| The quality of an object’s surface influences its using function characteristicsdirectly and it is also an important standard of processing technology evaluation.Meanwhile, with the development of mechanical processing technology, opticaltechnology and microelectronics technology development, higher request oftechnology and method for surface topography is put forward. The white light verticalscanning is a kind of non-contact face scanning interfering method which is widelyused in the surface technology optical measurement because of its advantages of highprecision, large arrangement, non-fuzzy phase, and high speed, high stability in the3D surface topography measurement.Nowadays, most of the white light interferometers focus manually. As theinterference length of white-light just ranges from a few microns to a dozen microns,it is may easily missed for the reason that manual adjustment is too fast or thehuman’s reflect is too slow or just because of visual fatigue. What’s more, the verticalscanning trip determine manually won’t be so accurate. In order to avoid the abovedisadvantages, a method of auto-focus scanning with a change of image grayvariance is presented in this paper. This method is able to focus measured object tothe interference range of interference scope automatically and determine the verticalscanning trip according to the arrangement of the interference fringes. For a morestable performance of the interferometer, a double level motion control mechanismof coarse and fine is developed in this paper, which means using the separated controlmechanism to realise the function of automatic focus and vertical scanningrespectively.The main contents and innovations of the paper are as follows:1) A white light vertical scanning interferometer with a control structure of driveninterference object lens is developed according to theory of white light verticalscanning. It is able to realize the automatic focus scanning function. The maintechnical indicators: vertical scan range:0-14μm, vertical scanning accuracy:±0.03μm, field area of view range:0.36*0.27mm2 2) A two-stage coarse and fine motion control agencies is developed and make fulluse of respective advantages of coarse and fine agencies. Coarse-level institution withstepper motor single axis drive is used to realize interferometer auto focus function.Its journey is150mm.and the single-step length is0.3μm. Fine-level institution is apiezoelectric ceramic micro-displacement, which is used to interferometer verticalscanning. Its journey is14μm, and the precision is±0.03μm.3) The white light auto focus scan algorithm based on the image gray variancechange is put forward. It includes the following steps: firstly, compute the image grayvariance change ratio; Secondly, compare this ratio with the predetermined threshold，so as to judging the initial and ending positions of the interference fringes; And finally,determine the interference scanning range according to the two positions. Thefeasibility of the algorithm is proved in experiments, and the paper also analyzes thefactors affecting the threshold, which providing the basis for the choice of thresholdsfor later measurement of other objects. |
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