Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-06-04
2001-02-13
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06188483
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a method and apparatus for determining deformation and elongation on curved bodies, and more particularly a method of determining displacement of at least a part of the surface of a measurement object between an initial condition and a measurement condition and the shape of said surface of the measurement object and apparatus for carrying the method into effect. The term measurement object is used herein to denote any appropriate object, deformation and the shape of at least a surface portion of which is to be measured.
BACKGROUND OF THE INVENTION
An operation of ascertaining, in respect of surface area, displacement of a part of or also the entire surface of an object, including a three-dimensional body of any shape, is a task which is essential in the context of many industrially relevant questions. Thus for example partial displacement of the surface of an object occurs in testing components for possible weak points, for the stress analysis of complex systems, and in checking components in regard to complying with certain stress limit values, insofar as, due to a loading, for example a compression loading, a body being observed is elongated slightly at the surface in the region of a weak point in its wall or its layer structure, or a hollow body also experiences complete elongation by virtue of a compression loading.
Known strain measurement methods are either very expensive in terms of application (for example wire strain gauges—only punctiform measurement, surface layer processes), highly inaccurate (brittle lacquer method) or can only be used under particular conditions (cyclic loading in the case of thermoemission analysis).
Speckle interferometry permits contact-less areal detection of displacement and/or deformation on any components. 2D- and 3D-speckle interferometers make it possible to determine deformation in two or three co-ordinate axes. For that purpose the components of the displacement of points of the surface, measured with the speckle interferometer, in one or more directions, for a very large number of points, are converted together into the co-ordinate system of the object or a spatial co-ordinate system. That will be described in greater detail with reference to the Figures.
Thus EP-A0 731 335 already shows such a method of detecting undesirable deformation of an object, which generally occurs under loading, in which speckle interferometry is used employing the specific method of shearography. In that procedure however the configuration of the object is not determined. The apparatus has two separate cameras and a double-armed Mach-Zehnder interferometer.
In addition DE-A-41 02 881 also describes a method of detecting deformation by means of speckle interferometry, in which case the particularity thereof is that the illumination necessary for 3D-deformation is used from different directions with light involving different properties (wavelength, polarisation etc) in order to be able to implement at the same time the recordings which are based on the different illumination directions, which is necessary in particular for detecting deformation on objects whose deformation does not come to a halt or which are to be observed during the deformation.
Knowledge of the object geometry and the relative position with respect to the speckle interferometer are however desirable for accurately determining the three-dimensional displacement vector.
The geometry of the object can either be inputted manually or it can be detected by a measurement procedure using another measuring instrument, for example a fringe projection system. In that case, the surface of a body is radiated with a regular pattern, in particular a simple fringe pattern, which gives a modified pattern on the surface of the body. If for example a ball is irradiated with such a fringe pattern, then the result obtained is lines which are ever increasingly curved outwardly in a lens configuration on the surface of the ball.
On the basis of knowledge of the geometrical data of the irradiation pattern and the spacing relative to the object, it is possible to compute therefrom the shape of the surface of the object.
The disadvantage of that mode of operation is the high level of financial expenditure involved in providing two different expensive measurement systems for displacement measurement and shape measurement.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method with which both shape detection and also displacement detection can be effected and calculated therefrom elongation and stress.
Another object of the present invention is to provide a method of determining displacement of at least a part of the surface of a measurement object and the shape of the measurement object, which is simple to implement while affording reliable operating results.
Still another object of the present invention is to provide apparatus for determining the shape and displacement of at least part of a surface of a measurement object, operable to supply accurate information results combined with a straightforward and easily overviewable operating procedure.
The foregoing and other objects are attained by the method and apparatus of the present invention.
As will be seen from embodiments described hereinafter, that is possible in that, in the case of speckle interferometry for displacement measurement, the respective phase difference between two light waves impinging on the same pixel of the light-sensitive measurement surface of the camera, generally a CCD-chip, is ascertained in any case on the one hand for the initial condition and on the other hand for the measurement condition of the object, and the difference value between the two phase differences allows conclusions to be drawn in relation to displacement of the corresponding point on the object. That can be effected not only for a single point but for all individual points which can be distinguished by the light-sensitive layer of the camera, and thus also permits information to be afforded about the entire viewing surface, that is to say each individual distinguishable point on the surface of the object in the viewing region.
If the initial condition of the object is used instead of the measurement condition in which the object to be measured is displaced or acted upon by force in some form, but parameters at the viewing side are altered, for example by varying the illumination direction and/or varying the viewing direction and/or varying the distance of the illumination unit and/or the viewing unit and/or the direction-changing mirrors used in that case and/or the wavelength of the light employed, then by comparison of two measurement steps which differ in terms of those parameters, but in each case on the same object which is left in the initial condition, it is possible to ascertain the relative position of individual points on the surface of the object relative to each other—and in the case of large-scale determination of such relationships of the surface points—the shape of the entire surface in the viewing region.
That means that the shape of the object is known for that condition in which the shape of the object was determined—generally the initial condition—and in addition the displacement of each individual one of the surface points ascertained, due to the variation of the object between the initial condition and the measurement condition, is known, the measurement condition generally being a condition in which a force has been applied.
That displacement of each individual point occurs as a vector, that is to say given by the three partial vectors in the three directions in space x, y and z. That information, specified raster-like for each observed point on the surface, is to be referred to as the displacement range or field. In that respect elongation in a given point on the surface of the object is the difference in respect of the displacement of that point—between the initial condition and the measurement condition—relative to the displacements of the adjacent points.
In regard to as
Dr. Ettemeyer GmbH & Co.
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Font Frank G.
Natividad Philip
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