| Resonator Fiber optic gyro (R-FOG) is a high accuracy inertial rotation sensor based on Sagnac effect, and it plays a decisive role in navigation and guidance system. For the Sagnac effect is very weak, signal detection system shows its great significance in improving the performance of R-FOG. In order to achieve the high accuracy and reliability, digitalizing the R-FOG system and using the closed-loop operation must be the trends.This paper firstly introduces the design scheme of digital R-FOG system using triangle wave modulation technique. By adding triangle wave on phase modulator (PM) to make the lightwave of the laser which later injects into the Fiber Ring Resonator (FRR) alternate between a positive and a negative frequency increment, the photodetector (PD) will give out square wave signal. Next use the same frequency square wave as reference signal to demodulate the PD output signal and get the amplitude by lock-in-amplifier (LIA) thchnique. There are two optic circuits in FRR transmitting in clockwise (CW) and counterclockwise (CCW) respectively. The demodulation signal of one optic circuit is fed back to tune the fiber laser's frequency so as to track the resonance frequency change, and the other demodulation signal would be the open-loop rotation signal of R-FOG.During the process to track the resonance frequency, there is distortion phenomenon of PD's output signal, not like the ideal square wave or direct signal, but overdip/overpule included. Further analysis shows that this is the transient response process after triange wave modulation, and the processs is related to the parameters of FRR, so is the distortion degree. By sampling the stable signal of the distortion square wave to demodulate, or oversamling the distortion signal to get a number of data, then accumulating and averaging these data to demodulate, the distortion's effect can be considerably decreased. When using digital serrodyne modulation in R-FOG, the step time should be equal to the transmission time in the FRR, otherwise the PD output signal fluctuates and the demodulation curve deviates from the ideal one based on analyzing the phase relationship of the electric field. By averaing the signal and using symmetry positive and negative frequency modulation, such unideal effects can greatly be avoided.Because of the weak signal, it is very difficulty to measure the resonace frequency directly and necessary to make one optic circuit lock at the resonance frequency. This paper picks up the simple mathematic model of the feedback loop, designs the proportional intergrate (PI) control system, tests the actual closed-loop performance, and analyses how the noise in the loop can affect the output signal, especially the optical circuit noise. In the digital detection system, the main functions are realized in FPGA, and the noise resistance of digital LIA is calculated. By testing the delay time of signal to compensate the reference, the maxim demodulation ouput.is realizedWith the above analyses, the R-FOG system using digital triangle wave modulation is set up, and the detection circuit based on FPGA is developed. At last, the R-FOG resolution for 0.5°/s is obtained... |
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