Добірка наукової літератури з теми "Chatter suppression"

A mechatronic system of actuators, sensors, and analog circuits is demonstrated to control the self-excited oscillations known as chatter that occur when single-point turning a rigid workpiece with a flexible tool. The nature of this manufacturing process, its complex geometry, harsh operating environment, and poorly understood physics, present considerable challenges to the control system designer. The actuators and sensors must be rugged and of exceptionally high bandwidth and the control must be robust in the presence of unmodeled dynamics. In this regard, the qualitative characterization of the chatter instability itself becomes important. Chatter vibrations are finite and recognized as limit cycles, yet modeling and control efforts have routinely focused only on the linearized problem. The question naturally arises as to whether the nonlinear stability is characterized by a jump phenomenon. If so, what does this imply for the "robustness" of linear control solutions? To answer our question, we present an advanced hardware and control system design for a boring bar application. Initially, we treat the cutting forces merely as an unknown disturbance to the structure which is essentially a cantilevered beam. We then approximate the structure as a linear single-degree-of-freedom damped oscillator in each of the two principal modal coordinates and seek a control strategy that reduces the system response to general disturbances. Modal-based control strategies originally developed for the control of large flexible space structures are employed; they use second-order compensators to enhance selectively the damping of the modes identified for control. To attack the problem of the nonlinear stability, we seek a model that captures some of the behavior observed in experiments. We design this model based on observations and intuition because theoretical expressions for the complex dynamic forces generated during cutting are lacking. We begin by assuming a regenerative chatter mechanism, as is common practice, and presume that it has a nonlinear form, which is approximated using a cubic polynomial. Experiments demonstrate that the cutting forces couple the two principal modal coordinates. To obtain the jump phenomena observed experimentally, we find it necessary to account for structural nonlinearies. Gradually, using experimental observation as a guide, we arrive at a two-degree-of-freedom chatter model for the boring process. We analyze the stability of this model using the modern methods of nonlinear dynamics. We apply the method of multiple scales to determine the local nonlinear normal form of the bifurcation from static to dynamic cutting. We then find the subsequent periodic motions by employing the method of harmonic balance. The stability of these periodic motions is analysed using Floquet theory. Working from a model that captures the essential nonlinear behavior, we develop a new post-bifurcation control strategy based on quench control. We observe that nonlinear state feedback can be used to control the amplitude of post-bifurcation limit cycles. Judicious selection of this nonlinear state feedback makes a supplementary open-loop control strategy possible. By injecting a harmonic force with a frequency incommensurate with the chatter frequency, we find that the self-excited chatter can be exchanged for a forced vibratory response, thereby reducing tool motions.<br>Ph. D.

In metal cutting processes, excessive vibration or chatter has an adverse effect on productivity and product surface quality. Various studies have been reported in the literature over the past few decades. However, the real application of the outcome of these studies has been very limited. A new system has been developed in this study for chatter detection and chatter suppression. The coherence function values of the frequency spectra from two accelerometers in orthogonal directions were used as a chatter indicator. The vibration energy was used to offset the over-vigilance behaviour of the coherence function. A fuzzy logic control approach was used for chatter suppression based on both the coherence function value and vibration energy level. To improve the adaptability of the fuzzy controller, a self-learning algorithm has also been developed for on-line updating the fuzzy rule base. A direct output tuning method was also proposed to improve the responsiveness of the system. The proposed system has been tested using both steel and aluminium workpieces with and without thin-walls. The experimental results show that the proposed system worked reasonably well for on-line chatter detection and suppression. The thesis also explored the possibility of using the coherence function for chatter prediction. The verification of its feasibility may be carried out in the future.

In metal cutting processes, chatter has been recognized as one of the main factors that limit machining productivity and affect product quality. Two different categories of chatter were classified by researchers, i.e., regenerative chatter and non-regenerative chatter, and in this thesis the former is mainly studied. Over the past few decades, though various chatter detection and suppression methods have been developed, their industrial acceptance is still very limited. This research work presents a new system for on-line chatter detection and suppression. Its detection module implements a statistical index to identify chatters by performing wavelet transform and conducting statistical analysis of positive wavelet transform modulus maxima (WTMM). To suppress chatter, two versions of fuzzy control modules, i.e., plain fuzzy control and self-regulating fuzzy control have been implemented. Unlike the previous chatter suppression systems, the new suppression module features two-way adjustment, i.e., both increasing and decreasing the amount of adjustment. Along with the use of single or multi-output control variables to suppress chatter, productivity is preserved as much as possible. The proposed system is implemented on a SERVO 2000 milling machine. Extensive tests have been carried out. The experimental results show that the wavelet-based chatter detection index can not only detect the existence of chatters but also distinguish the severity levels. The new chatter suppression module works reasonably well in most tests. However, its performance is adversely affected in the presence of non-regenerative vibrations due to the lack of workpiece or clamping rigidity. Further improvements need to be carried out for industrial applications.

Chatter is one of the major problems in today’s milling processes. Theoretical models to calculate stability lobes are used to predict and avoid chatter onset. However, current predictions are not accurate enough and significant deviations between predicted and experimentally observed stability limits have been reported.The causes for these deviations are diverse and can be the result of the sum of multiple effects. According to previous works, main errors in stability prediction are related to lack of knowledge about double period instability (flip lobes) and inappropriate determination of dynamic parameters through standard experimental characterization techniques. This Thesis deals with these two problems that affect accurate chatter prediction, contributing with new knowledge and calculation methods for double period type lobes and developing a new methodology for a more accurate dynamic response identification. Nevertheless, an accurate chatter stability prediction does not necessarily imply an optimum use of the machine to maximize productivity, as it is required in current production environments. For this reason, three novel process stabilization techniques are proposed for those cases in which the designed machining process is subject to chatter vibrations.<br>El chatter és avui en dia un dels principals problemes en els processos de fresat. Per predir i evitar la seva aparició es disposa de models teòrics per al càlcul dels lòbuls d'estabilitat. No obstant això, les prediccions realitzades amb els models d'estabilitat de fresat no són robustes, presentant casos en què les desviacions entre la predicció i la realitat són importants. Les causes d'aquestes desviacions són variades i poden ser degudes a la suma de múltiples efectes. A la vista dels estudis previs realitzats, els principals errors es troben en l'omissió de lòbuls de doble període (lòbuls flip) i errors en la determinació experimental dels paràmetres dinàmics del sistema mitjançant mètodes tradicionals. Aquesta Tesi aborda aquests dos problemes principals en la predicció, aportant nous coneixements sobre el chatter de doble període i desenvolupant una nova metodologia per a un càlcul més precís de la resposta dinàmica del sistema. No obstant això, una predicció precisa de les condicions que donen lloc a un procés de fresat estable no garanteix l'aprofitament òptim de la màquina per maximitzar la productivitat, tal com s'exigeix en l'entorn productiu actual. Per això, es proposen tres noves tècniques per a l'eliminació de chatter en aquells casos en què, el procés de mecanitzat dissenyat estigui sota el perillós influx del chatter.

碩士<br>國立高雄第一科技大學<br>機械與自動化工程所<br>94<br>ABSTRACT Chatter is a nuisance to precision machining. Most previous research regarded the time-delay effect in the chatter problems as a disturbance. This research focuses on the investigation of this time-delay problem, and considers its effect for chatter controller design. In this way, one would achieve a better performance of improving the machining stability. First, the time-delay effect is included in the formulation of equations of motion for a machining process. Then the optimal control is adopted for the system with the time-delay term, and the Riccati equation is derived for the optimal controller. Finally the computer simulation is conducted for verification, and the improvement on stability lobes is then discussed.

碩士<br>逢甲大學<br>自動控制工程所<br>96<br>Chatter is a self-excited vibration during machining that causes violent vibration between the tool and the workpiece. Chatter degrades surface finish, causes wear or breakage of tools and limits the material removal rate. This phenomenon is more conspicuous on slender workpiece. Therefore, the ability to suppress chatter can improve machining performance significantly. In this study, the chatter suppression problem is investigated for slender workpieces in turning. A tool holder driven by a piezoelectric actuator is designed and controlled. Based on the H∞ controller may change the chip width dynamically by controller signal voltage for chatter suppression in the turning process. Experimental modal analysis and ANSYS finite-element modal analysis are carried out for obtaining accurate frequency response functions of the workpiece and the cutting tool for designing controllers. According to the chatter theory, the happening of chatter has important relation to the structure’s dynamic transfer function of the workpiece and cutting tool. Based on the model matching conception and applied H∞ control theory to design a controller to have the higher critical stabile value of the structure’s dynamic transfer function. The performance of controller is tested first in a simulative environment, then an experimental structure is built by utilizing dSPACE, include of the real cutter and the computer-modelling workpiece and cutting status. To proceed with the machining experiment, the lathe was refitted to mount the piezo-actuated tool holder. Compared with the results of cutting by traditional tool holder and uncontrolled piezo-actuated tool holder and controlled piezo-actuated tool holder under the same cutting condition, to make sure the H∞ controller possess the ability of chatter suppress effectively.

碩士<br>國立成功大學<br>機械工程學系<br>105<br>In this study, effect of sinusoidal spindle speed variation on end milling is investigated based on semi discretization method. Tooth passing period is changed by period spindle speed variation, then result in interruption of regernerative effect and supress chatter. In order to clarify effect of system parameters on variable speed machining stability, effect of variation amplitude, variation frequency, modal parameters, shearing constant and process damping coefficients on stability of variable speed system is investigated in the present study. The results of simulation show that the dominant parameters are variation amplitude, variation frequency and process damping coefficients. Appropriate amplitude and frequency should be choosed to supress chatter at different nominal spindle speed. Process damping effect increases milling stability at low speed region dramatically and increases the asymptotic speed, which is absolutely stable speed. But semi discretization method including process damping also takes longer simulation time. The results of simulation also show that variable speed machining suppress chatter effectively at local worst speeds, that is, the speeds that have lowest stable cutting depth. But such method cause negative effect on sweet spot, transform the machining system from stable to unstable. Modal parameters, flute number, and shearing coefficients do not affect the efficiency of improving the milling stability by variable speed machining. Experimental results also show that variable speed machining can reduce vibration amplitude when chatter is occurring.

In the last decades, unstable vibrations originated in the milling process, often referred to as chatter vibrations, have collected the interest of several researches, mainly driven by the detrimental effect this phenomenon generates on productivity, surface finishing and tool wear. Although several approaches and techniques have been developed nowadays, their industrial application is still limited by the required expertise, time-consuming procedures or relevant interventions on the machine tool structures. This research is focused on the investigation and design of active fixtures to mitigate chatter vibrations in milling, considering that this kind of devices could represent an appealing industrial alternative, due to the fact that they can be directly retrofitted to different machine tools and applied to different machining operations. The aim of this thesis was to improve the performance of intelligent active fixtures by carefully addressing the specific design challenges, both in terms of mechanical design and control aspects. The main focus was put in extending the device bandwidth in accordance with the requirements of a general chatter mitigation application, where chatter frequencies can easily reach and exceed several kilohertz. In particular, specific design guidelines and simplified modeling strategies, aimed at supporting the definition of an adequate mechanical design, are presented and discussed along with the selection of suitable actuation devices capable of granting the needed reliability, even when operated at high frequencies in demanding dynamic applications. Moreover, this work presents the development of a novel control strategy aimed at exploiting low-frequency excitation to disrupt chatter vibrations, without requiring the further extension of the device bandwidth nor the preliminary system identification and modelling, as generally needed for renowned model-based control techniques.