Mode Coupling Chatter Research Articles

Improving Robotic Milling Performance through Active Damping of Low-Frequency Structural Modes

Industrial robots are increasingly prevalent due to their large workspace and cost-effectiveness. However, their limited static and dynamic stiffness can lead to issues like mode coupling chatter and regenerative chatter in robotic milling processes, even at shallow cutting depths. These problems significantly impact performance, product quality, tool longevity, and can damage robot components. An active inertial actuator was deployed at the milling spindle to enhance dynamic stiffness and suppress low-frequency vibrations effectively. It was identified that the characteristics of the actuator change with its mounting orientation, a common scenario in robotic machining processes. This variation has not been reported in the literature. Our study includes the identification of model parameters for the actuator in both horizontal and vertical mountings. Additionally, the novelty of the present work lies in the specific design and implementation of compensation filters tailored for the active inertial actuator in both horizontal and vertical configurations. These filters address the unique challenges posed by low-frequency vibrations in robotic milling, offering significant improvements in dynamic stiffness and vibration suppression. Traditional model-based compensators were effective for vertical mounting, while pole-zero placement techniques with minimum phase systems were optimal for horizontal mounting. These compensators significantly enhanced dynamic stiffness, reducing maximum low-frequency robot structural modes by approximately 100% in horizontal mounting and approximately 214% in the vertical configuration of the actuator. This advancement promises to enhance industrial robot capabilities across diverse machining applications.

A novel mode coupling mechanism for predicting low-frequency chatter in robotic milling by providing a vibration feedback perspective

The low-frequency chatter (LFC) of the robot body during milling severely constrains the machining efficiency. Taking the classical mode coupling mechanism as the underlying foundation, previous studies have developed various stability models to predict the LFC in robotic milling. However, the classical mode coupling mechanism has recently been proven to be applicable only to operations such as threading and boring, showing significant deviations in robotic milling. In response, this paper proposes a novel vibration feedback-based mode coupling (VF-MC) mechanism applicable to robotic milling based on robotic modal directionality, which describes the coupling and feedback process of modal vibration among multiple modes through dynamic cutting forces. The feedback process is modeled considering the time delay effect. The LFC can be judged by the trend of the amplitudes of the feedback vibrations. The effectiveness and efficiency of the VF-MC model for predicting the LFC are verified by comparing with the milling experiment results and the traditional mode coupling chatter model. Furthermore, the dynamical conditions for the LFC are investigated based on the VF-MC mechanism with the experimental evidence. The core contribution of this paper is to quantitatively reveal the feedback mechanism of low-frequency self-excited vibration among modes in robotic milling, and further develop the VF-MC model to efficiently predict the LFC. The code for the VF-MC model is available at https://github.com/Dr-JwWu/VF-MC__A-chatter-model-for-robotic-milling.

Robotic milling stability optimization based on robot functional redundancy

PurposeCompared with the high stiffness of traditional CNC machine tools, the structural stiffness of industrial robots is usually less than 1 N/µm. Chatter not only affects the quality of robotic milling but also reduces the accuracy of the milling process. The purpose of this paper is to reduce chatter in the robotic machining process.Design/methodology/approachFirst, the mode coupling chatter mechanism is analyzed. Then the milling force model and the principal stiffness model are established. Finally, the robot milling stability optimization method is proposed. The method considered functional redundancies, and a new robot milling stability index is proposed to improve the quality of milling operations.FindingsThe experimental results prove a significant reduction in force fluctuations and surface roughness after using the proposed robotic milling stability optimization method.Originality/valueIn this paper, a new robot milling stability index and a new robot milling stability optimization method are proposed. This method can significantly increase the milling stability and improve the milling quality, which can be widely used in the industry.

Tuning vibration absorbers to mitigate simultaneous regenerative and mode-coupling chatter

This paper examines a set of parametric tunings for vibration absorbers to enhance stability in the event of simultaneous chatter during machining and proposes a novel tuning criterion of minimizing the definite integral of frequency response function, while accounting for damping of the absorber’s base component. Chatter vibrations are an inherent characteristic of machining processes, and a common tool for mitigating chatter is a tuned mass damper (TMD), which functions effectively when its dynamic characteristics are appropriately tuned to the natural frequency of the vibrating base component. Robotic machining is a recent cost-effective method of material removal, but simultaneous regenerative and mode-coupling chatter can arise on robot tools. TMD tuning criteria for mode-coupling and regenerative chatter are different, and tuning to suppress one may prompt the other. Moreover, conventional analytical tunings do not consider damping of the base component, which may be a valid assumption for low-damped machine tools but not for highly-damped robot tools. The possibility of simultaneous chatter and base component damping effect necessitates the exploration of alternative tuning criteria.

Research on reliability of mode coupling chatter of orthopedic surgery robot

In this paper, the reliability probability method is introduced into the prediction of mode coupling chatter of the orthopedic surgical robot milling to avoid the influence of the randomness of the input parameters on the chatter prediction results. The reliability of mode coupling chatter is defined to represent the probability of the stability (no mode coupling chatter occurs) of milling system. Probability model (reliability model) of mode coupling chatter is established to predict robotic milling chatter, in which the modal stiffness K x, K y and cutting force coefficient Ks involved in the prediction of chatter vibration during robotic milling are considered as random variables. The fourth moment method is adopted to solve the reliability model of milling system to obtain the reliability level. The mode coupling chatter reliability circle diagram (MCCRCD), which is a contour line with a specified reliability level as a function of the angle between the main stiffness direction and the cutting force and the depth of cut, is presented to identify the mode coupling chatter reliable and unreliable regions. An example studies the influence of random variables on the reliability of milling processing, and the time-domain simulation method and experiment method are used to verify the possibility and effectiveness of using MCCRCD to predict the stability of the robotic milling. It can be concluded that MCCRCD can be effectively used to predict the stability of robotic milling.

Can mode coupling chatter happen in milling?

In milling, two different self-excited vibrations have been reported; regenerative and mode coupling chatter. The regenerative chatter mechanism has been extensively studied and validated with tests whereas mode coupling chatter mechanism was reported a long time ago for threading operations. The presented mode coupling chatter models were based only on current vibrations of the system but not on the delayed vibrations. With the increase in the research carried out on robotic milling operations, the low frequency severe self-excited vibrations at high spindle speeds were claimed to be mode coupling chatter by many researchers. However, the justification of mode coupling chatter mechanism is absent from predicted stability boundaries and a strong evidence distinguishing it from regenerative chatter mechanism is not present. Additionally, mode coupling chatter models applied to milling was based on threading operations and hence they were not capturing the characteristics of intermittent milling process. Therefore, this paper focuses on the diagnosis of mode coupling chatter in robotic milling. Mode coupling chatter principles are applied to milling process considering its intermittent characteristics. The zeroth order approximation for mode coupling chatter mechanism is adapted for milling and extended to multi frequency approximation. Mode coupling chatter stability boundaries are calculated, explored with tests and compared with regenerative chatter stability boundaries. Results show that mode coupling chatter stability boundaries are very low and vaguely dependent on the spindle speed. This contradicts the stability observed in the experimental tests. Hence, it is concluded that mode coupling chatter in milling is not possible, because the assumption of the chip thickness depending only on current vibrations does not apply to milling operations. The novelty of the presented paper is the theoretical and experimental justification that mode coupling chatter is not possible in milling operations.

Chatter stability and precision during high-speed ultrasonic vibration cutting of a thin-walled titanium cylinder

Titanium alloys are widely used in the aviation and aerospace industries due to their unique mechanical and physical properties. Specifically, thin-walled titanium (Ti) cylinders have received increasing attention for their applications as rocket engine casings, aircraft landing gear, and aero-engine hollow shaft due to their observed improvement in the thrust-to-weight ratio. However, the conventional cutting (CC) process is not appropriate for thin-walled Ti cylinders due to its low thermal conductivity, high strength, and low stiffness. Instead, high-speed ultrasonic vibration cutting (HUVC) assisted processing has recently proved highly effective for Ti-alloy machining. In this study, HUVC technology is employed to perform external turning of a thin-walled Ti cylinder, which represents a new application of HUVC. First, the kinematics, tool path, and dynamic cutting thickness of HUVC are evaluated. Second, the phenomenon of mode-coupling chatter is analyzed to determine the effects and mechanism of HUVC by establishing a critical cutting thickness model. HUVC can increase the critical cutting thickness and effectively reduce the average cutting force, thus reducing the energy intake of the system. Finally, comparison experiments are conducted between HUVC and CC processes. The results indicate that the diameter error rate is 10% or less for HUVC and 51% for the CC method due to a 40% reduction in the cutting force. In addition, higher machining precision and better surface roughness are achieved during thin-walled Ti cylinder manufacturing using HUVC.

Three-dimensional stability analysis of robotic machining process

PurposeSince robot’s structural stiffness is usually less than 1 N/µm, mode coupling chatter occurs frequently during robotic milling process, and chatter frequency is close to the natural frequency of the robot itself. Chatter not only affects the surface quality but also damages the robot and reduces the positioning accuracy. Therefore, it is necessary to predict chatter in robotic machining process.Design/methodology/approachA three-dimensional dynamic model for robot’s spatial milling plane is established, and a corresponding stability criterion is obtained. First, the cutting force in milling plane is transformed into the coordinate system of the robot principal stiffness direction based on homogeneous transformation matrix. Then the three-dimensional stability criterion under milling process can be obtained by using system stability analysis. Furthermore, the circle diagram of mode coupling chatter stability is drawn. Each feeding direction’s stability under the two processing forms, referred as spindle vertical milling and spindle horizontal milling, is analyzed.FindingsThe experimental results verify that the three-dimensional stability criterion can avoid chatter by selecting machining feed direction in stable area.Originality/valueThis paper established a three-dimensional dynamic model in robot’s spatial milling plane and proposed a three-dimensional stability criterion according to the Routh criterion. The work is also expected to be an efficient tool in the development of robotic milling technology.

Mode coupling chatter prediction and avoidance in robotic machining process

Robots are widely used in automation because of their flexibility. However, there are still challenges to enhance the effective use of robots in machining process due to unexpected vibration/chatter. Instead of time-consuming trial and error, this paper proposes an expanded mode coupling chatter theory that suits robot kinematics and enables industrial application due to accurate mode coupling chatter prediction. This research allows determination of mode coupling chatter occurrence with respect to constantly changing parameters of robotic kinematic and practical contemplation before setting up the physical robot, workpiece and tool for machining. Firstly, the mathematical model of the robot structure is established and combined with chatter theory. A deepened analysis of stability criteria is carried out followed by experimental validation of the theory. By carrying out this chatter analysis, a software tool based on an algorithm was developed to help determine the stable setup, including machining parameters, robot pose, travel direction and workpiece setup, for a certain robotic machining application. The developed prediction algorithm ties vibration occurrence to machining force direction and magnitude that makes it applicable to various machining operations.

A chatter-free path optimization algorithm based on stiffness orientation method for robotic milling

Robots used in machining processes are more prone to mode coupling chatter due to low rigidity and asymmetrical structure. In this paper, a new stiffness orientation method is proposed to optimize milling path and improve stability of the robotic milling system. First, the principal stiffness directions of the robot are determined through experimental or calculated modal analysis. Then, based on kinematics, the principal stiffness directions are projected onto the processing plane. Second, a mode coupling chatter stability criterion for robotic milling is obtained based on machining force analysis and system dynamics stability. Finally, a robotic milling path optimization algorithm based on the stiffness orientation process is proposed. Experiments were performed to verify the benefits of pre-optimizing the machining path to avoid mode coupling chatter and improve machining accuracy.

A Review on Chatter in Robotic Machining Process Regarding Both Regenerative and Mode Coupling Mechanism

During the last few decades, industrial robots have been widely used in various applications to develop a flexible and efficient manufacturing process such as material handling and welding. However, as a high value-added application, few robotic machining systems have been installed mainly due to the limitation from the chatter, which leads to the poor product quality and low productivity. Although researchers have been continuously investigating the robotic machining chatter, there is still a lack of understanding due to the complexity of the issue. This paper provides a comprehensive review on chatter-related issues during robotic machining tasks, including mechanisms, mitigation strategies, and identification methods regarding both regenerative chatter and mode coupling chatter. Due to the low stiffness and couple structure of industrial robots, both regenerate and mode coupling chatter can occur at different cutting conditions. The difference between two chatter mechanism in the robotic machining process are compared and a list of guidelines is provided to help in distinguishing these two types of chatter. Systemic analysis of the mechanisms of the chatter identification and suppression is presented providing a research basis for future studies.

Mode coupling chatter suppression for robotic machining using semi-active magnetorheological elastomers absorber

Abstract Chatter is one of the major barriers for robotic machining process. As the dominant vibration frequency of the chatter varies under different working conditions, Magnetorheological elastomers (MREs) whose stiffness is adjustable is an ideal device to be used for chatter control. This paper presents a new mode coupling chatter reduction scheme by assembling an MRE absorber on the spindle to absorb vibration with a specific frequency range. Firstly, a MRE absorber was designed and fabricated to suppress the target chatter according to the robot model and then a test was implemented to obtain the frequency shift property of the designed MRE absorber. Subsequently, robotic milling of an aluminium block using ABB IRB6660 robot was tested under various conditions to demonstrate the performance of the MRE absorber under different constant currents. After that, a semi-active controller was established to control the electrical current applied to the MRE absorber to trace the chatter frequency. The experimental results show that the semi-active MRE absorber performs better on the chatter reduction during robotic milling than passive absorbers.

A Method for Mode Coupling Chatter Detection and Suppression in Robotic Milling

A new method for online chatter detection and suppression in robotic milling is presented. To compute the chatter stability of robotic milling along a curvilinear tool path characterized by significant variation in robot arm configuration and cutting conditions, the tool path is partitioned into small sections such that the dynamic stability characteristics of the robot can be assumed to be constant within each section. A methodology to determine the appropriate section length is proposed. The instantaneous cutting force-induced dynamic strain signal is measured using a wireless piezoelectric thin-film polymer (polyvinyldene fluoride (PVDF))-based sensor system, and a discrete wavelet transform (DWT)-based online chatter detection algorithm and chatter suppression strategy are developed and experimentally evaluated. The proposed chatter detection algorithm is shown to be capable of recognizing the onset of chatter while the chatter suppression strategy is found to be effective in minimizing chatter during robotic milling.

PVDF sensor based on-line mode coupling chatter detection in the boring process

Dynamic instability in the form of chatter vibrations during boring is detrimental to machining process. Therefore, an on-line chatter detection system is required to detect chatter before damage occurs. This paper evaluates a Polyvinylidene Fluoride (PVDF) piezoelectric thin-film based strain sensing system to detect mode coupling chatter in the boring process. In addition, this paper compares the performance of time domain based (autocorrelation), time–frequency based (wavelet transform), and frequency based (Fast Fourier Transform) methods for detecting mode coupling chatter. The PVDF sensor system is demonstrated to be a low-cost solution for detecting chatter in the boring process.

CCT-based mode coupling chatter avoidance in robotic milling

Currently, large aerospace structures are machined using large multi-axis CNC machining centers. In comparison, milling with a multiple degree-of-freedom (dof) articulated robotic arm has several advantages due to its lower cost and versatility. The low stiffness of an articulated arm robot, however, gives rise to severe low frequency mode coupling chatter during machining. Previous studies have shown that such chatter can be suppressed by minimizing the angle between the average resultant cutting force direction and the direction of maximum principal stiffness of the robot. This approach limits the range of permissible robot motion, and therefore its flexibility of use. This paper presents a new method for avoiding mode coupling chatter in robotic milling using the Conservative Congruence Transformation (CCT) stiffness model, which does not require changing the tool feed direction or the workpiece orientation. Robotic milling experiments show that mode coupling chatter is significantly reduced when using this approach.

A novel deep groove machining method utilizing variable-pitch end mill with feed-directional thin support

A novel method for deep groove machining is developed which utilizes a long-shank variable-pitch end mill with a feed-directional thin support in this research. Recently, thin and tall ribs are required for many parts to reduce their weight and material consumption without sacrificing their stiffness and strength. It leads to necessity of deep and narrow grooves in dies and molds for their mass production. However, such deep groove machining is difficult, since long flexible end mills cause severe chatter vibrations induced by regeneration and mode-coupling. There have been many studies on the regenerative chatter vibration, while there have been few studies on the mode-coupling chatter vibration. Both chatter vibrations need to be suppressed in the deep groove machining. In this study, the regenerative chatter vibration is suppressed by employing one of the conventional methods, i.e. a variable-pitch end mill. On the other hand, there is not a good method to suppress the mode-coupling chatter vibration in the deep groove machining. Therefore, a new suppression method is proposed in which the long-shank variable-pitch end mill is supported with a thin plate in the feed direction. The support device and the long-shank variable-pitch end mill are developed and applied to machining of hardened die steel. Validity of the proposed method is verified both analytically and experimentally in this study.

Chatter analysis of robotic machining process

The need for flexible automation keeps moving downstream in the foundry production process, such as casting parts cleaning and pre-machining. One of the most challenging issues in practice is to know the vibration/chatter characteristic of any given machining process. To reduce the trial and error frustration, this paper presents the underline mechanism and theoretical analysis to provide physical understanding for the onset of chatter problem and principles to prevent that. First, the cutting force model and robot structure model are established for a systematic analysis of chatter mechanism. Completely different from common woes of regenerative chatter in conventional CNC machine paradigm, another type of chatter, namely, mode coupling chatter was identified as the dominant source of vibrations in robotic machining, largely due to the inherent low structure stiffness of industrial robot. In-depth analysis for stability criteria and experimental verifications are then presented followed by the guidelines of process configuration and parameter selections to achieve chatter free machining operation.

Eigenvalue Analysis of Mode-Coupling Chatter for Machine-Tool Stabilization

The mode-coupling chatter phenomenon, due to self-excited vibrations occurring during many machining operations, is studied in this paper. Although mode-coupling chatter has been widely treated in the specialized literature, the authors propose here an approach that is rather different from the classical one, namely, an eigenvalue analysis of chatter based on system theory. A mathematical model of the workpiece- tool system is established first, so as to define the system equations. A closed-form equation, expressing the stability condition of the system as a function of the system parameters, and particularly of the angle between the feed direction and the piece surface, is then obtained from the eigenvalues of the system. Moreover, a study of the eigenvectors of the system enables one to analytically express the tool trajectories, both in the stable and in the unstable case. The model thus formulated and the related considerations about the system stability are then employed in a real case. Namely, a wood-cutting machine subject to mode-coupling chatter is considered, and a technique for stabilization, based on the considerations made on the model, is presented. The authors believe that the eigenvalue analysis of mode-coupling chatter presented in this paper provides a more detailed and complete analysis of the phenomenon than what is currently present in the literature.

A System Theory Approach to Mode Coupling Chatter in Machining

Chatter is an undesired phenomenon of self-excited vibrations that occurs during many machining operations. This paper analyzes mode coupling chatter from the point-of-view of the system theory. A simplified mathematical model of the cutting process is established, from which the equations of the system are obtained. Then, the eigenvalues and the eigenvectors of the system are evaluated, and a simple stability condition is formulated. The tool trajectories both in the stable and unstable case are studied. Finally, an example of mode coupling chatter in a machine for wood cutting and its stabilization is presented.

Detection of Self-Excited Chatter Caused by Mode Coupling. 1st Report. Study on Criterion of Detection.

The purpose of this study is to define criterion of detecting self-excited chatter caused by mode coupling of workpiece system on a lathe. Theoretical study on relation between cutting conditions (direction of principal axes of workpiece system and width of cut) and vibration properties of workpiece system was carried out, on assumption that workpiece system is two degrees of freedom and is composed of a mass and two kinds of spring and damper system assigned to their principal axes. Theoretical stability limit of mode coupling chatter and vibration properties versus direction of principal axes of workpiece system were calculated. The following conclusions were reached as a result of some comparative consideration about them. (i) Difference between natural frequencies of workpiece system decreases with increase of width of cut under stable cutting conditions in the direction of principal axes of workpiece system where mode coupling chatter may occure. (ii) Phase angle between the vibrations in the directions of principal axes of workpiece system is from 30 degrees to 90 degrees under unstable cutting conditions.