Research On Key Points Of Digital Demodulation Of Open-loop Fiber Optic Gyro

Interferometric fiber optic gyroscope which is used to measure the angular velocity of rotation of moving objects is a typical Sagnac interferometer.It is widely used in military and civilian applications.Open-loop fiber optic gyroscopes have obviously cost advantages compared with the closed-loop fiber optic gyroscopes,and their accuracy requirements are lower,which have broad space in the civilian field.In this paper,the digital demodulation method of open-loop fiber optic gyroscope is studied.Firstly,the optical structure of the gyro and some error mechanisms are introduced.Then,the modulation technology of the open-loop fiber optic gyro and several existing demodulation methods are analyzed.According to theory of the digital phase sensitive detection method,some components and design points of the digital signal processing circuit are analyzed.In order to eliminate the influence of temperature,the influence is compensated by changing the amplitude of the modulation wave,which is realized by changing the reference voltage.After that,the digital demodulation process based on FPGA is analyzed in detail.In addition,this paper proposes a new demodulation method.For this method,we firstly analyze its principle and advantages in theory,then introduce the key steps and precautions of the algorithm implementation,and finally compare with the digital phase sensitive detection method,and then analyze their advantages and disadvantages respectively.Another main content of this paper is the study of the demodulation error of open-loop fiber optic gyroscopes.Different from the previous error analysis,this paper discusses how the non-ideal effects(source coherence and nonlinear phase modulation)caused by the two main components of the optical structure affect the whole system.The effect of the filter on the modulation depth is then analyzed.Finally,the effects of white noise,quantization noise and CORDIC algorithm noise on the output are analyzed.The experimental results are as follows:Under normal temperature conditions,the digital phase sensitive detection method has a scale factor stability of 787ppm,a bias of 6.4°/h and a bias stability of 6.7°/h in the open-loop fiber-optic gyro prototype with a fiber loop length of 275 m,an outer diameter of 33 mm,and an inner diameter of 22 mm.This results consistent with the results obtained by using the SR830 lock-in amplifier.The new demodulation method proposed in this paper has a scale factor stability of 3100ppm in the integrated gyro which has 500m fiber length,a bias of 18°/h,and a bias stability of 0.64°/h.