Design And Implementation Of Phase Laser Ranging System For LIBS Detector

The automatic focusing system of LIBS detector must accurately obtain the distance between the emitted laser and the measured sample in order to make the laser accurately focus on the sample surface and ablate.The phase laser ranging module is very suitable for LIBS detector because of its fast ranging and high precision.This paper studies a phase laser ranging system based on FPGA applied to LIBS detector.PLL technology is used to generate dual frequency modulation signal.TDC time measurement chip is combined with digital circuit phase discrimination algorithm to calculate the phase difference and solve the distance.Compared with the commonly Mileseey S2 rangefinder(ranging accuracy ±2mm,size115*49*26mm)and Bosch GLM500 rangefinder(ranging accuracy ±1.5mm,size169*100*37mm),the accuracy of the ranging system designed in this paper can reach ±1mm and the test distance can reach 20 m,which is higher than the accuracy of the rangefinder on the market.At the same time,the ranging system in this paper is small(73*63*25mm)and can be placed inside the LIBS detector.On the premise of ensuring the ranging range and accuracy,the hardware design is simplified as much as possible to reduce the cost.The research contents are as follows:(1)Based on the characteristics of LIBS automatic focusing system,which requires high range accuracy and continuous and variable ranging range,the phase laser ranging method based on dual frequency ruler technology is selected.The overall scheme of laser ranging system based on FPGA is designed,and the design and manufacture of laser modulation circuit and APD photoelectric detection circuit are completed.In order to take into account the test distance and test accuracy,a coarse frequency and fine frequency PLL frequency synthesis circuit based on SI5351 A is designed.Considering the influence of different properties of target reflector on laser modulation signal,mixing circuit,waveform shaping circuit and time digital conversion circuit are designed to realize the phase discrimination processing of the test signal.(2)The functional module control of ranging system with FPGA as the core is completed.The FPGA configures the register of PLL chip and generates specific modulation frequency signal and controls the laser transmitting and receiving module.The FPGA configures TDCGPX2 chip to complete the precise time measurement of ranging signal and reference signal.The phase difference is measured by digital phase discrimination method,and the actual distance is calculated according to the principle of phase ranging.In order to reduce the error influence of phase difference measurement,phase calibration is carried out to make the ranging value more accurate.Finally,the ranging results are displayed on the PC through serial communication.(3)The circuit performance of each part of the hardware is tested,the waveform output of each part of the circuit is observed with an oscilloscope,and the ranging experiment is carried out for the overall debugging of the ranging system.The experimental results show that the phase laser ranging system designed in this paper can meet the index requirements of measuring range 20 m and accuracy ±1mm.