Detection Of Conical Thread With Digital Image Processing Technology

1 Measure principle and system constructionsAn detecting system with digital image processing technology used for conical thread consist of the light source , clamps ,CCD camera ,PCI card ,computer and output equipments .Figure 1 show the system concrete construction . Work principle is: first make use of CCD camera to take a conical thread picture, then transport the picture into the computer by PCI card, finally get the contour picture by digital image processing technology. The contour in picture is proportional to the actual one, and the ratio is a constant K determined by CCD camera magnified ratio. Calculating the geometry parameters and comparing with the actual size. 　　2 Digital image processing The detecting software is developed by VC++6.0 , Fig 2 show the main process, it consist of the image pre-processing, getting the contour and calculating the geometry parameters.2.1 Collecting and pre-processing image The picture is transferred into the computer by DH-CG400 PCI card, its rate could reach to 132MB/S and the support with CPU is not needed . The method of image pre-processing is smoothing image, and the purpose is to reduce the interferers. The ordinary interferers include spiced interferer, impulse interfere, and gauss interferer etc. If there are random black & white points in picture, it is the spiced interferer. If there are random black or white points in picture, it is the impulse interferer. The character of gauss interferer is that the distributing of interfere points in the picture is normal. The method of smoothing image is consistent to the character of the interferer. In the space area averaging the value of the neighborhood points might reduce the interferer, and in the frequency area the method of reducing interferer is filtering the wave with a low frequency value. Smoothing image usually make use of the template, the template operation is a kind of neighborhood operation, its work principal is that wiping off the variety point by operating the value of neighborhood point. But smoothing image can make itself illegibility, so the different template is used to reduce the relevant interferer. The templates such as , are common used, the first is 3×3 mean template, and the last is gauss template. Fig 4 is the 3×3 mean filter result, fig 5 is 3×3 gauss filter result, and fig 6 is the mean filter result. We can make the conclusion that the mean filter result is better than other results, because the image is clearer and the contour line is well preserved. 　Fig.3 Original image　 Fig.4 3×3 mean filter result　 Fig.5 3×3 Gauss filter result　　Fig.6 Mean filter result Edge detection Unless getting the contour line, you can not detect the geometry parameters of conical thread. Edges are areas in an image where rapid changes occur in the intensity function or in spatial derivatives of this intensity function. The goal of edge detection is to recover information about shapes and reflectance or transmittance in an image.The methods using edge detection operator to get the contour is applied to the simple image in which the measured object is obviously contrasted to the background. But the calculation quantity of operator is very great and the efficiency is lower, so this method is not suit to the measured request. The work principal of the threshold method is : first get the threshold in the histogram, then convert the image to the binary one, at last search the object edge and get the contour line. Edge detection operator There are many kind of edge detection operator, such as Sobel operator, Prewitt operator, Roberts operator, and Gauss-Laplace operator etc. (1)Sobel operator. Fig 7 show that the Sobel operator consist of two templates, the one respond for vertical edge, and the other respond for horizontal edge. The maximal value of two results is put to the output value. Fig 10 is the Sobel edge result. (2)Prewitt operator. Fig 8 show that the Prewitt operator include two templates too. Its w...