Research On The 3D Profile Measurements Of Coal And Liquid Film Based On Structured Light Projection

China is the largest coal consumer in the world.The efficient and clean use of coal is very important to China.Coal combustion/gasification is a complex physicochemical process.Thermal expansion,volatilization release and crushing are often accompanied by changes in surface morphology,measurements of which are helpful to understand the mechanism of coal combustion/gasification.Liquid film is also widely used in thermal engineering,aerospace and other fields,such as falling film cooling.In a word,3D morphology measurement has important application value in industrial field.Based on structured light projection technique,the 3D surface profile of coal and liquid film,two important research objects in the field of energy,has been measured and studied,and the software has been developed.In this paper,the following three aspects were studied.For the N-step phase-shift profilometry,an improved fringe model is proposed.Additional phase is introduced to make the formula of wrapped phase more applicable.By constructing a complex number,the principal value of the complex argument is used to represent the wrapped phase,which solves the problem that the value range of the arctangent function does not match the range of the wrapped phase.Based on Qt and OpenCV libraries,the structured light processing software ScanPro is developed using C++ language.The software mainly includes three functional modules: fringe generation,system calibration and multi-frequency phase-shifting profilometry,which can switch between Chinese and English.Based on multi-frequency phase-shift profilometry,the surface profile of coal blocks of centimeter size was measured and studied.Compared with ordinary objects,the biggest problem in the measurement of coal is that the surface of coal block is generally dark,while some local surface is strongly reflective due to the influence of some minerals,which leads to a large number of invalid phase points clustered in a small range in the process of phase unwrapping.After analyzing the causes and distribution characteristics of invalid phase points,it is found that invalid phase points mainly include background area,shadow area and other invalid phase points caused by random noise.Compared with normal phase points,the distribution of invalid phase points caused by random errors presents the characteristics of general random dispersion and local small-scale aggregation.Based on the intensity of fringe background,the background and the strong reflective points on the surface of coal are removed.The shadow area is removed by modulation contrast.The reliability criterion is defined according to the phase gradient of unwrapped phase,and the residual invalid phase points are removed based on the connected domain analysis.The other two reliability-oriented correction algorithms in literature are compared and analyzed.For smooth surface coal,the three algorithms can get good correction results,but for non-smooth surface,the improved algorithm in this paper has better correction effect and higher efficiency.The results of software operation show that the invalid phase correction scheme proposed in this paper is not only applicable to the 3D profile measurement of black objects such as coal blocks,but also to ordinary objects.The system error is analyzed from two aspects of phase error and calibration error.The non-sinusoidal error of DLP4500 projector is relatively small.When the fringe period is small,the error caused by fringe discretization reaches 0.06 rad.The calibration accuracy of camera and projector is the similar,and the RMS error of feature points is about 0.1 px.So the errors in the measurement of coal mainly come from fringe discretization error and random error.The measurement accuracy of the coal block profile is verified by the gauge block,and the accuracy in the height direction is about 80 μm.For the three-frequency phase-shifting method used in this paper,when the image resolution is 1280×960 px2,the processing speed of the software can reach 3 Hz at the fastest.The time of phase calculation and system calibration accounts for more than 80% of the whole calculation time,which is the main factor restricting speed of the software.Based on Fourier transform profilometry,the profile of falling liquid film on vertical wall was measured and studied.The effects of height change rate,random noise and fringe frequency on the accuracy of three-dimensional reconstruction are verified by simulation calculation.With the increase of the standard deviation of random noise,both the phase RMS error and the height RMS error increase.For slowly varying surface objects,when the noise standard deviation is equal to 0.1,the phase RMS error is 0.019 rad and the height RMS error is 0.062 mm.When the fringe frequency increases,the height RMS error decreases gradually.The experimental results show that the maximum liquid film thickness increases first and then decreases as the liquid film moves downward along the wall.The average measurement error of height is about 0.10 mm,while the RMS error is about 0.081 mm.