Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-01-25
2004-11-23
Glick, Edward J. (Department: 2882)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S485000, C356S492000, C356S497000, C356S511000
Reexamination Certificate
active
06822745
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to optical methods and means for determining and/or verifying the geometric dimensions of precision-engineered parts.
The fabrication of precision-engineered components is governed by standard practice in geometric dimensioning and tolerancing (GD&T). Metrology for GD&T requires accurate determination of surface form as well as relationships between part surfaces. The metrology must be accurate and conform to international standards, and preferably takes no more than a few seconds. Surface forms of interest include, for example, opposing plane parallel surfaces, orthogonal plane surfaces, disconnected planar, cylindrical and spherical surfaces, and component surfaces of an assembly.
SUMMARY OF THE INVENTION
The invention features optical systems and methods that determine absolute positions of points on potentially disconnected surfaces on a test part with respect to a common reference frame. Such systems and methods allow a user to verify, for example, that the location, relative orientation, and form of part features conform to specifications.
The invention includes an optical system having one or more optical profilers adapted to view a test part from different perspectives. Each profiler is capable of measuring absolute positions of surface points in three dimensions with respect to a coordinate system local to each profiler. The invention further includes initialization and calibration procedures to relate the coordinate systems of each profiler to the other, so as to relate each measured surface position to all others. Such procedures can employ, for example, mechanical standard artifacts or a distance measuring laser interferometer, to provide information regarding the separation and relative orientation of the two corresponding optical profiler coordinate systems. Suitable optical profilers include triangulation systems, time of flight systems, and optical interferometers, such as height-scanning interferometers, which employ mechanical or equivalent scans perpendicular to the surface to obtain a localized (e.g. coherence-limited) interference pattern for each image pixel. As described in greater detail below, height-scanning interferometers that employ infrared sources can be particularly advantageous when working with test parts having relatively rough surfaces.
In general, in one aspect, the invention features a method for determining a geometric property of a test object. The method includes: interferometrically profiling a first surface of the test object with respect to a first datum surface; interferometrically profiling a second surface of the test object in a second coordinate system with respect a second datum surface different from the first datum surface; providing a spatial relationship between the first and second datum surfaces; and calculating the geometric property based on the interferometrically profiled surfaces and the spatial relationship between the first and second datum surfaces.
Embodiments of the method can include any of the following features.
The interferometric profiling of the first surface can provide a distance to each of a plurality points on the first surface from a corresponding point on the first datum surface. Similarly, the interferometric profiling of the second surface can provide a distance to each of a plurality points on the first surface from a corresponding point on the first datum surface.
One or both of the datum surfaces can be a portion of a plane, a curved surface, or have a structured profile.
The first surface of the test object can be spaced from the second surface. The first and second surfaces can correspond to opposite faces of the test object. The first and second surfaces can correspond to adjacent faces of the test object. The first and second surfaces can be adjacent faces separated by a step height. The first and second surfaces can be displaced from one another by a distance greater than a range of the interferometric profiling of the first surface and greater than a range of the interferometric profiling of the second surface.
The interferometric profiling of the first surface can include directing electromagnetic radiation to the first surface along a first direction and the interferometric profiling of the second surface includes directing electromagnetic radiation to the second surface along a second direction different from the first direction. The interferometric profiling of the first surface can include positioning the test object relative to an interferometry system and the interferometric profiling of the second surface includes repositioning the test object relative to at least one component of the interferometry system. For example, the repositioning of the test object relative to the interferometry system can include moving the test object or moving the at least one component of the interferometry system. In the latter case, the method can further include measuring the movement of the at least one component of the interferometry system to determine the spatial relationship between the first and second datum surfaces.
The relationship between the first and second datum surfaces can be defined by a distance between corresponding reference points on the first and second datum surfaces and two angles defining a relative orientation of the first and second datum surfaces.
The method can further including determining the spatial relationship between the first and second datum surfaces.
For example, determining the relationship between the first and second coordinate system can include: interferometrically profiling a first surface of a reference object with respect to the first datum surface; interferometrically profiling a second surface of the reference object with respect to the second coordinate system; providing at least one calibrated dimension for the reference object; and calculating the spatial relationship between the first and second datum surfaces based on the profiled surfaces and the at least one calibrated dimension. The reference object can be selected according to approximate dimensions of the test object.
The method can further including determining the spatial relationship between the first and second datum surfaces based on at least one interferometric displacement measurement. For example, the spatial relationship can be determined based on the at least one interferometric distance measurement and an initial calibration. The method can further include adjusting at least one of the first and second datum surfaces to accommodate the interferometric profiling of the first and second surfaces of the test object and interferometrically measuring the adjustment of the at least one of the first and second datum surfaces to determine the spatial relationship between the first and second datum surfaces.
The spatial relationship can also be determined by: interferometrically profiling a first surface of a initialization artifact with respect to the first datum surface; interferometrically profiling a second surface of the initialization artifact with respect to the second datum surface; calculating an initial spatial relationship between the first and second datum surfaces based on at least the profiled surfaces of the initialization artifact; adjusting the first and second datum surfaces to accommodate the first and second surfaces of the test object, and interferometrically measuring at least one displacement corresponding to the adjustment of the first and second datum surfaces. For example, the first and second surfaces of the initialization artifact can be the front and back of a common interface. Furthermore, the method can provide at least one calibrated dimension for the initialization artifact, and the calculation of the initial relationship can be based on the profiled surfaces of the initialization artifact and the at least one calibrated dimension.
The geometric property can be any of: flatness of the test object; thickness of the test object; parallelism of the test object; a step height; the angular orientation of the first
Biegen James
Colonna De Lega Xavier
De Groot Peter
Grigg David
Artman Thomas R
Fish & Richardson P.C.
Glick Edward J.
Zygo Corporation
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