Study Of High-speed Three-dimensional Shape Measurement Using Rotary Mechanical Projector

Optical three-dimensional(3D)sensing technology,with the excellence in noncontact,high-precision and high reconstruction efficiency,has been widely used for industrial manufacturing,biomedicine,virtual reality,cultural heritage and fossil protection etc.With the penetration and application of optical 3D sensing technology in other fields,the measured scene gradually expands to the dynamic processes,no longer limited to measuring "stationary" objects.Especially in the field of industrial processing,it is often necessary to reconstruct 3D shape of a product or an operated object whose surface shape is changing rapidly.For example,in-line product testing,autonomous navigation of mobile robots,obstacle detection in automatic driving,automotive crash testing,tracking the trajectory of weapons(guns)and rockets,separation and deformation detection of stamping sheet under pressure,stress concentration caused by object geometry or load discontinuity,as well as the state law of moving objects in aerodynamics and kinematics,such these dynamic processes are ecpected to accomplishing the corresponding 3D shape reconstruction in high-speed.Fringe structured light projection based 3D shape measurement technology projects grating fringes to the surface of measured object actively,and obtains 3D shape data through fringe analysis,phase unwrapping,phase-height mapping.It has the advantages of simple structure,high precision,high-speed,low-cost,easy implementation,and easy to popularize and apply.In recent years,with the rapid development of high-speed projection and imaging hardware,the flexibility,high accuracy and high-speed of 3D shape measurement technique has been guaranteed.These advantages and enough auxiliary equipments make fringe projection technology very suitable for the current development direction of high-speed and high-precision in the field of 3D measurement,satisfied with the actual production and scientific research needs.For fringe projection based high-speed 3D shape measurement technology,the performance of hardware equipment--high-speed projection and high-speed imaging device will directly determine the upper limit of the accuracy and speed of the measurement system.Compared to the high-speed,high-resolution,successfully commercialized high-speed imaging device,high-speed projection has been restricted due to the relatively low-speed projection speed,high prices,limited spectral range and core technology subject to foreign companies.Therefore,it is of great significance to study how to break through the limitations existing in the current high-speed fringe projection technology.After the research and analysis of the existing high-speed projection equipment and technology,aiming at the problem of current technology,this thesis proposes a self-made high-speed projection equipment--rotary mechanical projector(RMP).Combined with a single high-speed camera,a monocular high-speed 3D shape measurement system using a self-made rotating mechanical projector is realized.According to different measurement needs,this thesis provides a variety of RMPbased 3D shape measurement technical solutions.Realization of single-frame algorithm 3D reconstruction using RMP system;Realization of multi-frame algorithm3 D reconstruction using RMP system;RMP system working with phase-shift information feedback and in flexible projection mode.These technical solutions make full use of the projection flexibility of RMP and can cover a required wide range of fringe high-speed projection situations,i.e.from single-frame fringe projection to phase-shift fringe projection with arbitrary shifting step;from a lower projection frequency of tens or hundreds Hz to tens of thousands Hz.Using these technical solutions,3D shape reconstructions of a variety of static objects,slow and fast changing objects have been realized in this thesis.The main work and innovation of this thesis are summarized as follows:(1)Aiming at the limitations of the current high-speed fringe projection technology,a rotary mechanical projector is designed and proposed.The working principle and technical realization of the rotary mechanical projector are introduced in detail from the aspects of projection principle,hardware selection,disc design,device realization and so on.When the disc remains at rest,the single-frame fringe projection can be realized.When the disc rotates under the drive of the motor,the fringes are continuously projected to the surface of the measured object through the projection lens,and the high-speed sinusoidal fringe projection is realized.The projection test experiment proved that the RMP can realize single-frame and multi-frame fringe projection with high speed,low cost,sinusoid and high contrast(the camera’s exposure time is set at 200 μs).(2)The possible system error in the multi-frame fringe projection of the RMP system is obtained by comparing the measurement results of the fixed plane using the different projection areas of the rotating grating disc.The source and range of these system errors are analyzed.For the system error that mainly comes from inaccurate phase-shifting information feedback,probability distribution function(PDF)is used to find the precise phase-shifting value and phase correction.Corrected RMP system can complete the high-precision phase acquisition with the multi-frame algorithm.The projection performance of the corrected RMP system is systematically evaluated.The repeatability of the RMP system is verified by multiple measurements on the same object.And comparisions with the commonly used high-speed projection equipment DLP4500 in detail prove the performance of the RMP system in high-speed projection.(3)Using the RMP monocular measurement system,single-frame and multipleframe algorithm reconstruction is realized.Aiming at the problem that phase unwrapping is restricted when complex dynamic surface shape is measured,a reference plane-assisted 3D phase unwrapping algorithm is proposed.Combined with temporal Fourier transform profilometry,this thesis finishes the point-to-point measurement of the whole process of dynamic scenes,including 3D reconstruction of the isolated objects and the impact process of rapid changes.Using multi-frame algorithm,the 3D shape reconstruction of hand swing and statue movement scene are realized at the reconstruction rate of 296fps(frame per second),which provides an effective technical scheme for high-speed and high-precision 3D shape measurement.(4)Based on multi-frame(phase-shifting)algorithm reconstruction method of the RMP system,a phase-shifting fringe projection mode with flexible and arbitrary steps is proposed.Through adjusting the speed of motor,the fringe projection with different phase-shifting steps can be realized.The phase recovery results of the ceramic plate with different phase-shift fringe projection steps demonstrate that the method can be succesfully applied to the flexible phase-shifting fringe projection.At the same time,the reconstruction algorithm that is most suitable for the high-speed and high-precision 3D shape measurement of the current RMP system is selected.Using this algorithm,the static and dynamic 3D shape measurement with complex surface is completed.In the phase-shift fringe projection mode with flexible steps,the projection speed of RMP in different projection modes is analyzed,and a high-precision 3D measurement technical scheme that can be used for fast-changing dynamic scene measurement is proposed.Three-dimensional shape measurement of high-speed vibrating objects with rich details is completed at 1600 fps,which provides a new technical support in vibration mechanics,aerodynamics and other related research fields.The proposal and improvement of the devices and theories in this thesis are all based on the technical goals--to achieve high-speed and high-precision 3D shape measurement.The presentation of each method or system is to improve the RMPbased high-speed 3D measurement technology.All the actual experiments designed under each technical scheme,the verification experiments after the new proposed method,or the comparison experimental results with the existing technology show that the RMP-based high-speed projection technology can realize a variety of different projection modes and corresponding different measurement scheme.The high-speed fringe projection technology proposed in the thesis breaks through the bottleneck of the existing technology and realizes high-speed,high-precision,lowcost,autonomous high-speed fringe projection and 3D shape measurement.The research results of this thesis are expected to be applied to national defense construction,industrial manufacturing and processing,equipment performance testing,moving target detection,force analysis,and modal analysis of motion mechanics,vibration mechanics,aerodynamics,etc.