Key Technology Research Of Polymer Film Defects Detection Based On CCD Scanning

The performance and selling price of transparent and semi-transparent polymer film or substrate materials, such as polyester substrate using for X-ray film, cine film, liquid crystal display(LCD) plate substrate, printing polymer and well-packed film, will be affected by the internal defects or the surface defects. At present the polymer film or substrate materials are always produced by rolling, casting, blowing and other methods. The common form of defects is as gel, extrinsic contaminants, surface scratch, etc. And the defects are always granular or irregular shape. Therefore, it makes sense to design a high speed and high resolution detection system for 100% online detecting and comprehensive controlling the product quality.Key technologies of CCD scanning polymer film materials defects detection are deeply investigated in this thesis. A polymer film materials defects detection system is established. The low-pass filter effects of lens are analyzed. Time and spatial frequency domain responses of CCD signal are analyzed using the average sampling principle. The filter cutoff frequency in an ideal condition is obtained according to the characteristics of signal frequency spectrum extension caused by CCD sampling. At the same time, the image high frequency changes caused by the average sampling are got. Also the detection resolution and image high frequency changes affected by different cutoff frequency of low-pass filter are analyzed, which shows the significance of adjustable parameters filter to optimizing system resolution. Relationship of the CCD output signal and optical integration time, and relationship of CCD driving frequency are described. In order to obtain the most defects image information, as well as the optimal signal-noise ratio, a follow-up signal processing scheme is established. The CCD driving and finite impulse response(FIR) filter units with adjustable parameters are designed on one field programmable gate array(FPGA) chip, satisfying the high speed and real-time requirements of the system. A computer may configure the CCD integration time, CCD driving frequency and the filter parameters through USB2.0 bus, which make the detecting system flexible. So this CCD signal acquisition system with adjustable parameters can be widely used in different CCD scanning image detecting systems.There are two outstanding characteristics in polymer defects detection system. One is that region of interest(defect region) in a whole image is a small probability event. The other is big gray scale difference between defect region and background image. According to the two characteristics, the image gray scale is set to represent background gray scale and the region of interest image is picked up by FPGA in real time. Thus the image data is greatly compressed on the premise of retaining useful information. A dynamic extraction image method is designed under uneven background gray scale conditions. A threshold calculation unit and state machines are set on the FPGA to implement image compression and data packages. And the computer reconstructs the defects images according to the uploaded data through USB2.0 bus. Commonly used image matrix operators in the image processing field are studied in order to further compress data. Image pre-processing technology is achieved on FPGA chip according to the characteristics of the matrix operators such as large amounts of data, simple arithmetic and parallel structures. This method provides a solution for the massive image processing data. The experiments proved the polymer film materials defects detection system presented here can accurately detect defects in size and gray information in real-time.