Research On Improving The Measurementaccuracy And Speed Of The FMCW Laser Ranging System

The subject of our group is developing a high precision spherical coordinate measuring instrument,which is based on frequency modulated continuous wave(FMCW)laser ranging techlonogy and can measure large non-cooperative target,and the FMCW-based laser absolute distance measurement system is an importanct part of the instrument.Because of the advantages such as high pricision,large range,no measurement blind area and not need cooperative target,the FMCW-based laser absolute distance measurement system has an important application value in metrology,aerospace and precision manufacturing and had been a research hot-spot in the field of absolute ranging.The theoretical precision and resolution of the FMCW-based absolute distance measurement system are both limited by the frequency sweeping bandwidth of the laser.Because the external cavity laser(ECL)has a very large bandwidth(100 GHz),it enables the measurement accuracy of the FMCW-based laser ranging system to reach micron magnitude.However,there are several problems in the actual measurement situation.Firstly,the factors,including the tuning nonlinearity of the laser,the noises and the mode hopping,the picket fence effect,the fiber dispersion and the doppler effect,could lead to the decline of measurement accuracy.Secondly,the common frequency extraction methods have the problems of low precision or needing too much computation,so these methods can not meet the requirements of the high precision and real time measurement.Thirdly,the scattering characteristics of the object surface can affect the pricsion of the FMCW-based laser ranging system when it measures the noncooperative targets.In view of the above three problems,this paper studies the factors that affect the measurement accuracy and speed of the FMCW-based laser ranging system.The main research contents are as follows:1.The principle of FMCW-based laser ranging technology is studied and the influence of the laser tuning nonlinearity on the ranging accuracy and resolution is analyzed.Then a FMCW-based laser ranging system with a dual-interference optical path is built and the influence of laser tuning nonlinearity is eliminated by frequency sampling method.In addition,the influences of the noises and mode hopping,the picketfence effect,the fiber dispersion and thedoppler effect on FMCW-based laser ranging system are analyzed briefly.2.The influences of the noises and the mode hopping in the auxiliary interference signal and the measurement interference signal on the measurement accuracy of FMCW-based laser ranging system are analyzed in detail.According to the characteristics of noise and mode hopping,an adaptive filtering method and a phase matching mosaic algorithm are proposed.Simulation and experimental results show that these methods can effectively eliminate the influence of the noises and the mode hopping and improve the measurement accuracy.3.According to the characteristics of the interference signals when measuring noncooperative targets and aiming at the problems of low precision or needing too much computation of thecommon extraction algorithms,a non-cooperative real-time processing algorithm is proposed.In order to prove that the algorithm can meet the requirement of real-time and high-precision measurements for non-cooperative targets,the simulations results of the non-cooperative real-time processing algorithm and the several common algorithms are compared and analyzed.In addition,the real time ranging experiments without cooperative target were conducted.4.The influences of the surface scattering characteristics on the power of the echo measurement signal and the measurement accuracy are analyzed theoretically.On this basis,several typical machining roughness samples are measured in real time.