Study On Automatic Control Technology Of Helical Milling On CFRP/Ti Composite Structures

Due to the excellent physical and chemical properties,Carbon fiber reinforced plastics(CFRP)and titanium alloy(Ti)have been widely used in aircraft manufacturing industry.CFRP/Ti composite structure is usually employed in key load-bearing components,and the CFRP and Ti phasers are joined by mechanical fastening technique,such as bolting,riveting.The drilling precision and quality directly affect the connection strength and fatigue life of the aircraft components.However,the CFRP laminate exhibits anisotropy nature,low interlaminar strength characteristic,and the Ti alloy presents high hardness,low thermal conductivity,high chemical affinity,the difference between the material properties has pushed the high precision drilling technology of CFRP/Ti composite structure become an urgent issue to address in the aircraft industry.Helical milling technology,which is proposed in recent years,is an effective way to solve the drilling problem of typical aviation difficult-to-machine materials.For the novel helical milling system developed by Zhejiang University,this dissertation analyzes the helical milling mechanism of CFRP/Ti composite structure,establishes the three-dimensional model of cutting force,combines with cutting experiments,in-depth studys on several crucial control issues such as bore diameter automatic control,tool deflection compensation,stack interface detection.The main research contents are as follows:According to helcial milling mechanism,the dissertation analyzes the relationship between the cutting parameters,tool geometry,and undeformed chip.Considering the cutting action of the bottom edge and side edge,the three-dimensional cutting force model of helical milling based on instantaneous rigid mechanical model and the solid tool discreting method.Carrying out the helical milling cutting experiments on CFRP and titanium alloy,the model coefficients are identified by the average cutting force method,and the accuracy of helical milling cutting force model is verified under different cutting parameters.Proposed a novel helical milling effector,which employes double eccentric sleeve mechanism with direct drive rotary motor to adjust tool eccentricity.Considering the variation of the cutting force interference in different cutting conditions,the dynamic imbalance caused by the tool revolution,and the uncertainty of system parameters,the eccentric axis control system,which is used to automatically adjust and precisely control the bore diameter,is desgined based on the adaptive robust control theory.Helical milling cutting experiments with different tool eccentricity and revolution speed are carried out on CFRP laminate and titanium alloy,the results show that the system keeps the tool eccentricity average error and the maximum error under 0.5?m and 1.6?m,respectively,and the proposed method which satisfies the accuracy requirements of the connecting holes of aircraft structures is reasonable.To eliminate the adverse effects on the diameter precision of the tool deflection in helical milling process,the method which fuses cutting force model estimation and real-time observation is proposed to observe the radial cutting force,the tool deflection is calculated by the equivalent diameter cantilever model,and then adjusting the eccentricity to achieve the real-time compensation of tool deflection.The tool deformation compensation experiments show that the diameter error is reduced 50%by the proposed method in the helical milling operations of CFRP/Ti composite structure,the IT9 accuracy requirements can be satisfied after a single shot,in the situation that the cutting force increased to 2 times its initial value caused by the tool wear,it still guarantees the diameter accuracy.An adaptive disturbance observer with forgetting factor based on dynamic tool feed system model is designed to estimate the axial cutting force,the sudden changes of the cutting force would be detected by the moving linear regression algorithm,accordingly,the CFRP/Ti stack interface position information could be obtained.Thus,providing key technical support for the realization of varing-parameter drilling on CFRP/Ti composite structure.The CFRP/Ti composite structure drilling experiments show that,position error of the stack interface can be detected within±0.05mm,when the cutting tool feed through the stack interface at the speed of 0.2mm/s,the control system can get its location information within±0.25s,it is viable to achieve the automatic control of process parameters to adapt the cutting stage.Finally,the whole work described in this dissertation is summarized,and the future work is also discussed.