DE3600672A1 - Computer-aided determination of deformation and movement from images using projected patterns - Google Patents

Abstract

For measurement purposes, a method is presented for determining deformation and movement from images using projected patterns. The method is based on the combination of the following features: a) producing a displacement of the projected pattern relative to the measurement object, b) recording and storing images of the measurement object for different positions of the pattern and for different deformation states or location states of the object, c) punctiform calculation of deformation and movement components by means of the stored images.

Description

genus

The procedure concerns quantitative, computer-aided Determination of deformation and / or movement by means of Images of the measurement object. The measuring principle is based on the Projection of patterns onto the measurement object; from the spatial change in the pattern can be due to deformation and Movement of the measurement object can be calculated back.

State of the art

The pattern change is generally evaluated with Help of the Moir´ technique. This technique, which is regarding photographic materials and execution of a great variety, can always on the following Principle can be attributed:

From the object on which a pattern is projected, at different deformation or location state two photographic Recordings made on translucent material. The overlay of the two recordings leads to Moir´- Lines that cover the image of the object.

The Moir lines are generally evaluated in two ways:

• a) visually-manually by counting the lines, by Tracing the lines using optical Tools or graphics tablets. An increase in Evaluation accuracy for individual points is determined by mutual movement of the recordings achieved.
• b) Computer-aided by reading the digitized images into data storage and evaluating them according to the skeletonization method using line recognition programs.
  Lit.:.2V. Perzborn and K.-H. Laermann, VDI reports 552
  9th GESA Symposium 1985
  T. Yatagai and M. Idesawa, 1982
  Optics and Lasers in Engineering 3 1982 73-83

criticism

The visual-manual procedures are time consuming subjective and error-prone. A high evaluation accuracy is only achieved and required for a few image points specially qualified personnel. The skeletonization process are computing time consuming, sensitive to disturbances in the Stripe pattern, e.g. B. when shadows or reflections occur, and fail with certain types of images. Both procedures evaluate only parts of the information contained in the sample and require the environment of a Pixel.

Object of the invention

The object of the present invention is the entire Information content that the pattern change contains complete or in any size with high Accuracy, insensitive to interference, such as B. Shadows or reflections, and free of subjective assessment criteria to determine.

solution

The intensity image I ( x, y ) of an object illuminated with a pattern, which is recorded with a camera, is known to be composed of various components, namely a background intensity H ( x, y ), a contrast intensity K ( x, y ) and a sample image M ( ϕ ( x, y )). The function M is determined by the type of pattern generation; ϕ ( x, y ) describes the shape of the pattern on the object, and x and y denote the coordinates of the pixels. In the intensity image, the sizes H, K and ϕ are linked as follows:

I ( x, y ) = H ( x, y ) + K ( x, y ) · M ( ϕ ( x, y )) (1)

By means of equipment measures and methods, as characterized in claims 1 and 5, a targeted displacement of the pattern can be achieved relative to the object; the intensity image I thus becomes dependent on a known position parameter δ :

I _δ ( x, y ) = H ( x, y ) + K ( x, y ) · M ( ϕ ( x, y ), w ) (2)

By choosing suitable sets of δ _i , equation 2 can be expanded to an equation system that provides an exact resolution according to the unknowns H, K and ϕ from the known image intensities I _{δ 1} , I _{δ 2 ,.} . . . allowed.

For this purpose, the intensities I _{δ i} for different pattern positions are recorded in practice and stored on data memories, in accordance with point 1b of the claim. The calculation of the shape function ϕ ( x, y ) and / or the contrast image K ( x, y ) and / or the background image H ( x, y ) is carried out point by point using the stored image contents, in accordance with point 1c of the claim.

The same can be done for the deformed or moving object. The information about the deformation or movement of the object lies in the measuring principle in the change of the shape function ϕ ( x, y ). The deformation can be calculated for each point with high accuracy using the two precisely determined shape functions and the data of the optical structure.

The calculation of the deformation does not necessarily have to be carried out using the individually determined shape functions, rather there is a great variety with regard to the sets of δ _{i that can be used} and thus a multitude of possible solutions for the systems of equations. As is demonstrated in the exemplary embodiment, there is the possibility, by skillful choice of the position parameters δ _i , of designing the system of equations in such a way that it can be solved directly after the difference in the shape functions.

The calculation of the exact solutions can be done in suitable Computer systems for the entire picture with little time and Carry out program effort. Essential, and of the Differentiating previous methods is that the Calculation at each individual pixel exactly and independently from the surroundings of this point.

Design

Adding an apparatus parameter is a methodical one Process that allows automated evaluation or, after appropriate instruction, also less qualified Personnel enables the evaluation.

Achievable advantages

There will be a full evaluation of the entire picture achieved with high accuracy and great reliability, the evaluation is free of subjective assessment criteria. The data is in a machine-readable form for display and further processing available, e.g. B. for illustration the deformation by means of 3D plots.

Embodiment

The determination of the z component of a deformation is chosen as an exemplary embodiment; an arrangement which can be used for this purpose is shown in Fig. 1. A laser beam strikes a slightly misaligned two-beam interferometer ( 2 ), passes through an expansion lens ( 4 ) and then illuminates the measurement object ( 5 ) at an angle with an interference fringe pattern. The strip position on the object can be adjusted by controlled movement of a piezo mirror ( 3 ). An electronic camera ( 6 ) is used to read and store images in various strip positions and deformation states in an electronic image processing system consisting of a computer, data memory and monitor.

The intensity distribution that is captured by the camera can be represented as follows in the case described:

I _δ ( x, y ) = H ( x, y ) + K ( x, y ) cos ( ϕ ( x, y ) + δ _i ) (3)

In the present case, four pictures are taken; the first in the state of deformation A and the strip position w ₁ = 0 °, the other three in the state B and strip positions of δ ₂ = 0 °, δ ₃ = 120 ° and δ ₄ = 240 °. This creates the following system of equations:

I ₁ ( x, y ) = H ( x, y ) + K ( x, y ) cos d _A ( -x, y )
I ₂ ( x, y ) = H ( x, y ) + K ( x, y ) cos d _B ( -x, y )
I ₃ ( x, y ) = H ( x, y ) + K ( x, y ) cos ( d _B ( -x, y ) + 120 °) (4)
I ₄ ( x, y ) = H ( x, y ) + K ( x, y ) cos ( ϕ _B ( -x, y ) + 240 °)

In the first calculation step, the amounts of the difference images I ₁ - I ₂ , I ₁ - I ₃ and I ₁ - I _{4 are} formed and low-pass filtered to suppress terms with ( ϕ _A + ϕ _B ).

In the second calculation step, the quantity ϕ _A - ϕ _B relevant for determining the z deformation is calculated point by point using the following equation:

In the third step, the z deformation v _{z is} calculated from the image ϕ _A - ϕ _B using the geometry data of the optical structure:

d is the strip spacing and α is the angle between the direction of illumination and observation. Image v can also be stored on data storage and is available for further processing or data output in digital form.

Claims (7)

1) A method for computer-aided determination of deformation and movement components from images with projected patterns, characterized by the combination of the following three features:

• a) generating a displacement of the projected pattern relative to the object,
• b) pattern-resolved recording and storage of images of the measurement object for different positions of the pattern and for different states of the measurement object,
• c) point-by-point calculation of deformation and movement components or derived quantities such as stretch or speed using the stored image content.

2) A method according to claim 1, characterized in that the pattern is a parallel, equidistant stripe pattern. 3) A method according to claim 1, characterized in that the pattern is caused by interference is generated by light. 4) A method according to claim 1, characterized in that the pattern by an or multiple light beams or laser beams is generated. 5) A method according to claim 1, characterized in that the displacement of the pattern is generated by changing an optical path.   6) A method according to claim 1, characterized in that the inclusion or the Reading of the pictures with an electronic camera he follows. 7) A method according to claim 1, characterized in that several simultaneously Patterns are projected or that at the same time multiple cameras are used.