31 December 2013 Sub-millimeter servo system for sample positioning based on thresholding of defocused laser spot
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Abstract
Accurate sample positioning and automatic sample operation can improve the performance of measuring instruments. A Sub-millimeter servo system for sample positioning based on thresholding of defocused laser spot is proposed. The effective laser spot image is extracted by thresholding of the light cone section on the sample surface. By estimating the section area and centroid of the spot, the defocus status and position of the measured sample can be acquired. A focused light cone at visible wavelength is cast onto the surface of sample, forming a marked laser spot as the indicator of the measurement point. A CCD camera is used for visual imaging, and a high-precision three-dimensional motorized translation stage is used for the accurate servo control. The marked spot is real-time monitored and processed in the platform of LabVIEW. The Autonomous Thresholding Image-Processing Algorithm (ATIPA) is proposed to detect and analyze the defocused marked spot, through which system creates a servo whereby accurate position control of the sample can be achieved. The measurement point on the sample can be accurately located by computing the center coordinates of the marked spot. And a focus function is implemented by measuring the size of defocused spot. This focus function is then used within an improved climbing search algorithm to obtain the focused sample position via moving the sample stage. Experimental results show that the system could measure the laser spot and control the sample in a robust, repeatable way within reasonable errors. The accuracy of the sample autofocus reaches 0.1 mm.
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Zheng Wang, Liangcai Cao, Enyao Zhang, Guofan Jin, "Sub-millimeter servo system for sample positioning based on thresholding of defocused laser spot", Proc. SPIE 9042, 2013 International Conference on Optical Instruments and Technology: Optical Systems and Modern Optoelectronic Instruments, 904203 (31 December 2013); doi: 10.1117/12.2037990; https://doi.org/10.1117/12.2037990
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