Research On Measuring Method Of Nonholonomic Arc Based On Machine Vision

Incomplete circular arc,that is,incomplete circular arc,commonly seen in the body of the "four doors and two covers",such as door edge,body edge,et al.In recent years,the incomplete circular arc on the car body has gradually become an important component to express the design concept of the body shape.The consistency of its contour size is one of the important criteria to measure the exquisite and beautiful appearance of the car.The traditional R gauge manually measures the radius of non-holonomic circular arc,but there are some problems such as low accuracy,poor reliability,difficult data summary and high labor cost.The use of machine vision is the main trend of quality control and intelligent measurement at present,but at present,there are few measurement schemes for the incomplete small arc on the car body,and the actual measurement process mainly exists the following two problems: Firstly,the contour data collected by the machine vision camera includes the arc feature part and the straight line part.In order to measure the radius of the incomplete small arc,the point cloud of the arc feature part should be accurately extracted first.Second,only a small number of effective point clouds remained after the completion of arc feature extraction and denoising,making it difficult to carry out accurate fitting.In this study,the measurement of small arcs is divided into two categories according to the number of effective point clouds(or the degree of central Angle)of arcs that can be collected:For the angular and enclosing nonholonomic circular arcs with a large number of effective point clouds,the basic measurement scheme of nonholonomic circular arcs is designed.First,slope screening is carried out to remove most of the lines in the point cloud contour curve by using the slope change characteristics of each point in the line part and the arc part.Then,curvature screening was used to solve the curvature equation of the point cloud in a small range,and the curvature variation characteristics were used to remove the part of the line point cloud connected with the arc and the part of the line point cloud at the corner,to complete the rough extraction of the arc features.Finally,the circular arc features are extracted precisely by using RANSAC algorithm to match the characteristics of the optimal model and the radius of the circular arc is estimated by using least square method.For the gap type circular arc with few effective point clouds,the limit measurement scheme of nonholonomic circular arc is designed.In other words,the steps of multi-angle data acquisition,point cloud rough registration and Laplacian depth smoothing are added to the basic measurement scheme.The details are as follows: Firstly,multi-angle data acquisition is carried out,and the arc contour is extracted.Then,multiple groups of point clouds with small central corners and circular arcs were registered to increase the density of point clouds with small central corners and circular arcs and to increase the number of available information points.Combined with Laplace depth smoothing,the point cloud is smoothed to remove the fluctuating point cloud.Finally,the radius of the small circular arc is obtained by fitting the basic measurement scheme.As the radius of the incomplete small arc of the body is usually 1-2 mm,the stamping accuracy of sheet metal parts is not high,and the car paint will affect the measurement accuracy,so it is difficult to pass the actual measurement(due to the small parts,highprecision coordinate machine is also difficult to measure),so this study adopts standard parts for accuracy verification,and then uses the real car covering parts for stability verification.The experimental results show that the absolute accuracy of circular arc fitting can reach0.024 mm and 0.059 mm respectively under the basic and limiting conditions,and the effect is good.The stability and efficiency of the proposed scheme are also proved by the results of real vehicle experiments.The proposed scheme can accurately measure most of the non-holonomic arcs in the process of automobile manufacturing,and the measurement results can guide the stamping process of automobile enterprises and improve the sensory quality of automobile.Further,it can promote the quality inspection level of manufacturing industry.In addition,the project practical application value is high,wide range of application,can also be applied to parts roundness,coaxiality measurement and so on.