Image measurement device and method for the surface deformation of specimen based on sub-pixel corner detection

Patent metadata

Field	Value
Publication number	US-11119016-B2
Application number	US-201716648177-A
Country	US
Kind code	B2
Filing date	Oct 18, 2017
Priority date	Oct 18, 2017
Publication date	Sep 14, 2021
Grant date	Sep 14, 2021

Abstract

A digital image measurement device and method for the surface deformation of specimen based on sub-pixel corner detection is disclosed. This digital image measurement device is composed of a new type of image pressure cell, a complementary metal-oxide-semiconductor (CMOS) camera, a camera bracket, a flexible lens hood, a computer and matching measurement software. This method discretizes the specimen into several four-node finite elements by printing grids on the specimen and takes corners of the grids as the nodes of the finite elements; tracks the deformation of the feature points in real time by edge detection and corner detection based on sub-pixel; captures the deformation of the whole surface of the specimen by the two flat mirrors which are at an 120° angle behind the specimen; achieves the observation of the deformation of the whole surface by conducting splicing and error correction on the three images.

First claim

The invention claimed is:

1. A digital image measurement method for a surface deformation of specimen based on sub-pixel corner detection, wherein, taking grids of a specimen as identifying features and tracking and recording nodes in real time by using a sub-pixel corner detector algorithm; and, an image of the whole surface of the specimen is captured simultaneously by two flat mirrors of an image pressure cell; at the meantime monitoring axial strain, radial strain and volumetric strain of the specimen; obtaining partial and whole strain fields of the specimen according to displacement of the nodes by introducing finite element method and finally deformation of the whole surface of the specimen can be obtained; detailed measurement process is as follows:
1) installing the specimen
the specimen is installed between a specimen pedestal and a specimen cap of the image pressure cell; taking grids on the specimen as identifying features and the features are facing a complementary metal-oxide-semi conductor camera; images captured by the two flat mirrors behind the specimen is required to be symmetrically distributed to ensure that a coordinate of each corner of the grid on the specimen is tracked and recorded in real time by the complementary metal-oxide-semiconductor camera with sub-pixel recognition accuracy;
2) adjusting the complementary metal-oxide-semiconductor camera
adjusting a camera bracket through a vertical shaft, including adjusting an angle of rotation and height of the camera; the specimen and images captured by the two flat mirrors are all located in a window of the complementary metal-oxide-semiconductor camera; then, a flexible lens hood is installed on the front panel of the image pressure cell to shield external light; fine-tuning a universal material testing machine, so that a loading rod can slowly contact with a specimen cap; adjusting aperture and focal length of the complementary metal-oxide-semi conductor camera to make a resulting image bright and clear; conducting” Start Selecting Points” in advance; all corners of “Start Selecting Points” can be identified and “Start Selecting Points” have good repeatability;
3) starting experiment
after conducting “Start Selecting Points” in advance, starting the experiment and measurement; the image of the front surface of the specimen is captured by the complementary metal-oxide-semiconductor camera; the deformation of the whole surface of the specimen is captured by the two flat mirrors which are at an 120° angle behind the specimen; finally, conducting splicing and error correction on three images and the real time deformation of the whole surface of the specimen is obtained, achieving observation of the deformation of the whole surface; disassembling the whole measurement device at the end of the experiment;
the measurement method is implemented based on a measurement device, including an image pressure cell, a complementary metal-oxide-semiconductor camera and a computer;
the image pressure cell is composed of an image pressure cell pedestal, a specimen pedestal, flat mirrors, a specimen, a specimen cap, a semi-cylindrical cavity, a piece of flat tempered glass; a front panel of the image pressure cell is a piece of flat tempered glass which is fixedly connected with the semi-cylindrical cavity forming a sealed pressure chamber; there are two flat mirrors at a 120° angle inside the pressure chamber at a back of the specimen; the specimen is printed with white grids or is wrapped in a black rubber membrane which is printed with white grids and fixed on the specimen pedestal, while the specimen pedestal is mounted on the image pressure cell pedestal, and there is a specimen cap mounted on a top of the specimen to transfer a load of the loading rod; the whole surface of the specimen is divided into grids and discretized into several four-node finite elements; corners of the grids are the nodes of the finite elements; the coordinate of each node is tracked and recorded by using sub-pixel recognition accuracy and then deformation process of each node can be obtained;
the image pressure cell is located on the universal material testing machine; a camera bracket is fixed on a vertical shaft of the universal material testing machine, and can rotate freely on the vertical shaft; a complementary metal-oxide-semiconductor camera is fixed on the camera bracket and its position is adjustable; the complementary metal-oxide-semi conductor camera is located in front of the image pressure cell, and its optical axis is perpendicular to the flat tempered glass of the front panel of the image pressure cell; then the image of the whole surface of the specimen is captured simultaneously by the two flat mirrors of the image pressure cell coordinating with the complementary metal-oxide-semiconductor camera; finally, the computer displays the image in real time after processing;
the sub-pixel corner detector algorithm is as follows:
the detection of sub-pixel corners is to determine the sub-pixel precision position of corners in gray-scale images through an iterative algorithm; assuming that an exact position of the corner is at point q, then any vector that points from q to another point in its neighborhood is perpendicular to the image gray gradient at pi; because of noise, the dot product of the two vectors is not equal to 0, resulting in errors ε i :
ε i =∇( I ) T ●( q - p i )

where ∇(I) T is the image gray gradient at pi; the value of the point q should be the coordinate of the point which can minimize ε i ; then, simultaneous equations containing all the p i points in the detection area, assuming there are n points, are converted to solve for X so as to minimize AX=b; here A is the matrix of ∇(I) T , and b is the column vector of ∇(I) T ; when x=(A T A) −1 A T b, the ∥Ax−b∥ can be minimized; in this way, a new point q is obtained; the algorithm takes the new point q as the center of the area and continues to use this method for iterative operation to obtain a higher sub-pixel accuracy; conducting iterative operation until the center q is kept in a given threshold range and finally, the point q is determined.

2. The digital image measurement method for the surface deformation of specimen based on sub-pixel corner detection according to claim 1 , wherein, it also comprises a flexible lens hood; the flexible lens hood is used to shield external light interference and its one end is fixed to the camera bracket; the other end is freely adjustable; the flexible lens hood is manually installed on the flat tempered glass during the experiment and removed at the end of the experiment.

3. The digital image measurement method for the surface deformation of specimen based on sub-pixel corner detection according to claim 1 , wherein, circular light-emitting diodes are specially designed to withstand high pressure and placed on the top and bottom of the semi-cylindrical cavity to ensure an unchanged lighting environment for photographing of the complementary metal-oxide-semiconductor camera.

Assignees

• Suzhou H C Soil & Water Science And Tech Co Ltd
• Univ Dalian Tech

Inventors

• Shao Longtan
• Guo Xiaoxia
• Liu Yonglu
• Huang Chuan
• Wu Mingming
• Li Xicheng
• Xia Pingxin
• Ju Peng
• Liu Xiao
• Wang Chun
• Xue Jie
• Wang Ying

Classifications

• H04N23/55

  Optical parts specially adapted for electronic image sensors; Mounting thereof · CPC title

• G01N2203/0647

  Image analysis · CPC title

• G06T2207/30172

  Centreline of tubular or elongated structure · CPC title

• G01N3/08

  by applying steady tensile or compressive forces (G01N3/28 takes precedence) · CPC title

• G01N3/068 Primary

  with optical indicating or recording means · CPC title