Modified adaptive filter for digital subdivision of a four-frequency differential laser gyro

Inertia navigation systems are widely used in various industrial and military applications. Among the available optical gyroscopes, ring laser gyro (RLG) is commonly used in the inertial measurement unit to determine the systems’ performance due to their wide dynamic range, digital output, and good linearity features. Recently, the four-frequency differential laser gyro (FFDLG), a solid-state type of laser gyroscopes, was developed to address some of the limitations of the traditional gyros, such as the locking problem. Besides being all-solid-state, FFDLG exhibits high sensitivity, high-performance index, and remarkable random walk, making it attractive for broader applications.

FFGLGs depend on the generated constant Faraday bias frequency to overcome some of the limitations of the traditional gyros. Over the last decades, extensive research has been conducted to improve the efficiency and performance of FFGLGs. This includes enhancing the resolution, reducing the quantization error based on frequency measurements and using the digital subdivision to enhance its performance by obtaining higher angular resolution. Unfortunately, based on the literature, most of the proposed resolution improvement methods are limited by noise interference. This results in poor quality of the input signal due to poor resolution and low measurement accuracy.

To address the issues mentioned above, Dr. Weibin Zhu and Mr. Gang Zheng from China Jiliang University, Mr. Linfeng Chen and Mr. Ke Liang from AVIC Xi’an Flight Automatic Control Research Institute, together with Dr. Yao Huang and Professor Zi Xue from National Institute of Metrology PR China proposed a modified adaptive filter to eliminate or suppress the beat noise of the four-frequency differential laser gyro. Due to the significant effects of the noise interference on the eightfold digital subdivision, the authors’ main objective was to analyze the influence of the beat noise to digital subdivision to optimize the adaptive algorithm and improve the demodulated signals. Their work is currently published in the research journal, Applied Optics.

In their approach, a demodulated signal model of FFDLG was constructed to analyze the dependence of the digital subdivision on the beat noise. The modified adaptive algorithm constituted a dead-zone operator of the error and a signal reconstruction process based on the least mean square adaptive algorithm. The authors also discussed the structure of the proposed filter and the criteria for selecting its parameters to ensure high performance. The feasibility of the modified adaptive filter was validated by implementing it on a field-programmable gate array (FPGA) chip.

Results demonstrated an improvement in the digital subdivision of the FFDLG. The filtering and signal division processes were successfully realized on the FPGA chip. The digital circuit structure replaced the sign-bit multiplication with a 2:1 multiplexer which improved the operation speed of the circuit, reduced its complexity and decreased the resources required. The modified adaptive filter increased the signal-to-noise ratio of the demodulated signal from 20 dB to 40 dB by suppressing the beat noise without changing the optical structure of the FFDLG.

In summary, the study reported the design of a modified adaptive filter to enhance the digital subdivision of FFDLG by suppressing the beat noise. The design was rather simple, less complex and robust; thus, it reduced the consumption of the hardware resources. Unlike the optical demodulation method in which the optical path needs to be adjusted to achieve the same effects, the proposed method required no change in the gyroscope structure to suppress the beat noise. The remarkably high signal-to-noise ratio was necessary to improve the performance of FFDLGs. In a statement to Advances in Engineering, Dr. Weibin Zhu explained that the presented new approach is promising for mass production as it reduces the production time for gyroscopes.