Research On Spatial Filtering Velocimetry Based On Image Sensors

Optical velocimetry has a great advantage over many other traditional velocity measurement techniques due to the non-contactness.Therefore,optical methods are developed to be one of the new prospects in the field of velocity measurement.Among these methods,spatial filtering velocimetry has received wide applications in industry and scientific research since its simplicity,stability and high presicion.In this dissertation,the evolution of spatial filtering velocimetry is retrospected and the current research status abroad and home is introduced in detail.Then,spatial filtering velocimetry is investigated thoroughly,and an engineering prototype of spatial filtering velocimeter is developed.It is clarified that the basic principle of spatial filtering velocimetry is to utilize the fact that the light intensity of the moving image of the measured object is modulated by the periodic transmittance of the spatial filter.The mathematical model of spatial filtering effect is established to investage the principle of spatial filtering velocimetry theorectically.The power spectra of three kinds of spatial filters,common spatial filters,differential spatial filters and spatio-temporal filters,are obtained from that model.From the aspect of power spectrum,the spatial filtering characteristics,the spectrum bandwidth,deviation of central frequency and peak transmittance,of spatial filters with a rectangular window and rectangular transmittance are deeply analyzed,providing theorectial basis for parameter optimization in spatial filters.The configuration of a vehicular spatial filtering velocimeter based on a linear image sensor is designed.The utilization of an image sensor as a spatial filtering element enhances the flexibility of spatial filter design.The image sensor can also be used as a photodetector,simplifying the structure of spatial filtering velocimeters.The configuration of a spatial filtering velocimeter with bi-channel and bi-detctor is proposed to enhance the depth of field of the measurement system.A spatial filtering velocimeter integrated with a laser displacement sensor is proposed not only to compensate the fluctuation of scaling factor caused by the variation of the working distance but also to overcome the disadvantage that the scaling factor has to be recalibrated after every installation.A spatio-temporal filter is designed based on an image sensor,which can remove the direction ambiguity in two opposite directions,double the velocity measurement range,and reduce the uncertainty of velocity measurement.By full comprehension of principles of spatial filtering velcimetry and laser Doppler velocimetry,a fringe image velocimeter based on a digital micromirror array is proposed.The principle of the digital micromirror array is introduced,and a method of generating fringe image is suggested.Three configurations of the fringe image velocimeter based on a digital micromirror array are put forward,including common,frequency shifting,and Janus configuration.An integrated solution for signal processing in spatial filtering velocimeters is proposed according to the characteristics of spatial filtering signals.The integrated solution is composed of three modules,including signal conversion and pre-processing,frequency calculating,and results output.The signal conversion and pre-processing module realizes output conversion of the image sensor,analog-digital conversion,spatial filtering operation,and signal pre-processing.The frequency calculating module accomplishes fast Fourier transform and frequency correction on a field programmable gate array(FPGA),obtaining the frequencies of spatial filtering signals fast and accurately.The results output module decides the validity of signals via validity criterions and makes corresponding output.Non-stationary spatial filtering signals resulting from large acceleration of vehicles are processed by quadrature demodulation to get instantaneous frequencies.The two orthogonal signals used in this method are obtained by two orthogonal spatial filters.An engineering prototype of spatial filtering velocimeter based on an image senor is developed.The engineering includes four stages.Firstly,the design of a telecentric imaging system,which has an aperture stop in the back focal plane.Secondly,the FPGA-based design of an industrial camera,including the driving circuit and acquiring circuit of a high speed linear image sensor.Thirdly,the FPGA-based design of real-time and accurate signal processing system,which realizes spatial filtering operation,fast Fourier transform,frequency correction based on energy centrobaric correction,and validity judgement and output of signals.Lastly,the configuration design of the velocimeter.In order to evaluate the performance of the designed spatial filtering velocimeter,laboratory experiments were conducted with a high precesion rotary table.The results show that the designed velocimeter has a relative velocity measurement uncertainty better than 0.54%,linearity better than 0.734%,and a field of depth as much as 110 mm.Furthmore,to evaluate the dynamic characteristics of the vehicular spatial filtering velocimeter on actual road surfaces,field tests were carried out together with GPS and a self-developed laser Doppler velocimeter,whose measurement uncerainty is better than 0.08%.The results show that the vehicular spatial filtering velocimeter has the advantages of full independence and more data output compared with GPS,and that the drop-off is less compared with that of the laser Doppler velocimeter.To examine the feasibility of the velocimeter for testing on different roads,field experiments were carried out on common asphalt road,sandstone road,soil road,and road with mixed sandstone and soil,showing robust performance on different roads.Vehicular integrated navigation tests were conducted with velocimeters and inertial measurement unit.The calibration process of the scale factor of the spatial filtering velocimeter and the installing declinate angle of inertial measurement unit is discussed.In the vehicular integrated navigation tests of spatial filtering velocimeter and IMU,the maximum positioning error of the vehicle obtained by inertial measurement unit is reduced from 10 468 meters to 54 meters in two hours.In the vehicular integrated navigation comparing tests of spatial filtering velocimeter and inertial measurement unit,laser Doppler velocimeter and inertial measurement unit,the maximum positioning errors of the vehicle obtained by the spatial filtering velocimeter and the laser Doppler velocimeter are 43 meters and 20 meters,respectively,in 1.8 hours,while that obtained by inertial measurement unit is 7 216 meters.Experimental results fully verify the feasibility and validity of the application of the spatial filtering velocimeter to vehicular integrated navigation system.