Real-time Direction Judgment System of Sub-nanometer Scale Grating Ruler

This paper presents the design of a real-time direction judgment system for a sub-nanometer scale grating ruler based on field-programmable gate array (FPGA) technology. On the basis of the working principle of the grating ruler reading head, the characteristics of the grating ruler phase signal are analyzed. In view of the frequency uncertainty and nonlinear error of grating scale phase signal laser, a preprocessing structure based on biorthogonal lock-in amplifier is designed. Aiming at the problem of low data update rate and insensitivity to direction switching in short stroke of traditional grating direction judgment method, a real-time direction judgment method is proposed. On the FPGA platform, the signal generator is used to simulate the actual photoelectric signal of grating ruler, and the calibration experimental platform of electrical analog signal is built to verify the effect of the real-time direction judgment system on the FPGA platform. The results show that compared with the traditional direction judgment system, the data update rate is improved by 256-fold. Moreover, a method for using the result of the direction judgment system to compensate the count value of the electronic subdivision when the object is moving backward is proposed.