On-machine Measurement With 2D Laser Profilometer And Adaptive Milling Of Large Thin-wall Curved Parts

Large thin-walled curved parts are widely used in high-end manufacturing fields such as aerospace industry.However,due to the feature of large span and poor stiffness,traditional CNC(Computer Numerical Control)machining methods are difficult to meet the needs of efficient and high-precision machining of large thin-walled curved parts,leading to problems such as low productivity and high reject rates.Closed-loop control of processing systems through advanced sensing technology is a common idea in current industrial upgrading of intelligent manufacturing and flexible manufacturing,and it is also a key research direction to solve the problem of processing large-scale thin-walled curved parts.This paper discusses how an on-machine measurement system with 2D laser profilometer for largescale thin-walled curved parts is established to realize the integration of "measurement-processing-compensation".Based on this measurement system,two practical problems about 3D measurement and adaptive milling are solved as specific application examples: one is about the blank part measurement and benchmark processing of the tube section parts,the other is about welding seam measurement and compensation milling of rocket tank bottom part.This paper mainly includes the following three aspects:1.Establish an on-machine measurement system based on 2D laser profilometer.Hardware system and communication system of the onmachine measurement system based on 2D laser profilometer are completed first.Then supporting measurement software based on secondary development of the numerical control system is further developed to realize basic measurement function of on-machine measurement.Conversion from laser sensor measurement results to the workpiece coordinate system has been completed.Calibration method to find the pose of the sensor after installation has been studied.Then measurement coordinate system and processing coordinate system have been unified,which facilitates subsequent compensation processing.2.Aiming at the practical problem that it is difficult to determine reference plane during rough machining of barrel part blank,the measurement point cloud of the blank part obtained by scanning with onmachine measurement system and the theoretical model are matched to find optimal reference plane position under the constraint of allowance.This paper presents an entire algorithm flow which takes the blank measurement point cloud and the theoretical model of the part as input and the matching result expressed by the best transformation matrix as output.The procedure is mainly divided into two steps: rough matching and optimal matching.Rough matching is mainly for unconstrained overall matching,and it uses ICP(Iterative Closest Point)algorithm to achieve fast alignment.The optimization matching mainly uses PSO(Particle Swarm Optimization)algorithm to achieve the goal of least matching error under the specific minimum allowance constraint,and finally to obtain the optimal transform matrix of the measurement point cloud of the blank.The main method using multiplier method to deal with constraints and BFGS(Broyden-FletcherGoldfarb-Shanno)quasi-Newton method to achieve unconstrained optimization are analyzed as a comparison.Transform matrix will be settled to each axis of the machine tool to find reference plane for processing.3.In order to solve the practical processing problem that small assembly gap for friction stir welding of rocket tank bottom parts(about0.3mm)is difficult to measure and uneven distribution of the gap width affects the welding strength,the on-machine measurement method is used to measure the assembly gap,and then compensation processing is completed to improve gap width consistency.Based on the measurement results of the gap,shoulder point recognition algorithm with a single-frame line laser measurement result as the input and the gap shoulder point coordinates as output is proposed.Errors between the theoretical position and the actual position are calculated and fitted as error-position curve using spline curve fitting,finally compensated to machining toolpath.The machining experiments have been performed to prove the effectiveness of this proposed method.Experiment results show that the consistency of the width of the assembly gap of the tank bottom parts is improved.