A Digital Auto-focus Method

Auto-focus is a key technology of automatic control. With development of science and technology, auto-focus is used more and more in many areas, such as industry, daily life, and military affairs. It inversely require more improvement of this technology, such as higher precision, higher speed, better stability, and lower cost, so more research is needed.The basis of our paper is a auto-focus system applied to microscope, which uses SCM controlling and CCD analog signal processing. The system firstly processes analog video signals with a multi-feedback band-pass active filter. The filtered signal which represents the image details is magnified and then adjusted by integrator. And this signal after A/D converting is regarded as the focus evaluation of estimate. Then the focusing is controlled by SCM and executed by actuation mechanism.In the paper, we improved the original system in some aspect and add some new function. Firstly We changed the work pattern from one sampling per step to successive sampling with continuous rotation of actuation mechanism, which shortens the focusing time. Secondly we changed the sampling mode, using dual monostable multivibrators to select the focusing window, which solves the sampling drift problem raised by SCM break. Then we integrated the switch power supply to the PCB, making the control box much smaller. Finally we added the PC into our system to control auto-focusing through serial port which makes long-distance remote control through internet become possible.The other important work of this paper is that we propose and implement a digital auto-focus system. Its special advantage is we can use various focus evaluation measure function to fit different samples, and we can select focus window wherever we want in the image by dragging our mouse. It will be useful for some rough sample. The system uses digital video board to get image data from CCD, and process the data in PC with focus evaluation measure function, then control the actuation mechanismto implement the auto-focus. Finally we make some experiment to prove the system's availability and stability. The results are analysed.