High Precision Hybrid Pulse and Phase-Shift Laser Ranging System

With the rapid development of military, aerospace, and precision manufacturing technology, a multitude of situations need to carry out a large-range and high-precision distance measurement. The growth of measurement applications has led to a higher requirement for the laser ranging technology which can be accomplished by using different patterns. At present, the pulse laser ranging method is widely used for medium-range and long-range measurement because of the fast measurement speed and considerable measurement range. However, the ranging precision is low. The short-distance measurement mostly adopts the phase-shift laser ranging method which has high ranging accuracy but limited measurement range. Therefore, the research on lifting the accuracy of pulse laser ranging method and extending the measurement range of the phase-shift laser ranging method will be carried out. In this thesis, combining the existing pulse laser ranging system and phase-shift laser ranging system, dual-frequency and single-frequency hybrid pulse and phase-shift laser ranging systems are proposed. The basis for solving the current problems of poor measurement precision in pulse laser ranging method and short measurement distance in phase-shift laser ranging method are provided. Also, the designed structures have a broad application prospect in the fields of industrial production, military, and aviation. At the beginning of the thesis, the principle and characteristics of the current typical laser ranging methods are introduced and analyzed. According to the Fourier Series theory, the spectrum analysis of the pulse signal and the relationship between the pulse signal and the same-frequency sinusoidal signal, the idea of phase-shift laser ranging based on pulse modulation signal is generated. Instead of a continuous sinusoidal signal, the laser is modulated with a periodic pulse signal. Distance measurement by calculating the phase difference on the sinusoidal signal extracted from the pulse signal with the same frequency at the receiving end. Based on the principle of conventional dual-frequency phase-shift laser ranging method, a dual-frequency pulse laser ranging method is proposed. The distance to be measured is obtained by transmitting two periodic pulse signals with different frequency and then combining the implementation of rough and accurate measurement outcomes. Afterward, a single-frequency pulse laser ranging method is introduced. After receiving the pulse signal, the direct counter method is used to realize rough measurement and phase-shift of the co-frequency sinusoidal signal is utilized to improve the ranging accuracy. This proposed model has the advantages of high ranging precision and long-distance measurement without any other auxiliary frequency. The accuracy of the phase difference calculation is the most critical element in both the dual-frequency and single-frequency laser ranging systems. Currently, the commonly used phase difference calculation methods operated in phase-shift laser ranging system are digital synchronous detection, fast Fourier transform method, and all phase fast Fourier transform method. Published works have discussed the performance of frequency estimation and initial phase calculation using these approaches. In this thesis, the precision of phase difference measurement based on these methods above is compared. The effects of normalized frequency deviation, white Gaussian noise, harmonics are simulated in MATLAB. Simulation results show that all phase fast Fourier transform method has a superior anti-noise ability so that exceptional accuracy of phase difference measurement can be achieved. Furthermore, as the number of sampling points increases, all phase fast Fourier transform method will obtain a more accurate calculation consequence. Finally, this thesis carries on the co-simulation test of the designed dual-frequency and single-frequency hybrid pulse and phase-shift laser ranging systems in Optisystem and MATLAB. The transmitting frequencies of pulse signals operated in the dual-frequency method are 15 MHz and 150 KHz. The pulse used in the single-frequency method is set to 15 MHz. In the simulation, the performance of proposed methods is tested by setting various measuring distance. When the number of sampling points is 1024, the standard deviation and ranging error of the dual-frequency method are 3.72 cm and 13.6 cm within 963.15 meters. For the single-frequency method, the results show a 3.78 cm standard deviation and 14.6 cm ranging error. Simulation results illustrate that the proposed ranging methods have lower ranging error compared with recently published works. It means that the combination of the pulse method and the phase-shift method can achieve high-accuracy and long-range measurement