Non-contact On-machine Measurement And Systematic Error Analysis Of Large-diameter Aspheric

Aspheric surfaces have excellent optical properties.Large-diameter aspheric components are widely used in military and civil equipment,such as astronomy telescopes,laser weapons,medical imaging equipment,etc.However,the fabrication and measurement of large-diameter aspheric surfaces are the main factors affecting their development.It is of great significance to improve the measurement accuracy and reflect the large-diameter aspheric surface shape which is closer to the true one.After obtaining the measurement data,it is necessary to compensate the errors according to the requirement of theoretical contour surface accuracy,to improve the processing accuracy,to reduce the surface error and to establish a precise measuring system which improves the shape accuracy of large-diameter aspheric surface.According to the state of machining,large-diameter aspheric surface measurement can be divided into three methods,i.e.off-line measurement,on-machine measurement and on-line measurement.Traditional three-coordinate measuring machines are usually used for off-line measurement.For surface profile measurement,this measurement method not only requires repeated positioning,but also brings in new errors,such as positioning errors caused by repeated clamping of workpieces.Difficult to meet the requirements of on-site processing measurement.Aiming at the problems of low off-line measuring efficiency and unsuitability for multiprocess on-site machining of large-diameter aspheric surface parts in multi-process,this paper proposes a non-contact measuring method for measuring large-diameter aspheric surface,implements on-machine measurement,improves measuring efficiency and accuracy,and adapts to processing requirements on-site.The main research contents and achievements include:Path planning is proposed for non-contact measurement;Suitable measurement path is proposed according to aspheric surface type and contour shape.Measurement path planning of off-axis aspheric surface and symmetric aspheric surface are studied.The contour surface of aspheric surface is sampled and measured according to measurement path.Both sampling quantity and measurement efficiency are considered.Key parameters of NURBS curve fitting;According to the fitting process of NURBS curve,the key parameters such as discrete data points,data parameterization,node vector,control vertex and weight factor are determined step by step.During the determination of key parameters,sampling is carried out based on the planned path to obtain discrete data points.An innovative sampling point optimization method,i.e.sampling method with equal tangential interval distance,is proposed to improve the fitting efficiency of NURBS curve.The fitting NURBS curve is inserted into nodes,the shape of NURBS curve is changed,the method of local optimization is used to optimize the measuring path,and the NURBS curve is inserted into the node vector to generate the most reasonable measuring path for aspherical surface.Analysis of several error sources and error evaluation in non-contact measurement;NURBS fitting error and movement table guide rail error treatment methods,adjust the shape of NURBS curve and guide rail error of moving table by separating the guide rail error,and use least square method to evaluate the error.Through experimental analysis,non-contact on-machine measurement is carried out on off-axis aspheric surface and large-diameter aspheric surface.Compared with NURBS curve fitting and common fitting measurement accuracy,the PV value of surface error is significantly reduced after NURBS curve fitting.