientation, such that a set of known reference points may be established. For airfoil measurement, six physical contact points are utilized, defining a “six point nest”. This set of six data points establish the position and orientation of the airfoil in its physical holder and enable the contact points to be translated to any other coordinate system. CMM&#39;s provide high quality (i.e. highly accurate) measurements of the sample points. However, the time to scan an airfoil is relatively slow as the process of positioning the airfoil in the six point nest is time-consuming, and the sensor probe must be continually repositioned to obtain data. Once the high quality surface points are collected, software processes these points into deviations from values generated in a computer assisted drawing (CAD) model of the object and analyzes the deviations in terms of process-based shape deformations. Current CMM processing software, however, is also relatively slow. 
     An alternate method of obtaining measurements representing deformation parameters of an object employs full-field non-contact range sensors. Non-contact full-field sensors can quickly scan the external surfaces of opaque objects, using laser or white light, significantly faster than CMMs. Examples of non-contact sensors include sensors that project laser line gratings onto the surface of an object and process detected images thereof using stereo triangulation; and those based on single laser line scan plus rotation of the object. Additional non-contact sensors are based on phase-shifted moir é patterns and white light. While these sensors are capable of scanning the part quickly and obtaining large quantities of data, the level of accuracy is affected by undesirable reflections of the scan light from shiny or prismatic surfaces on the object. 
     To compensate for these undesired reflections, shiny or prismatic surfaces on the object are traditionally coated with a diffusing material such as a paint or powder to non-contact gauge measurement. This additional step adds uncertainty to the measurement, and increases measurement time which is highly undesirable. Accordingly, there is a need for eliminating or reducing the effect of undesired reflections from shiny or prismatic object surfaces in a non-contact measurement system in a manner which does not require application of diffuse coatings to surfaces of the object being tested. 
     SUMMARY OF INVENTION 
     Briefly stated, the present invention provides a method for the inspection of shiny metal prismatic objects having uncoated prismatic surfaces, such as compressor blades, using polarized light in an optical three-dimensional structured light measurement system. 
     In one embodiment of the present invention, a structured light measurement system projects a structured light pattern onto the surface of an object, parallel to a plane bisecting one or more prismatic features of the object. An imaging system receives the structured light pattern reflected from the surface of the object and analyzes the deformation of the reflected light pattern to calculate the surface features of the object. The projected light is polarized at a known polarization angle, and the reflected light is polarized to a related angle, such that light reflected directly and indirectly from the planar surfaces of the object is separated by the imaging system. Using multiple images of the object obtained with polarized light, difference images and region masks can be generated to reduce or eliminate undesired reflections and noise from the resulting images of the object. 
     The foregoing and other objects, features, and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     In the accompanying drawings which form part of the specification: 
     FIG. 1 is a block diagram of an apparatus for effecting the method of the present invention for measuring surface features of an object under test; 
     FIG. 2 is a side sectional view of an object under test, illustrating single-bounce and double-bounce light paths; 
     FIG. 3 is a top view of an object under test, illustrating changes in reflected light polarization for light following the paths shown in FIG.  2 . 
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. 
     DETAILED DESCRIPTION 
     The following detailed description illustrates the invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the invention, describes several embodiments, adaptations, variations, alternatives, and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. 
     FIG. 1 is a block diagram of an apparatus  10  for measuring surface features of a manufactured object  12 , according to one embodiment of the present invention. Apparatus  10  is adapted to inspect and determine surfaces of the object. These surfaces may include features such as tilts, bends, twists, or warps when compared to a reference model or other ideal representations of the object. 
     In one embodiment of the invention, object  12  comprises a blade, e.g., a compressor blade of an aircraft engine, having an airfoil  14  extending from a platform  16 , and having an axis  18 . While the following description is directed to inspecting blades, one skilled in the art will appreciate that the apparatus and the method of the present invention may be utilized to improve structured light imaging for any object having similar characteristics. 
     An object  12  to be inspected is positioned within the sensing range of an imaging system  20 , preferably a full field, non-contact, laser line grating range sensor mounted on a translation stage. The imaging system comprises a structured light emitter  22  and one or more imaging sensors  24  configured to receive structured light reflected from object  12 . One or more computers  25  are utilized to process images received from the sensors  24 , and a monitor  27  may be utilized to display information to an operator. 
     Full field, non-contact range sensors suitable for use as imaging systems  20  are currently readily available from commercial sources. For example, the model 4DI sensors sold by Integrated Automation Systems may be employed with the present invention. The 4DI sensors utilize the projection of a laser line grating onto the surface on an object  12 , and the stereo triangulation of the projected lines by one or more imaging detectors. Other suitable sensors employ single laser line scans with rotation of the object, or phase-shifted Moir é and white light. 
     To improve the performance of the full-field, non-contact range sensors on objects  12  having shiny or prismatic surfaces, the object is positioned in a preferred orientation relative to the source of structured light. As seen in FIG. 2, the angle α of orientation is selected so as to present a view to the imaging sensors  24  in which a plane β defined by light emitter  22  and imaging sensors  24  approximately bisect one or more prismatic features on the surface of the object  12 . For example, if the object  12  comprises a compressor blade, airfoil  14  and platform  16  define a prismatic feature of the blade, as a portion of light projected onto the airfoil may reflect onto platform  16 , thus producing a double reflection back towards the imaging system  20 . 
     To reduce and identify reflections of the projected light from prismatic features of object  12 , the structured light projected onto the surface of the object from light emitter  22  is polarized with a polarizing filter  26  oriented at a known angle θ relative to a the plane β defined by light emitter  22  and imaging system  20 . A second polarizing filter  28  is placed between imaging sensors  24  and object  12  and oriented to block any light reflected back through two surface reflections. Now, any light reflecting off two facets of the prismatic surfaces of the object is reduced in intensity or completely blocked from the imaging sensors. 
     The imaging sensors  24  obtain an image of the structured light. projected onto a surface of object  12 . This image is composed of an array of pixels, with each pixel representing the intensity of received light at that particular point in the image. If some portions of the image are either over or under saturated, i.e. the intensity levels exceed a maximum observable level or does not reach a minimum threshold level, additional images of the object can be taken with the structured light projected at different levels of intensity and/or with different polarization angles so to provide additional data in the usual manner. 
     Only directly reflected light from a surface of object  12  can be readily interpreted in a structured light imaging system  20 . A directly reflected light path is indicated as SB in FIG.  2 . Light which has reflected off of a prismatic surface on the object, as indicated by light path DB in FIG.  2 ,(i.e. “double-bounce light”), produces light patterns observed by the imaging sensors  24 , but at erroneous orientations. This can lead to an incorrect interpretation of the surface features of object  12 . 
     As shown in FIG. 3, the polarization of any light which has bounced off of two or more surfaces of object  12  is changed so as to be different from directly reflected light. By selecting polarization angle θ for the light illuminating the surface of object  12 , and subsequent filtering of the light as seen by the imaging sensors  24 , single bounce and double bounce light is effectively separated. 
     Reflected light which has bounced off of two prismatic surfaces of object  12  is enhanced by positioning the second polarization filter  28  in front of imaging sensor  24  so to be 90° out of phase with the first polarization filter  26  which is placed in front of the structured light emitters  22 . Other orientations of the second polarization filter  28  can provide additional information on the angle and number of bounces through which light received by the imaging sensor  24  has passed. 
     By taking multiple images of the structured light projected on a surface of object  12 , and using different polarizations angles of second polarization filter  28 , reflections from single bounce light, double bounced light, or other variations, are readily identified and selectively extracted from the image using conventional image processing techniques. 
     Suitable image processing methods include subtraction or difference imaging between two images, masking of certain areas of an image, as well as correlations of image information. For example, it is known that if two or more images of an object are obtained, and are correlated such that they have common reference points or are from the same position and orientation, digital processing techniques permit one image to be “subtracted” from the other, so to obtaining a resulting difference image. This resulting difference image includes only information on those regions in the field of view which have changed between the first and second images. 
     Alternatively, with prior knowledge of the shape or configuration of the object undergoing measurement, or two or more images, specific regions in an image known to contain erroneous or irrelevant information may be digitally masked or blocked from further processing. For example, using subtraction techniques, regions of an image containing background can be identified in a difference image, and then utilized to mask out background regions in subsequent or current or subsequent images. 
     Similarly, using known information or multiple images, an image of an object undergoing measurement may be correlated or registered to a stored reference image, facilitating identification of differences between the object and an ideal model or representation of the object. 
     In this manner, even if the second polarizing lens  28  does not completely block unwanted light reflections from imaging sensor  24 , the effects of the these reflections can be analyzed within the image and incorrect information sorted from correct information for use in the surface inspection of the object. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results are obtained. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.