Development Of Linear CCD Video-extensometer

The measurement of force and strain is core work in the field of material mechanics testing. There are many obstacles in using the traditional strain measurement. Such barriers range from low accuracy, difficult operation, to failure to adopt the traditional contact clamp for samples under test and to measure percentage total extension at fracture. Therefore, it is economically beneficial to develop a non-contact extensometer that can not only overcome the shortcomings of traditional extensometer, but also have a very significant economic benefit.This dissertation aims at analyzing the current demand and status of strain measurement in the field of material testing and researching on the related technology development of the latest video extensometer, so as to develop a systematic plan which bases on linear CCD and can directly output strain signals for high precision video extensometer. The hardware circuit of the linear CCD camera design is introduced, and the selection and noise of the linear CCD video signal processing achievement is presented in the dissertation. The structure and process of software system are introduced and it has achieved functions such as controller initialization, CCD drive plus control, ADC control, FIFO data reading, image processing and analysis, goal displacement calculation, image display and communication. While demonstrating the mechanical structure design of the linear CCD camera, the denoising algorithm, which affects the accuracy of the system’s CCD digital signals, is analyzed by building an ideal edge model basing on Sigmoid function. The comparison is performed for the advantages and disadvantages of algorithms of method of median filter, method of average value wave filt er and method of wavelet filter. The algorithm of image edge detection is studied, and the sub-pixel detection algorithms of Gaussian fitting method and least square method are compared. Automatic lens distortion correction algorithm is proposed. Finally, the experiments have verified the automatic lens distortion correction algorithm, calibrated the prototype of the extensometer, and performed the ordinary carbon steel dumbbell specimen tensile test.The video-extensometer developed in this paper does not upload images to a computer for data processing to obtain the strain value, but calculat e the strain by itself. The strain values are uploaded to the computer of the tensile testing machine via a serial port. This innovative modular design and simple way o f communication is helpful for the promotion and application of the video extension project. The high resolution linear CCD and appropriate hardware circuit and software algorithm are selected for the signal processing. The video extensometer developed in the paper meets the high precision requirements of grade 1.