Research On Key Technology Of Moire Fringe Nanoscale Subdivision

Grating sensor has such advantages as long range, strong ability to adapt to the environment and low cost, which is widely used in precision displacement measurement, digital readouts and numerical controls, long stroke precision positioning, precision processing and microelectronics industry, etc. Current industries put forward higher requirements of nanoscale and long range and so on to grating displacement measurement.Moire fringe subdivision is the key to the grating nanoscale measurement. High density grating can achieve nanoscale measurement only with low subdivision, but the low density grating needs high subdivision to achieve nanoscale measurement. Low density grating has such advantages as long range and low cost, low density grating sensor of50lines/mm is chosen as the research object. At present, Moire fringe subdivision methods at home and abroad mainly focus on the optical and electronic method, which has such main problems as serious effect by Moire fringe signal quality and difficult error correction, etc. The key problems of Moire fringe subdivision and pitch error correction are studied, and the new subdivision method and the pitch measurement method are proposed, completed work as follows:Firstly, wavelet subdivision method is studied. The method subdivides grating sensor’s output signal, because the optical part of sensor is fixed, the nanoscale measurement is achieved by high electronic subdivision. The unequal amplitude and non-orthogonality are corrected in order to adapt to the poor quality signal of sensor output.The optical structure model of grating sensor is built with Fresnel diffraction theory, low density grating based on the transmission principle can’t ignore the influence of interference and diffraction effect when it achieves nanoscale measurement, and the light intensity variation law of low density grating is analyzed by establishing the mathematical model of modulated light field by grating. Light intensity has directly relationship with such factors as the pitch, the distance between index grating and scale grating, the distance between scale grating and the photoelectric converter and so on, those influence factors are change with the movement of the grating sensor, so the output waveform of grating sensor is very complicated. The moving speed of grating sensor is not always a constant speed in the measuring process, especially in the process of start, stop and the reversing process, the signal is random. The output signals are analyzed with random theory, the results show that the frequency and amplitude is not wide-sense stationary and phase is wide-sense stationary. The wavelet subdivision method is proposed based on the randomness of grating signal.Wavelet subdivision method modulates the phase and frequency of the wavelet base function with grating output signal and then completes wavelet transform to modulated signal, phase corresponding to the maximum ridge reflects the displacement, and scale corresponding to the maximum ridge reflects the frequency that is the speed value. Modulated wavelet base and wavelet base can obtain the biggest similarity in the process of transform, which can improve the measurement accuracy. The method solves the problem to choose wavelet base, it is not necessary to choose the wavelet base again, whatever the method is applied in any the field, practicability is strong. At the same time identification direction can be realized according to the positive and negative of the phase difference of the adjacent moments and increase and decrease of modulated wavelet base signal frequency. The experiment system is constructed based on the synchronous data acquisition card and Lab VIEW, which can acquire the grating sensor output signal and preprocess the non-orthogonal correction and filtering, experiments verify the feasibility of wavelet subdivide method. The phase modulation and frequency modulation belong to phase subdivision, which has better anti-interference performance than the amplitude subdivision.Secondly, subdivision method is studied based on the space Moire fringe signal acquired by CCD. The method uses CCD to replace the four photoelectric cells inside traditional sensor, which can obtain the integral period space Moire fringe light intensity signal, and improve the space resolution. On the basis of the analysis of the pitch lines error and CCD acquisition error, subdivision method of the single spectral line algorithm and phase correction is proposed with a period of space Moire fringe signal, which realizes the displacement measurement through calculating the phase difference of Moire fringe. The experiment system is constructed based on FPGA, experiments verify the feasibility of subdivide method. This method has advantages of calculating with one way signal and unaffecting by unequal amplitude, non-orthogonal problems of the four way signals. Finally, measurement and error correction method of grating pitch are studied. Grating pitch is the benchmark of accurate grating displacement measurement, and the pitch value is the key to further improve displacement measurement accuracy. According to the dynamic characteristic of the grating Moire fringe, the measurement method to convert pith quantity to time quantity is proposed in the movement process of grating sensor. Based on the high order cumulant principle the time delay within which the Moire fringe signal passes through the two fixed distance photoelectric cells is measured, and then the sensor moving speed can be obtained, the time corresponding to grating pitch can be obtained according to the two adjacent minimum values of the Moire fringe signal that are determined based on the least square principle, the pitch value is determined according to the product of the moving speed and the corresponding time of pitch. The experimental results show that the measuring error is less than0.08μm for the grating sensor with the pitch of20μm, which verifies the feasibility of the method. This method can measure each pitch, and reduce the accumulation error and subdivision error based on each pitch error correction, which laid a foundation for large range and high precision measurement.