Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1997-02-19
2001-01-16
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
Reexamination Certificate
active
06175415
ABSTRACT:
DESCRIPTION
1. Technical Field
This invention relates to optical sensors, more particularly to optical sensors for determining a profile of an object.
2. Background Art
Precision components must often meet stringent requirements for dimensions, profile, and surface finish. For example, an edge on an aircraft engine blade is processed in accordance with strict standards for all of the above. Thus, an inspection system capable of making such measurements is needed.
The blade or airfoil is sometimes inspected by templates or gauges which are placed on the feature of interest and visually checked for fit. Another inspection process, involves pressing a wax stick on the blade edge to thereby get an impression of the blade edge. The wax impression is removed from the blade, backlit, magnified, and projected on a screen for a dimensional check with a plastic template. However, neither of these methods are accurate enough to meet the stringent inspection requirements required for these critical components.
Alternatively, blades are often inspected by a coordinate measuring machine (CMM) equipped with a touch probe. However, measurement of a blade edge by a touch probe is a difficult task. Furthermore, a CMM machine generally takes hours to measure a single blade edge. Collectively, these short comings suggest that, mechanical inspection devices are inherently limited in their ability to accurately and quickly measure components having complex profiles. Therefore, an optical rather that mechanical, sensor is sought.
An optical sensor for measuring the profile of a blade edge has been suggested in a prior art paper (see Chiarella, M., and Pietrzak, K. A., “An Accurate Calibration Technique for 3-D Laser Stripe Sensors”,
Proceedings of the SPIE Conference on Optics, Illumination and Image Sensing for Machine Vision IV
, SPIE Vol. 1194, Nov. 8-10, 1989, pp 176-185.) This sensor uses a light source to project a plane of light toward the surface of a blade, thereby illuminating the surface, in the form of a stripe. The profile of the illuminated surface is then viewed by a camera. However, such a sensor has limited capability because it can only illuminate and view a small portion of the blade edge at a time. While it can be moved to view a different portion of the blade edge, the views cannot be combined because each view represents a different perspective, and there is no common reference for combining them. Therefore it does not measure the overall profile of the blade edge.
Various other optical sensors exist, although, none are well suited to determining the profile of the edge of a blade. For example, stereovision combines images from multiple cameras, but in order to do so, it requires that a particular distinguishable feature be within the view of all the cameras at the same time. This is not practical for blade edges because there are no such features on the edge of a blade. Optical triangulation schemes can be used to illuminate, view and determine the distance to a point on a surface. A surface contour may be determined by scanning the surface, i.e. successively illuminating, viewing and determining the distance to different points on the surface. However, this is difficult at best for a surface with a complex profile such as that of a blade edge. In addition, triangulation requires that the positioning of all the optical components be precisely known. Furthermore, triangulation typically can not compensate for such optical imperfections as lens distortion. A special effects generator can be used to combine images from cameras viewing different portions of an object, but such a technique has limited accuracy unless all of the cameras are precisely oriented relative to the object and each other. Because of this limitation, such technique is generally not suitable for use in determining the profile of a blade edge.
In a scheme somewhat related to stereovision, a sensor projects reference features onto the surface to be profiled, to provide references for combining views from multiple cameras. One such sensor, for example, uses two light (line) sources and two cameras to measure the transverse profile of the head of a rail of a railroad track (U.S. Pat. No. 4,531,837). The head has a generally flat top. The light sources are projected at the head, from angles above, to illuminate the profile. A discontinuity in the illumination, i.e. a dark spot, is provided on the flat top of the head. Each camera views the dark spot and one side of the profile. The dark spot provides a common reference point used in combining the views. However, such a scheme can only be used on surfaces having particular geometries, e.g., a flat top. This scheme, however, is not practical for a complex profiles such as that of blade edge. First, the blade edge does not have a large flat surface suitable for projecting such a spot. Second, such a spot would obscure critical aspects of the edge profile.
Thus, an improved optical inspection device for quickly and accurately measuring blade edges, and other components, with critical dimensions and/or complex profiles is sought. As a minimum, such a device should be capable of determining the entire profile of a blade edge, i.e. be able to view both sides of the blade edge and accurately combine the two views together to provide a complete profile.
A method for quickly and accurately evaluating the profile is also sought. A traditional method requires that a cross section of the entire blade, not just the edge, be determined. Then, a best fit of the blade cross section is performed, and a normal deviation at every point on the edge is computed. This method has several disadvantages: it requires measurements of blade portions which are not of primary concern, it is slow, and it is prone to encounter errors during data manipulation. Thus, a better method is sought.
DISCLOSURE OF THE INVENTION
To overcome the problems described above, the apparatus of the present invention has at least one light source which projects a sheet of light at the surface of an object, thereby illuminating a profile, which is collectively viewed by at least two optical detectors, where each detector produces a signal representative of its view. An image processor receives the detector signals and uses calibration parameters to produce signals which collectively represent the profile. Such apparatus, in effect, “combines” views from multiple optical detectors so that a complex profile may be quickly and accurately determined. The calibration parameters are preferably predetermined. The calibration parameters may represent parameters derived by having the detectors view a known target and having at least one of the detectors view an intersection of the sheet of light and a target surface. The projected light sheet may be generally planar in a target region and generally perpendicular, to the surface of the object. The detectors may be angled nearly parallel to the surface of the object. The detectors may comprise video cameras. The apparatus may be used to obtain the profile of any suitable object including but not limited to edges of objects and the leading and trailing edges of airfoils. The apparatus may further comprise means for comparing a profile to a nominal profile and or means for making measurements on the profile.
In a second aspect of the invention, a method for evaluating a profile of a blade edge comprises the steps of: rotating the profile to orient it such that one of its two sides is substantially parallel to a coordinate axis, and locating the tip by determining which point has an extrememost coordinate value in one direction along the coordinate axis. This method quickly and accurately determines the location of the tip of the profile, and may serve as the basis for further evaluation of the profile. It is not prone to errors during data manipulation, nor does it require measurements of other profile portions not of primary concern. The method may involve finding and determining a line of a relatively flat section on the side of the profile, and rotating the profile to orien
Bari Farooq
Pietrzak Kenneth A.
Puffer LeRoy G.
Font Frank G.
Smith Zandra V.
United Technologies Corporation
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