Design And Implementation Of Digital Closed Loop Control Scheme For Fiber Optic Gyroscope

The fiber optic gyroscope(FOG)was an all solid state sensor for measuring the angular velocity of carriers which had very high theoretical bandwidth and fast response speed.But bandwidth of the laboratory FOG was about 100 Hz.At present,method of establishing the dynamic model of FOG system was based on the equivalent ideal Z domain model,the proportional integral(PI)control was used as the closed-loop control scheme,and bandwidth of FOG was generally 100Hz~1KHz at home and abroad.However,there were nonlinear factors such as noise,delay and signal modulation-demodulation in the actual FOG system.Response speed and tracking ability of input signal were determined by control performance of the system which affected performance of the inertial navigation system.In this paper,the dynamic model of FOG system was improved,bandwidth of the system was improved,and the digital closed-loop control scheme was designed and realized.Firstly,according to working principle of FOG,function of each part was analyzed.Then ideal S and Z domain model were determined by the specific parameters of laboratory FOG.Matlab was used to simulate and analysis the ability in tracking step,slope,acceleration and sinusoidal input signals with the PI control scheme under the ideal conditions.Bandwidth of the system was given by S domain model.Then control scheme of second-type system plus zeros was proposed to improve performance of FOG.A Z domain model with noise,delay and modulation-demodulation of the system was established to simulate the real system.This model was used to simulate and analyse the tracking situation of ramp and sinusoidal input signals with different control schemes.The reason why second-type control scheme could not be applied in practice was that noise of the system was big while useful signal was small.To solve this problem Sigma-delta modulation technology was introduced to form a control scheme for improving the signal-to-noise ratio and reducing noise of the system.Simulation results showed that reasonable digital filter design of Sigma-delta control scheme could improve the tracking performance of FOG system.Secondly,the FPGA implementation scheme of digital closed-loop control of FOG was given.VHDL language was introduced for writing hardware modules of the timing control,digital demodulation,waveform synthesis,ladder wave feedback,Sigma-delta control CIC filter and half-band filter.Verilog was used to write the testbench file.Modelsim software was used to verify the correctness and feasibility of the implementation scheme.Then the whole FPGA signal processing integrated debugging was given.Finally,method of testing control performance of the system based on equivalent input was given.Overall scheme and data communication part of the test system were designed and verified.A practical test of dynamic response of FOG system with Sigma-delta control scheme was introduced under the designed step and ramp input test signal condition.According to the dynamic performance,bandwidth of the closed-loop system was estimated to approximately 208 Hz.Then Allan variance was used to analyse main kinds of errors and noise of FOG system with different control schemes.The results showed that the Sigma-delta control scheme could indeed reduce noise of the system.