Study Of Laser Diffracton Diameter Gauge Based On Fpga

Based on Fourier Optics Spatial spectrum analysis principle and diffraction theory a method of fine wire diameter measurement with laser diffraction is described, the calculation formula between fine wire diffraction intensity and its diameter within limited monochromatic parallel light field is identified. Fraunhofer diffraction diameter measurement system which is formed by the optical system, photoelectric conversion device and signal processing device is constructed in this paper. The difference between ideal and actual filament diffraction patterns is compared and the main reasons for the distortion of diffraction pattern is summarized as follows, the circular spots diffraction, the background stray light, self-noises of CCD, high-frequency noise inherent in the hardware circuit, etc. Circular spot diffraction interference model is established, simulation results and the measured diffraction patterns are basically consistent. FIR low-pass filtering method is proposed through analyzing the diffraction pattern spectrum mixed with the noise. 96-order FIR filter is designed using MATLAB according to filter technical specifications. Simulation result shows the filtering effect is ideal. Based on FPGA the overall scheme of diffraction patterns is designed, FIR filter is realized adopting improved distributed algorithm to save FPGA resource greatly. Accurate extraction of dark fringe is implemented using internal various logic resources of FPGA.In this paper, the FIR filtering results and the dark fringe extracting results of 80μm thin metal wire diffraction pattern is monitored in real time with embedded logic analyzer of QuartusII. Filtering results could meet the system requirements. System can achieve 0.87% relative error under 4MHz counting frequency. The main source of errors is analyzed and three methods for improving accuracy are given.