Research On Laser Doppler Velocimeter For Vehicle Self-contained Navigation System

Inertial navigation system has abtained widely application with the advantages of being fully autonomous, free from external interference and simultaneously output the position, velocity and attitude. However, the positioning error of pure inertial navigation system showed divergent trends over time due to the initial alignment error, device error and inertia calculation error. Laser Doppler velocimetry has been an important development direction in velocity measurement field with the advantages of high accuracy, fast dynamic response, wide measuring range, good linearity, etc. From the practical point of view, the dissertation has carried out theoretical and experimental studies on the principle, optical structure, signal processing of the vehicle laser Doppler velocimeter, and preliminary vehicle laser velocimeter integrated with inertial navigation experiments have been conducted.The positioning error of pure inertial navigation system shows divergent trends over time. To improve the accuracy of the vehicle autonomous navigation, the laser Doppler velocimeter and inertial measurement units integrated navigation system is proposed. The development in optical structure, signal processing method, application areas of laser Doppler velocimetry have been reviewed briefly and the present researches in laser Doppler velocimetry are described.The basic principles of light Doppler effect and the laser Doppler velocimetry are expounded. It is pointed out that the optical heterodyne detection is advantageous comparing with direct spectral detection. The principle of the balanced optical detection is reviewed.The classic dual-beam optical structure, the backscatter reference-beam optical structure and the multi-layer type structure are reviewed. Considering the limitation of the reference-beam structure, the split reuse-type structure is proposed to improving the signal to noise ratio; To eliminate the Doppler frequency error caused by the laser wavelength drift, the wavelength-insensitive optical structure has been proposed; a dual-detector optical structure based on a dual-longitudinal-mode laser source is proposed which dramatically reduces the scale factor between Doppler frequency and the measured velocity, and is more suitable for high-speed measurements.An integrated solution for laser Doppler signal processing is proposed based on the establishment of a laser Doppler signal model. The integrated solution is composed of three modules which are signal conditioning, Doppler frequency solver and results output. The signal conditioning module realizes the photoelectric conversion, preamplifier and analog-digital conversion. The Doppler frequency solver achieves fast and accurate Doppler frequency solving on a digital signal processing chip. Signaloutput module determines the validity criterion validity of the Doppler signal and makes the corresponding output. It is proposed that Hilbert-Huang transform is employed to process the non-stationary laser Doppler signal. Simulation results show that the velocity and acceleration can be obtained through the ensemble empirical mode decomposition and instantaneous frequency. The errors of the vehicle laser Doppler velocimeter are quantitatively analyzed. And a variety of means to suppress or eliminate the errors are given. An engineering prototype of laser Dopple velocimeter is developed with the split-reuse type optical structure. In order to evaluate the performance of the developed laser velocimeter, laboratory experiments have been conducted with a constant speed turntable. The results show that: the measurement accuracy of the developed laser velocimeter is better than 0.08%; the measurement linearity is better than 0.082%, the low velocity limit of the laser Doppler velocimeter is measured to be 0.0678m/s. To evaluate the dynamic characteristic of the vehicle laser Doppler velocimeter on actual road surface, field test is carried out between vehicle laser velocimeter and a velocimeter from Beta laser Mike whose accuracy is better than 0.05% according to its parameter manuel. Results show that the drop-off of the vehicle laser Doppler velocimeter is fewer comparing with the one of Beta laser Mike. Vehicle navigation tests have been conducted with laser velocimeter integrated with inertial measurement unit(IMU). The dead reckoning navigation mode is selected. The calibration process of LDV scale factor and the installing declinate angle of IMU is discussed. Better results of vehicle navigation test has been obtained by employing the 1-D laser velocimeter and IMU taking the differential GPS as reference test. A normal distribution model of the LDV measuremens error and a simplified IMU heading error model are established. The dynamic measurement error of LDV is inversely estimated to be better than 0.08% by comparing the numerial simulation results and the positioning error of previous field test.