On The Key Technologies Of Automatic Measurement In Cabin Assembling

The docking assembly of cabin parts is a key process in the production of modern aviation,aerospace and navigation equipment.In order to upgrade the automation of this process,a stable and reliable measurement method should be put forward to guide the action of the pose adjusting device.Because it is difficult for current single measurement device to meet the requirements of non-contact,high-precision and automation for different cabins at the same time without measurement targets or probes,this dissertation studies a comprehensive measurement method of cabin pose based on line structured light scanning and distributed machine vision for the automatic docking of spacecraft,aiming to provide a feasible non-contact measurement approach for the cabin assembly.The researches of the dissertation can be divided into three parts:(1)Measurement for the pose of the cabin axis: Aiming at the measurement for the axis of the cabin,this dissertation proposes a measurement approach based on line structured light scanning.This approach employees a structured light scanner to obtain the point cloud from the side of the cylindrical cabin and divided them into a series of profiles in different parallel planes,in which these profiles indicate ellipses.Two feasible methods,the axis-fitting method and generatrix-fitting method are proposed to solve the pose parameters from these ellipse profiles.In the axis-fitting method,the centers of the ellipse profiles are estimated by direct ellipse fitting.And in the generatrix-fitting method,a special generatrix is picked and translated according to the spatial geometry to obtain the pose of the axis.In order to compare and analyze the accuracy and precision of the two methods,this dissertation designs a method based on Monte Carlo simulation,which finds that the precision of the generatrix-fitting method along the horizontal axis and axis-fitting along the vertical are both high.As a result,a synthesis method combining the advantages of the above two methods is proposed.In this method,the vertical coordinate obtained by the axis-fitting method and the horizontal coordinate obtained by the generatrix-fitting method are taken as the center of the fitted ellipse,and the spatial position of the axis is fitted based on these estimated centers.For the verify the above method,a prototype is designed and manufactured.It is proved that the accuracy and precision of synthesis method is high,in which the error of axis position is less than 0.03 mm,and the error of axis attitude is less than 0.03 °(2)Robustness enhancement for the measurement of cabin axis: In the application of line structured light scanning in practical engineering,two aspects of interference should be considered,i.e.the optical interference and the unstructured measurement environment.Optical interference refers to the measurement error caused by uneven exposure of the camera's sensitive elements when scanning the smooth metal surface by line structured light.In this dissertation,BRDF(Bidirectional Reflectance Distrbution Function)is introduced to analyze this situation,the cause of the error is explained and the solution is given.The unstructured measurement environment refers to the interference caused by the adhesion,bulge,depression and occlusion which will introduce measurement error.Therefore,based on M-estimation and random sampling consistency algorithm(RANSAC),an improved robust ellipse fitting algorithm is proposed to enhance the measurement process.In this method,the interference data points are eliminated by iterative fitting.Then the threshold of inner point is introduced to eliminate the points with large error in engineering.The algorithm is simulated based on the constructed data,and the effectiveness of the algorithm is verified.(3)Measurement for the relative angle between cabin flanges: To measure the relative angle between the two cabins,this dissertation proposes an approach based on distributed vision.Besides,considering the pose measurement of the flanges are a common problem in engineering,this dissertation generalizes the method on the basis of the actual needs.In this method,two groups of cameras arranged around the mating surfaces are used to take photos of the mating features on the surface of the cabin sections and get their positions.The relative positions between the two flanges can be easily obtained by converting them to the same coordinate system according to the pre-obtained registration matrix.In order to get this matrix,this dissertation uses a standard cabin and registration device to move in the cameras' FOV(field of view)and draw the trajectories of mating features on the cabin and device.Then the transformation relationship between cameras can be solved according to the determined relationship between the trajectories.Thus,experiments are carried out with the prototype to prove the error of the method,in which the error is proved less than 0.022°.Finally,in order to prove the effectiveness of the proposed conclusions in the actual engineering environment,an engineering prototype is developed based on the method proposed in this dissertation in cooperation with the relevant research units.Combined with the third-party measurement equipment,experiments have been carried out on it to prove the effectiveness of this dissertation in actual engineering process.The result proves that the method in this dissertation can effectively deal with most kinds of interference in the actual engineering through the robust measurement of the axis.Then the experiment of the angle measurement based on distributed vision is also proved.Finally,the assembly experiment proves that the method in this dissertation is able to effectively guide the adjustment device to complete the docking assembly.