Design Of Phase-shift Laser Ranging Device

Laser ranging technique is broadly used for its accuracy, speed and focusing ability. There are two major laser ranging techniques: pulse measurement and phase-shift measurement. Pulse ranging is mainly used in long distance ranging applications and is employed in many ranging devices. However, the phase-shift technique is more suitable for melee precise ranging applications as traditional analog phase measuring technique has been replaced with digital method to greatly improve the accuracy. Higher modulation frequency for laser beam is required, according with industrial applications'calling for higher accuracy, which is tough for most traditional optical and electronical parts. Modulation frequency up to 1MHz has been reported successful in the homeland.This paper presents a laser range finder based on sine waves. It can modulate the laser beam up to 10MHz by relatively simple circuits. Major researching achievements are:(1) The overall design of the whole system has been completed. The laser beam modulation frequency has been increased to achieve better accuracy. The circuit has been designed in a simple manner to reduce unexpected error and debugging complexity. Most of the electronical parts have a good cost performance, which makes the whole system very practical.(2) The hardware, which is the main part of the device has been carefully designed and implemented, including the laser-beam modulating and emitting circuit, then photo-electric detecting circuit, the frequency synthesis and mixing circuit and the phase-shift measuring circuit.(3) The software, which mainly operates in MSP430 in the phase measuring module, has been designed and implemented. The capture function of Timer_A in MSP430 counts the pulse width and period of input signal. In order to calculate the very small phase difference between the two signals, multi-cycle counting and calculating residue using period as divisor has been presented, which is superior to the traditional method.(4) By combining the designing experience and others'work, qualitative and quantitative analysis have been made for the sources of errors in the range finder. After that was some practical solutions for improvement.(5) Some basic research was done on the integration of the laser ranging system and vision inspection system. The basic structure of both the optic and electronic part was designed.Simulation in Pspice was made beforehand to validate the feasibility of the system. Actual testing implemented afterwards has seen good waveforms and results. The whole device maintains good stability, which paves the way for its integration with the computer vision system.