Research On Filament Diameter Measurement System Based On Double Grating

Diameter measurement of filament plays a vitally important role in laboratory studies and micro-processing technology, especially in micro-machines, biological equipment, optical processing, textile and other applications in various areas in the industry, where accurate measurement is required in such occasions to make sure of the quality of products.Based on a widely consulting of literatures at home and abroad, the main sections of work in this paper aimed at improving the accuracy of measuring the filament diameter are completed as follows:Firstly, a brand new method to measure the diameter of filament named "double grating" is proposed in this paper. A high-density grating, whose subdivision and amplification characteristics can be fully made use of to improve the measurement accuracy, is added behind the traditional diffraction model to diffract the spectrum of diameter and enlarge the distance between stripes to be measured.Secondly, the method to process the raw signal of diffraction spectrum measured by line scan camera is presented in detail. A zero-phase-shift filter consisted of two Butterworth low-pass filters is designed to filter the raw signal, and the filter's cut-off frequency is automatically confirmed by measuring the Extreme value of the Fourier space spectrum of its corresponding diffraction spectrum. The accurate positions of bright or dark stripes are calculated by twice quadratic fit on data near the initial extreme point. Among all the numerical methods to calculate the filament diameter according to the measured stripe spacing distance, the Newton Iteration Method is selected for its faster calculating speed.Thirdly, the real experimental system is introduced in detail, and the other parameters to be designed are calculated by the fixed parameters. The diffracting distances in the system are designed according to Fraunhofer's approximation condition. The distance between filament and the grating and the distance between the grating and the line scan camera are measured by a standard grating. The diffraction order m of diameter and n of grating are selected according to the measured distance. The size and opening width of filters are also designed at last.Finally, three standard filaments with nominal values 100.2?m,120.1?m and 140.8?m were respectively measured by the double grating method and traditional diffraction method under the same experiment conditions using the experimental system. The results of double grating method are 99.5?m,121.0?m and 141.6?m, while the results of traditional diffraction method are 102.4?m,121.8?m and 142.0?m. The relative measurement errors of the double grating method are less than 0.75% and its average relative error is reduced by 56% compared with traditional diffraction method. By analyzing the error caused by each factor in two methods, the advantage of the double grating diffraction method is verified for the fact that its uncertainties are respectively 0.83?m,1.16?m and 1.56?m for the standard filament while the uncertainties of traditional diffraction method are 1.01?m,1.41?m and 1.92?m.