Design And Implementation Of The Digital System For R-FOG Based On The FPGA

Fiber optic gyroscope is a highly accurate inertial rotation sensor based on Sagnac effect. It plays an important role in the field of sensing technology. Compared with the well-developed interference fiber optic gyroscope (I-FOG), resonator fiber optic gyroscope (R-FOG) shows advantages in miniaturization. Since the Sagnac effect is very weak, the signal detection system is essential for a high performance R-FOG. The detection system based on a digital circuit has a higher flexibility and a smaller size compared to that based on the analog circuit. It also outperforms the latter in the aspects of anti-interference and signal processing. Therefore, the digitalization of the signal detection circuit is in accordance with the development trend of miniaturizing the R-FOG. Using FPGA as the core hardware, we design and actualize a closed loop operation R-FOG system with an all-digital signal processing technique. Based on the serrodyne phase modulation with a positive and negative slope, the detection system is achieved with a single FPGA.In the paper, the system design is elaborated in three aspects: modulation, demodulation, and feedback. The design scheme of the digital closed-loop system based on the serrodyne phase modulation with a positive and negative slope is proposed. Based on the phase modulation scheme, the signal detection principle is analyzed and the selection principle of the modulation waveform parameters is proposed according to the demand of reduction of the backscattering induced noise. A digital lock-in amplifier (LIA) is used to demodulate the signal from the photodetector. The detection precision is 120 nV.The design of the feedback scheme consistes of three parts: the proportional-integral (PI) controller, serrodyne amplitude feedback controller, and the second closed-loop for the outputting of the Sagnac resonant frequency difference. The central frequency of the fiber laser and the resonant frequency of the fiber ting resonator (FRR) are susceptible to changes in environment and temperatures. The central frequency of the laser must be locked to the resonant frequency of the FRR in one direction, CCW, as an example. The PI controller is the essential part for the laser frequency-locking loop. It could efficiently eliminate the steady-state error and reduce the drift of the R-FOG by the PI feedback controller in the loop. The digital PI controller is designed and optimized. The control precision of the digital PI controller with FPGA is 8.7°/h.In the serrodyne phase modulation, it is very important that the amplitude of the driving waveform is exactly equal to 2π. Large reset pulse noise is induced when the serrodyne amplitude deviates from 2π. A feedback loop is setup to make the serrodyne amplitude exactly to 2πusing the transient responses of the FRR. A second closed loop to track the resonance of the other counterproprogating lightwave (CW) of the resonator is achieved with a serrodyne waveform adding to the two frequency modulation waveform. The new added serrodyne waveform acts as an equivalent frequency shifter.To increase the flexibility of the R-FOG system in debugging and data-processing, the communication and control system based on the FPGA is designed. The communication between the R-FOG detection system and the computer is realized.Based on the above design and works on the digital system with FPGA, the whole measurements on the gyro system with digital serrodyne modulation technique are carried out. A standard deviation of the zero-drift stability of 159.3°/h for two hours duration is demonstrated for the R-FOG with sensing fiber length of 18m.