Research On Environmental Error Of Fiber-optic Gyroscope And Suppressing Method

Fiber-optic gyroscope?FOG?has been widely used in various types of inertial navigation systems due to its state-of-the-art advantages such as small size,low weight,high reliability and fast start-up time.Though the development has been lasting for more than several decades,some engineering problems are still need to be further investigated in practical applications.Particularly,environmental influences such as thermal perturbation,vibration and magnetic field are primary factors that give rise to nonreciprocity phase shift in FOG and limit its further applications..This thesis mainly focuses on the mechanism of environmental errors in FOG and the corresponding suppressing methods.The main work of this thesis is listed as follows.The basic operating principle and the structure component of FOG are introduced in detail,and the control system model of digital closed-loop FOG is deduced and analyzed theoretically.The simulation schematic of all-digital closed-loop FOG based on square wave modulation is constructed then by Matlab-Simulink platform.The effectiveness of the control model is validated by parameters of the fiber gyro with reasonable selection.Frequency characteristic curves of the gyroscope control system are obtained based on the model.The dynamic performance of the fiber gyroscope under the excitation of step signal,slope signal and different frequency sinusoidal signals are simulated.Based on finite element method?FEM?and structure material of FOG,the temperature distribution in boundary and internal of fiber coil is obtained.Furthermore,the mathematical model of thermal drift in whole temperature range(1^oC/min,-40^oC to+60^o C)is established based on thermo-optic effect and elastic-optic effect of the optical fiber,and also based on the distribution of thermal expansion coefficient in radial direction,angular direction and axial direction of the fiber coil.Temperature experiments including temperature control condition and temperature variation condition are performed to verify the effectiveness of the model.Based on this model,further study of thermal drift is focus on:Suppressing the thermal drift by reasonably placing the position of the heat source relative to the coil;The influence of temperature variation rate on thermal drift and an effective method to evaluate the thermal performance of FOG is proposed;To optimize both thermal drift and thermally start-up time,the method by controlling the thickness of the insulation layer is studied.By taking these methods synthetically,the thermal stability of FOG can be reduced to 0.01^o/h or less in the full temperature range.The mathematical model of vibration error of FOG is established by considering elastic-optic effect of the fiber,the kinetic equation of each loop in the coil,and the quadrupole symmetrical winding method.The model is then verified by frequency-fixed vibration experiment with different amplitudes?1g-5g,50Hz?and FFT transformation.Based on this model,the influence of material parameters such as elastic modulus and density of the fiber coil is studied in detail.Then,the structure of FOG is analyzed by modal analysis and the material design is optimized.by Comsol multiphysics software,and the lowest natural frequency of the modified FOG is improved to above 2000Hz.Finally,the vibration error caused by the asymmetry of fiber tail of coil and waveguide is studied in detail.It is pointed out that the asymmetric length of the fiber tail is proportional to the vibration error.Through the comprehensive design to fiber coil,material,and structure,the output error caused by vibration could be greatly reduced.The magnetic error of FOG is studied based on Faraday magneto-optic effect and the circular birefringence effect of the polarization maintaining fiber.Firstly,the magnetic-induced error model based on single-loop coil is established.Then,the influence of fiber birefringence,fiber torsion rate,fiber length and magnetic field angle on the drift error is discussed and simulated respectively.Further,amother model based on multi-loop coil under random torsion is established.The influence of the radial magnetic field and the axial magnetic field on the output error of FOG is discussed respectively.To verify the established model,magnetic sensitivity test is performed for FOG which is mounted in Helmholtz coil.The results of the zero-stage magnetically shielded condition validate the effectiveness of the multi-loop coil model.The results of the two-stage and one-stage magnetically shielding conditions show that the single-stage magnetic shield has been fully satisfy the effect of magnetic shielding.Finally,a method by utilizing compensating fiber coil to suppress the radial magnetic susceptibility of FOG is proposed.