Active Vibration Control In Milling

The phenomenon of vibration is an inextricable part of any machining processes and modern machine shops are well aware of its detrimental effects. Cutting vibrations can desta-bilize a machining process with severe implications for machine tool, machining quality, tool life, productivity and operator's health. In extreme situations, uncontrolled cutting vibrations can lead to chatter and collapse the cutting system.Considerable research efforts have been put into controlling or reducing the extent of the problem to improve the productivity and machining quality in metal cutting. In general, re-search into vibration control in this area can thus be divided into two categories: classical cut-ting parameter adjustment and active vibration control. In the former category, the cutting pa-rameters (cutting speed, feed rate, etc.) are changed so that the cutting process adapts to dy-namic properties of the structure and the machining process always lies in a stable regime. For this approach, the lobe diagrame should be available to predict the stable cutting regime, and it is a tedious process since cutting dynamics usually variate with cutting conditions. Fur-thermore, the productivity may also be decreased since cutiign parameters are changed. Dif-ferent from the former method, in active method an auxiliary system is introduced to offer an active force to canceal the vibration. Comparing to the former method, it can improve the machining quality with no sacrifice of productivity. For the reason of convenience, active cut-ting vibration control systems for turning process are widely studied. Due to the complicated movements of milling tool, the relative vibration between the tool and workpiece is very dif-ficult to measure, and thus lead to the lack of essential feed back signals. Therfore, little atten-tions have been paid to the researchs in milling process.Milling is widely used in metal cutting, the milling productivy and quality in China is severely inferior to developed countries. Therefore,the study of active cutting vibration control system for milling process to improve the milling productivity and quality is an es-sential industrial requirement, and it has great economic and application metrits. In this reae-arch, the design and synthesis of active milling vibration control system which can attenuate the relative vibration between the tool and workpiece are presented, the study of cutting sys-tem dynamics and surface generation are also included.First, the domestic and oversea researchs in machine tool vibration have been summa-rized, the realization of vibration control systems and their characteristics are analysized, the modeling method, controller design, sensors and actuactors involved in this research area are also discussed.As aforementioned, the difficulty in tool vibration measurement results in the lack of study on active milling vibration control. In charpter II, one method can accurately measuring the vibration displacement of tool and workpiece is proposed, the experimental devices and program are also presented. The vibration displacement of tool and workpiece in different cutting phases under varing cutting parameters is acquired by using the method, and the reli-ability of the method is verified and the characteristics of cutting vibration are obtained through the analysis of sampled data.To study the coorelation between the cutting parameters and vibrationsd, cutting dy-namic model is built in charpter III. Modal experiment is carried out to obtain the displace-ment V.S forces FRFs of tool-workpiece-machien tool; parameteric models are obtained bas-ing on the measured FRFs. Combining the models with dynamic cutting forces model a cut-ting forces and vibrations prediction model is built. The validation of the mdel is proved by comparing the predicted and measured cutting force and vibration signals, and the influences of force dynometer dynamics are also discussed.The sudy of the coorealtion between the surface generation and cutting vibrations in pe-ripheral milling is presented in charpter IV. A geometric peripheral milling model and surface profile generation algorithm are presented. The surface topography is simulated using actual vibration signals and compared with the real ones, moreover, the effects of cutting vibration are also analyzed. Results show that the simulated surface profile gives a good agreement with the real one, and the vibration plays the leading role in surface generation in given cut-ting parameters.Since the tool vibration can not be controlled directly, the strategy for active vibration control milling is drving the workpiece to canceal the relative vibrations between the tool and workpiece. And hence in charpter V, basing on the characteristics of measured cutting vibra-tions, a two-axis piezo-actuators driven active workpiece holding stage, is developed as the executive unite of active milling vibration control system. The main performances of the stage are obtained through the finite element analysis. The stage is manufactured and assembled, followed by tests on the stage, the resuts show that the stage achieves the desired performance and can be used in activ milling vibration control.The system dynamics are analysized theoretically and experiment is carried out to derive system parametric model. In controller synthesis, the stage is modeled as controlled plant, while cutting vibrations are treated as disturbances to the system. Basing on the characteristics of the problem, the robust mixed sensitivity and robust disturbance rejection methods are em- ployed in feed back controller synthesis to achieve disturbance rejection and closed-loop sys-tem stabilization against variations of cutting vibrations and unmolded system dynamics in high frequency range. In this effort, the movements of worrkpiece are regulated by the devel-oped system to cancel the relative vibration between the tool and workpiece in cutting process. Numerical simulations are carried out to prove the validation of the designed controller. The hardware of the controller is developed by using NI PCI-6251 and Labview. Extensive cut-ting experiments are carried out to verify the performances of the control system, and the re-sults show that the developed system works well in cutting vibration cancellation and ma-chining quality improvement.The work presented in this thesis may beworthily used to study on the characteristics of cutting vibrations, modeling cutting process dynamics, surface generation and develop active vibration control system for milling.