Design Of High-precision Displacement Measurement System Based On Diffractive Grating

With the continuous progress of modern science and technology，microelectronics manufacturing industry, nanotechnology and technique of MEMShave been booming. It is based on the sophisticated measuring technology. Opticalmeasurement methods stand out, among all kinds of methods, because of thefeatures of high measurement accuracy, non-contact measurement, wide dynamicrange and easy-to-digital feature. Measurement system based on grating regards thegrating pitch as measuring basis. Therefore, it doesn’t request of high stability ofthe source frequency. Otherwise, the optical structure is relatively simple and small.It is able to do measurements in high-resolution, high speed and wide range in theultra-precision machining. Countries abroad are in the ability of mass production,while China is still a gap. It is significant to develop a kind of displacementmeasurement system based on grating in high-resolution and high-speedmeasurements.The main research content and work are given:Firstly, the moire fringe theory, Doppler shift theory, the diffraction of lightinterference theory and polarized light theory are elaborated. The measuring opticalsystem is designed and built according to these theories. The derivation of thesystem based on the Jones matrix theory is completed. The optical system can makethe frequency of signals become four times with less optics elements. Because ofthe feature of polarization, the system works well with the separation of measuringlight and stray light, and the effect of disturbance is reduced.Secondly, the signal processing methods of grating displacement measurementsystem, especially the electronic methods, are analyzed. The method of judgingmoving direction and segmentation is proposed.Thirdly, hardware circuit is designed for signal processing, and it is composedof units of FPGA and DSP. According to the error of grating measuring system,signal conditioning circuitry is designed, after which the unequal amplitude errorand non-orthogonal error is eliminated. The software program is also completed. Itcontrols high speed analog to digital convention, and calculates displacement, andcommunicates to the computer. In addition, display interface is realized usingLabVIEW.Finally, Error analysis and comparative experiments of the measurementsystem is done. It tells how grating, laser, optical path, circuit and environment effects the system accuracy and evaluates the system accuracy. The function of theoptical system and circuit system is verified, and the resolution of the measurementsystem is tested. Experimental results show that the system can achieve a2.6nmresolution and the maximum measurement speed is up to100mm/s.