Study On Videometrics For Wing Deformation Under High Subsonic Condition

While the aircraft is in flight,the wings often have to bear huge aerodynamic loads,causing excessive deformation of the wings,and ultimately affecting the performance,safety and life of the aircraft.Therefore,the actual wing deformations need to be measured quantitatively and accurately to prove the safety of the aircraft.Traditional methods like strain gauges and accelerometers as well as non-contact laser displacement sensors are difficult to use because of problems with their complicated equipment installation,the data transmission,providing their data only at the location where they have been installed,low accuracy,poor real-time performance.Furthermore,such sensors often yield to strong modifications of the aircraft structure.To address these limitations,the researchers have been actively exploring new technologies that can advance the current state-of-the-practice in deformation measurement of the aircraft structure.Due to the advances in cameras and computer vision algorithms,the videometrics are used increasingly and mainly for wing deformation measurement because of their advantages of only requiring simple equipment,ability to obtain non-contact measurements,wide range of usability,providing high-precision and robust results.Therefore,this paper investigated the major problems of in-flight wing deformation measurement based on videometrics and focused on developing reliable and easy-to-use measurement systems.The main contents and achievements are listed as follows:(1)To address the problem of aero-optical effects in optically measuring the wing deformation,the deformation error produced by the upstream and downstream non-uniform flow field of the aircraft wing were measured and studied by wind tunnel experiments.A method for evaluating the influence of aero-optical effects on the wing deformation measurement by videometrics,and two types of errors are identified: virtual displacement and virtual strain,which are proposed as the evaluation parameters.The digital image correlation technique is used on background dot patterns to quantitatively characterize the virtual deformation caused by imaging through the upstream and downstream region over the Mach number range from 0.3 to 0.7 and the angle of attack varied from-7.02° to 12.18°.The experimental results show that aero-optical effect is an important source of measurement error.The virtual deformation field exhibits some eddy and ribbon-like structures which is believed to be imposed by the coherent structures in the subsonic mixing layer.The beam propagation of the camera through the upstream region rather than the downstream region behind the wing might cause the virtual strain to fall by approximately 50%.The virtual strain varies linearly with the square of Mach number and reciprocal of the cosine of the angle of attack.(2)A numerical evaluation method based on CFD is established for evaluating the aero-optical effects caused by non-uniform flow field over the wing surface and the wing deformation measurement error.The influence mechanism of the aero-optical effect on the wing deformation measurement by videometrics is revealed.The basic process for evaluating the aero-optical effect in optically wing deformation measurement is given.Based on CFD simulation analysis,a ray tracing method in combination with triangular mesh interpolation is proposed.Also,three sets of simulation and real experiments are conducted to verify the effectiveness of the proposed ray tracing method.Based on the OA309 airfoil,the effects of the key parameters such as Mach number,angle of attack,and the placement of the camera on wing deformation measurement by videometrics are investigated through ray tracing simulation calculations.(3)Compared to laboratory applications of measuring wing deformation by optical methods,wind-tunnel and in-flight tests are facing with major challenges such as overexposure,underexposure,very dark background,image blur caused by camera motion and so on.To address these problems,an improved concentric circular coded target(CCCT)based on Schneider’s design is presented.Correspondingly,the positioning and recognition algorithms of CCCT are proposed to ensure high-precision results of optical wing deformation measurement even in extreme conditions.A novel and simple method on size selection of circular targets is proposed for high-precision measurement.To solve the problem of a serious image deformation of the CCCT due to a large imaging angle,the perspective transformation of the CCCT local image is used to achieve the image deformation correction and then to improve the recognition accuracy rate.To verify the proposed CCCT design and corresponding algorithm,the real experiments are carried out with a digital camera to capture the images under extreme conditions such as overexposure,underexposure,large imaging angle,complex background.The results show that applying them can obtain better positioning and recognition results even in extreme conditions,and it is quite robust and reliable.(4)Slight movements of the cameras can often occur due to elasticity deformation of the camera support and the surrounding structure or pulse and shock of the air flow,which will introduce a deformation error to measurement results of videometrics.A method based on camera realtime pose estimation is proposed to correct the errors caused by camera movements,and the performance evaluation of pose estimation algorithms is performed.The results show that the RPn P + LHM algorithm has the best comprehensive performance among 12 typical algorithms.Excellent,so in the method of correcting the errors caused by camera movements,the RPn P +LHM algorithm is selected for real-time camera pose estimation.Finally,through experiments,the comparison between the true value and the measured value before and after correcting the camera movements is given,which proves the effectiveness and feasibility of the proposed correction method and has important engineering application prospect.According to the above theories and technologies,this paper develops a wing deformation measurement system based on videometrics,which has been applied in wing deformation measurement for large UAV in order to verify the feasibility,accuracy,and reliability.The results show that when the developed wing deformation measurement system using two Basler industrial cameras with a resolution of 1280 × 1024 and a KOWA fixed focus lens of 8.5mm is applied to measure the deformation of a wing with a span of 3.49 m in a field of view of about 6m × 5m,the deviation between the displacement obtained and the laser displacement sensor is less than 0.8mm;Furthermore,this system is simple in equipment and easy to operate;the proposed CCCT design scheme,size selection and manufacturing method are convenient for engineering applications;the CCCT positioning and recognition,3D reconstruction,and deformation calculation are completely performed by the computer,which realizes the automation of the measurement.The obtained results are stable and can be displayed intuitively.In addition,without increasing the cost of hardware,the proposed method based on the realtime pose estimation is used to correct the influence of camera movements.After correcting,the displacement measurement deviation is about 1.5mm compared with the laser displacement sensor,which is 50% lower than the error before correction.The measurement accuracy in the moving state of the cameras can almost reach the accuracy of the stationary state.This method is simple and convenient,with good stability,and effectively improves the measurement accuracy when camera movements occur.Therefore,it can be used to measure the deformation of the wing during the flight of a large aircraft.