Active Chatter Suppression With Sliding-mode Control For Turning Process

Machining chatter,characterized by large vibrations among machine tools,cutting tools and workpieces in machining process,is an undesirable and complex phenomenon.Machining productivity is often limited by the occurrence of chatter,which accordingly induces low dimensional accuracy,material removal rate(MRR)reduction,poor surface finish and serious tool wear.With the ever-growing demand for good-quality products with high efficient manufacturing,the need to develop chatter suppression techniques becomes critical and pressing for modern manufacturing industry.Benefiting from the development of modern control theory and smart materials,active chatter control,including the control device and control algorithm,has received widespread attention and demonstrated potentials to improve machine tool performances.However,it remains a challenging task in active chatter control due to the following difficulties:complex cutting process models,time-varying paramters,unmeasurable system states and nonlinear characteristics,i.e.,input constraint and hysteresis nonlinearity,exsiting in actuators.It can also cause control performance degradation or even system instability.Sliding-mode control(SMC)has demonstrated itself an efficient nonlinear control method featuring robustness in the presence of disturbance.Hence,SMC is well-suited for chatter suppression since large disturbance exists in machining processes.With support of the National Basic Research Program of China“Basic Intelligent Manufacturing Research of High Quality and Complex Components”,this dissertation focuses on the research of chatter suppression in turning process,including the cutting tool chatter,thin wall plate chatter and piezoelectric actuatrs(PEA)in control device.This dissertation presents active chatter suppression approaches and high presion control methods for PEA.The main research contents and achievements are listed as follows:1.Two different sliding-mode reaching laws are developed to compensate the hysteresis nonlinearity without using the inverse hysteresis model.The PEA is modeled with a reduced dynamic model with unknown parameters and hysteresis nonlinearity.Firstly,in order to alleviating chattering,a new reaching law with improved quasi-sliding-mode domain(QSMD)is proposed for PEA systems with uncertainties.By redefining the change rate as the second order difference of the system uncertainties and adopting the continuous approximate function,smaller width of the QSMD can be guaranteed.Secondly,a novel reaching law is proposed to guarantee smaller width of the QSMD while decreasing the reaching time in the same time.The reaching law is established based on an exponential term.The ultimate magnitude of the QSMD in proposed method is of the order O(T~3).Both numerical simulations and experimental investigations on a piezoelectric actuator are employed to validate the effectiveness of the proposed method.2.Two active control algorithms are developed to release the restrictions exsits in active chatter control of cutting tools.Firstly,an active sliding-mode controller,which employs a dynamic output feedback sliding surface for the unmatched condition and an adaptive law for disturbance estimation,is designed for chatter suppression in turning process.Only displacement measurement is required by this approach.Secondly,aiming at the input constraint problem,a new adaptive sliding-mode controller is proposed for chatter suppression in turning process.Adaptive laws are employed to estimate uncertainties and state delay,while a sliding-mode controller(SMC),fulfilling the control input constraint,is designed to mitigate chatter.In order to compensate the hysteresis nonlinearity in PEA,the reaching law with improved QSMD is employed in both algorithms.The Lyapunov method is employed to demonstrate the stability of the closed-loop system.Numerical simulations and experimental results on a computer numerical control(CNC)lathe show that the chatter-free domain is substantially enlarged and the chatter can be substantially attenuated.3.An adaptive sliding-mode controller is presented for chatter suppression of thin plates in turning process.Unlike existing methods,a distributed-parameter dynamic model of a rotating thin plate with multiple vibration modes is used to analyze the machining stability with the proposed controller.In the designed SMC,adaptive laws are adopted to estimate the bounds of uncertainties and regenerative delay,while a sliding-mode controller is presented to mitigate regenerative chatter.In order to alleviating chattering of SMC,the reaching law with improved QSMD is adopted.Theoretical analyses and numerical simulations demonstrate the effectiveness of the proposed method.4.Since plate vibration at the cutting point is difficult to measure,the displacement field reconstruction method is adopted.Thus,the displacement is reconstructed to capture the plate dynamic behavior for feedback control during turning.A new active control method,consisting of an adaptive sliding-mode controller and the displacement field reconstruction,is presented for chatter suppression of thin plates in turning process.A fast tool servo has been used as an execution unit for the implementation of the proposed method.Theoretical analyses,numerical simulations and experimental evaluation on a lathe show that the chatter-free domain is substantially enlarged and the chatter can be effectively attenuated.