Papers by Keyword: Autofocusing

Abstract: The present study proposes a laser-based microscope system in which a precise autofocusing capability is achieved using a position feedback signal based on the distance L between the geometrical center (Xc, Yc) of the image captured by the CCD sensor and the centroid (xcentroid, ycentroid) of the image. The experimental results show that the proposed system has a positioning accuracy of 2.2 μm given a working range of ±200 μm. The autofocusing performance of the proposed system is thus better than that of a conventional centroid-based system, which typically achieves a positioning accuracy of around 5.2 μm.

Abstract: Large aperture collimator which has been widely used for calibrating and testing various optical devices plays an essential role in correlative laboratories. As being the basic testing and calibration equipment, the large aperture collimator’s accuracy should be much higher than the device under testing in order to ensure the accuracy of the measurement. However, the process of adjusting the collimator is extremely complicated due to the collimator’s large aperture and long focal length. So it is difficult to ensure the measurement’s quality and easy to cause the system being vulnerable to the surrounding environment. One of the most common problems is defocus. In order to solve the problem above, this issue presents a new type of autocollimator autofocusing system which uses pentaprism instead of using large-aperture plane mirror, semiconductor lasers as light source and CCD sensor as receiver. The system is smaller, lighter, and more convenient when using. The computer simulation shows that the autofocusing system’s resolution could reach the accuracy of 40μm. If we use the relevant algorithms to execute the sub-pixel scanning, the resolution could reach the accuracy of 10μm. It shows that the system could satisfy the required testing precision of testing large aperture optical device.

Abstract: DVD player is regarded as a mature and low-cost commercial product for multimedia and computer data storage. Fundamentally the pickup head is a key part of the DVD player and it uses focused laser beam to read the data on the DVD disk tracks whose width is approximately 0.74μm. This research focused on the control aspect applied to a DVD pickup head for the development of an auto-focusing laser probe. Practically, the laser pickup head has the S-curve linearity property with the measuring range of about 7μm [3]. To extend the measurement range, the feedback control of VCM in the focusing direction is needed. This study used Genetic Algorithms (GAs) to search for appropriate PID gains for VCM control. In MATLAB simulation, the GAs tuning scheme could produce the optimised gains within less than 0.02 sec. Moreover, the experimental results demonstrated that the VCM auto-focusing measurement range could be extended to 1350μm.

Abstract: The primary purpose of this research is to adopt a commercially available DVD pickup head and modify it to become a high-speed scanning nano-measurement probe. With the principle of astigmatism the probe can execute the autofocusing motion by an imbedded voice coil motor (VCM) following the height change of the tested object in the vertical Z-direction. Given high precision triangular current signal with appropriate frequency to the input ports of tracking, the same VCM can be moved along the lateral X-direction for profile scanning. Firstly this paper presents the structure of DVD pickup head, the theory of autofocusing and auto scanning, and the developed controller. Then experimental setups and accuracy calibration will be introduced. In order to achieve a bi-directional precision measurement in autofocusing and scanning, this study has developed a hysteresis error compensation scheme by a DSP-32 integrated system. In association with a high precision linear stage in Y-direction, this high precision micro/nano optical probe can measure the 3D profile of the miniature object successfully at fast speed.