Study On Strategy Of Long-Range Rapid Auto-Focusing For Image-Based Three Dimension Measurement System

With the development of computer technology and digital image processing technology, imaging measurement technology has considerably developed in the measurement field. It has a good many merits, such as simple structure, non-contacting, abundant of information, high speed, and high precision .At present, it has been widely applied to kinds of industrial measurement.Auto-focusing method based on image technology is investigated in this paper. Main research works are as follows.1) This paper introduces the current situation of auto-focus technology, outlines the advantages of auto-focus technology based on image technology and chooses this method as the theoretical premise, and expounds the source of this subject.2) This paper puts forward the technical indexes of the auto-focusing system, presents the holistic design, and analyzes the reason of design scheme and parts selection, including the transmission mechanism, CCD camera, image data acquisition card , motion control card, motor, optical lens , light source, and so on.3) After summarizing several common image pre-processing techniques and introducing kinds of clarity evaluation functions, this paper chooses the evaluation function based on edge gray gradient, describes the definition in detail, analyzes the characteristics, and studies the influencing factors, which lay a foundation for the proposal of the rapid auto-focusing technique in large range.4) The strongpoints and shortcomings of previous focusing methods are introduced detailedly in this paper. To overcome the shortcomings of previous focusing methods, such as low efficiency and small range, a rapid auto-focusing technique with a long-range is presented in this paper. According to the on analysis of focusing evaluation functions, the Variance function is chosen as the coarse focusing function, and the Tenengrad function is chosen as the accurate focusing function. After more deeply research, coarse focusing is gotten by using cubic spline interpolation according to Variance function in a large range, and accurate focusing is gotten by using cubic spline interpolation according to Tenengrad function in a small range. Lastly, Gauss fitting is accomplished by solving the overdetermined equation concerning the Gauss function according to Tenengrad function in a smaller range, and the focal plane is corresponding obtained. Factors that affect the auto-focusing precision have been analyzed through a series of experiments. The results of experiments indicate that this method can meet the demands of long range, high speed and high precision, and it has good generality.5) Factors that affect the auto-focusing precision have been analyzed through a series of experiments, and several solutions are proposed to reduce the corresponding influences.Main innovations are as follows.1) This paper investigates the characteristics of the evaluation function based on edge gray gradient in depth, and analyzes application of the function in kinds of industrial measurements sufficiently.2) This paper proposes a rapid auto-focusing technique in long range. Firstly, coarse focusing is gotten by using nine points interpolation according to Variance function. And then accurate focusing is gotten by using seven points interpolation according to Tenengrad function. Lastly, the focal plane is obtained by more accurate according to Gauss fitting. This method expands the focusing range from 1mm to 60mm, and it also has the advantages of high precision and high speed.