Design And Implementation Of Digital Signal Processor For Double Closed-loop RFOG

A resonant fiber optic gyro (RFOG), as same as an interferometric fiber optic gyro, is based on the Sagnac effect. It has the potential to achieve the high accuracy with a short sensing coil. The RFOG is a good choice of the fiber optic gyro with small volume and light weight. Compared with the analog signal processor, the digital signal processor has the advantages of smaller size, higher processing speed, better stability, stronger anti-interference ability and so on. Compared with the single closed-loop detection, the double closed-loop RFOG can improve the linearity and expand the dynamic range. In this article, a digital signal processor for the double closed-loop RFOG is designed and implemented based on a single field-programmable gate array (FPGA). The main achievements of this article are as follows:(1) A digital signal processor is designed and implemented for the RFOG based on the sinusoidal phase modulation technique. Both of the modulation signal generation and the synchronous signal demodulation are realized on the FPGA based on the CORDIC algorithm. The equivalent input noise of this processor is as low as9.95nV/√Hz.(2) A resonant frequency servo loop is designed and implemented for the RFOG based on the servo controller. To suppress the linear resonant frequency drift, a double integral term is added based on the traditional proportional-integral controller. As a result, a close-to-zero bias error is achieved at the demodulated output of the resonant frequency servo loop. To reduce the effect of the overflow resetting in the servo controller, an auto-controlled reset technique is proposed and experimentally demonstrated. As a result, the time for returning to the lock-in state is reduced to5ms from8s.(3) A high-accuracy frequency shifter for the RFOG is designed and implemented based on the digital serrodyne phase modulation. The sensitivity and dynamic range of the frequency shifter is improved by optimizing the digital serrodyne waveform. The frequency shifter has the frequency resolution of0.0075Hz, which is equivalent to a rotation rate of0.03°/h for an RFOG with a12-cm diameter fiber ring resonator.(4) The serial communication system between the computer and FPGA is designed and implemented. The computer can control the parameters in the FPGA in real time and sample the signals with high speed. The usability and flexibility of the system are improved.The digital signal processor for the double closed-loop RFOG is implemented based on the work above. With the integration time of1s, the equivalent accuracy of the resonant frequency servo loop is increased to0.18°/h (la). The gyro bias stability below0.5°/h is successfully demonstrated with the integration time of105s. Compared with the single closed-loop, the measured result shows that the double closed-loop RFOG improved the linear detection range from±215°/s to±1507°/s, which is improved by a factor of7. The scale factor nonlinearity is decreased from1.2%to0.02%(200ppm), which is improved by a factor of60.