Objective The most often used technique to lessen the lock-in effect of a ring laser gyroscope (RLG) is mechanical dithering. However, the RLG output will maintain the dither rate which must be demodulated to obtain the true body rate. The integer period sampling method, high-frequency sampling filtering method, and dither stripping method are commonly used in RLG demodulation. For high dynamic tracking applications, since the sampling frequency is low, the integer period sampling method cannot meet the requirement of high bandwidth. Thus, the high-frequency sampling filtering method is applied. Due to the characteristic of a finite impulse response (FIR) filter, the high-frequency sampling filtering method inevitably introduces a time delay, which will result in significant tracking errors. Based on the correlation of signals, the dither stripping method can remove the dither signal in real-time through the correlation cancellation algorithm. In order to solve the problem that the time delay affects the accuracy of tracking in a highly dynamic environment, this paper proposes a demodulation scheme that combines FIR filtering with dither stripping. The dither stripping method is employed in the delay period of the FIR filtering so that the RLG demodulation can be zero-latency.
Methods Aiming at the case that the RLG fitted on a radar antenna concurrently rotates, a non-delay measurement method based on the combination of FIR low-pass filtering and dither stripping is creatively proposed. To avoid the accumulation of stripping errors of angular increment during the time delay period, the dither stripping is directly carried out in the angle output of RLG. The angle output of RLG after bandpass filtering is taken as the dither feedback signal and the low-frequency angular acceleration is regarded as a random walk process. Based on the correlation between the dithering feedback signal and the dithering bias signal, the gain factor of dither stripping is dynamically tracked through Kalman filtering, and the amount of dither before and after delay is calculated in real-time.
Results and Discussions The RLG experiment is conducted to verify the effectiveness of the proposed non-delay measurement method. The test results reveal that the dither stripping gain factor can be tracked through the Kalman filtering in real-time (Fig.3), and the dither stripping error varies within one pulse. The method can eliminate the dither component in a FIR filter delay period of 10 ms, where cumulative angle increment error is less than two pulses (Fig.4) and stripping in the angle output can effectively limit the accumulation errors (Fig.5). Compared to the 80-order FIR filter, the dither stripping accuracy has a negligible residual error of about one pulse, corresponding to 0.466′′, which achieves accurate demodulation without latency.
Conclusions In order to achieve high accuracy of RLG demodulation in a highly dynamic environment, a non-delay measurement method is proposed in this paper. The combination of FIR filtering and dither stripping gives consideration to the characteristics of high-bandwidth and real-time performance, which eliminates the negative effect of time delay introduced by FIR filtering. The experiment results indicate that the accuracy of the proposed dither stripping method is better than 0.5″ during the delay period of 10 ms, which achieves zero-latency measurement. This paper provides a new demodulation scheme for RLGs applied in fast-tracking scenarios.
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