Speed Optimization And Contour Error Compensation Of High Precise CNC Cam Grinding

A major problem in CNC grinding is the excessive dependence on the manual operation and thus low efficiency is incurred.Objective of this thesis is implementation of optimization and intelligence in CNC cam grinding.The content of the research has the following aspects.This work is supported by the Jilin Province Science and Technology---The Speed Optimization and Contour Error Compensation in High Precision CNC Cam Grinding(Project No: 20150101031JC).Firstly,a mathematical model for mechanical structure in CNC cam grinding system is developed.Based on the mechanism analysis and nonlinear factor analysis,the mathematical model for single axis(Axis X and Axis C respectively)is established.Considering the repetition in CNC cam grinding system,a repetitive control mechanical system on basis of extended state observer is designed.It can improve the tracking precision of the two single axis and meanwhile provide a theoretical basis for the optimization of cam grinding.Secondly,the cross coupling control system based on iterative learning control is designed under the circumstance that sufficient accuracy of single axis has been obtained.Although the tracking precision of two axes was improved,the tracking error is still unlikely to be completely eliminated and as a result,lag is inevitable.Therefore,a contour error model based on lag synchronization is designed through analysing the geometric correlations between a servo tracking error and the contour error.Meanwhile cross-coupled control(CCC)based on this error model is adopted to realize real-time compensation.The repetitive learning control(RLC)and PID control are integrated to be as a new cross-coupled controller to further compensate the repetitive contour error.Detailed simulation results show that the designed control system is able to compensate the real time errors and to enhance cam contour accuracy.Thirdly,a new speed optimization algorithm is proposed.In CNC cam grinding,the contour error will inevitably exceed the regular range when two axes have different lags caused by the high-speed grinding even though the dynamic characteristics of an individual system is fairly good.To address such a problem,a new speed algorithm is proposed in this thesis.This model is based on a new contour error model using the idea of lag synchronization and can improve the contour accuracy.In this work,the mathematical relationship between the contour error and the speed of the two axes is deduced firstly.A new concept of relative speed between grinding carriage and the cam is firstly proposed,and the parametric equations of the relative speed are then developed.The intent of the speed optimization algorithm is to minimise the contour error by developing an exponential function and optimizing the velocity.It can be achieved that the velocity is reduced in the area where the lift curve of the given cam intensively changes and the velocity is increased when the lift gently changes.Hence the grinding accuracy can be improved and meanwhile the efficiency can be guaranteed.In comparison with the constant angular velocity grinding,significant improvements in contouring accuracy can be obtained with employment of such an approach.Furthermore,three practical examples are given to verify the accuracy and reliability of the proposed algorithm.The application results demonstrate the cam contour error can be well controlled within 0.012 mm.Fourthly,a two loops control scheme based generalized cycle-to-cycle(GCTC)feedback control is proposed.The size of the workpiece,however,is quite unlikely to be precisely measured until one process(or cycle)has been completely finished in CNC cam grinding.Although in-process measurement and control is possible,it is often economically prohibitive or practically complex to be implemented.To overcome this drawback,a double closed loop control is proposed in this study.Inner loop is the local controller to guarantee the tracking accuracy,GCTC feedback control is adopted,which serves as an optimization module for the closed-loop system under the local controller to update the reference in the outer loop.The equivalent dynamic model of the machining system can be obtained by the extended state observer and repetitive control in inner loop.The sufficient stability conditions and convergence of the GCTC control are derived in this study.The superiority is demonstrated by the simulations and experiments on CNC cam grinding based on GCTC control.The application results justify the effectiveness of the GCTC scheme.Fifthly,an integrated two-layer optimization strategy on basis of measurement is proposed for grinding system.An update strategy based on repetitive control is employed in the inner layer to improve the reactivity and convergence speed.In outer layer,cycle-to-cycle corrections which derive from cycle-to-cycle control and constraint-adaptation scheme are performed.The inner layer is the feedback of outer layer while the outer layer provide guidance for the inner layer.The application results demonstrate the effectiveness of the two-layer optimization scheme.