Auto-focusing Technique For Ultrasonic Microscope And Its Application

As an excellent non-destructive testing technique, ultrasonic microscope has been widely used in industry, medicine, biology and so on. It is worth mentioning that the focusing is an important link in the testing process. However, current focusing technique is not developed, cannot meet the development needs of automation. Therefore, research on auto-focusing technique for ultrasonic microscope is significant in both theory and application.In order to overcome the lack of present auto-focusing technique for ultrasonic microscope, this dissertation seeks to develop a fast, accurate and universal auto-focusing technique. Well-established theoretical models were applied to build the theoretical model of auto-focusing. Based on this model, auto-focusing method was devised, combined with coarse focusing and fine focusing. And a new approach was proposed for fine focusing of thin layer’s surface. Simulations and experimental inspection feasibility study were successful. The incident sound field of focused lens simulation using multi Gaussian beam method showed that energy is the largest at the focus. Reflectance spectrum in the case of oblique incidence was discussed. The result showed that reflection coefficient of the material surface is a real number in a small angle range, approximately equal to normal incident reflection coefficient. The lens system in different positions was analyzed by ray model. When material is at the focal, the most energy is received by transducer. V(z) curve model was established. It was proved that when the material is at the focus, the maximum amplitude of echo signal is received.The general formula of coarse focusing ensuring the speed of auto-focusing was derived. Fine focusing included three programs, Peak, Hilbert and FFT method. FFT method was the best method and could be well used for time-resolved acoustic microscope. Thin layers play an important role in the field of materials science and engineering. There is a need to inspect these thin layers, so a new method was used for thin layers’surface auto-focusing. The workflow of auto-focusing technique was designed.Finally, the study of the auto-focusing applications was carried out and related experiments were conducted on self-developed experimental platform (scanning acoustic microscope). Auto-focusing technique was successfully used for microscopic imaging and material evaluation of the acoustic characteristics. Through experiments on three aluminum sheets with different thickness, the new method for auto-focusing of films surface was verified.