Meng-Kun Liu

Multi-dimensional time-frequency control of micro-milling instability

Micro-milling is inherently unstable and chattering with aberrational tool vibrations. While the time response is bounded, however, micro-milling can become unstably broadband and chaotic in the frequency domain, inadvertently rendering poor tolerance and frequent tool damage. A novel simultaneous time-frequency control theory is applied to negate the various nonlinear dynamic instabilities including tool chatter and tool resonance displayed by a multi-dimensional, time-delayed micro-milling model. The time and frequency responses of the force and vibration of the model agree well with the experimental results published by Jun et al. A multi-variable control scheme is realized by implementing two independent controllers in parallel to follow a target signal representing the desired micro-milling state of stability. The control of unstable cutting at high spindle speeds ranging from 63,000 to 180,000 rpm and different axial depth-of-cuts are investigated using phase portrait, Poincaré section, and instantaneous frequency (IF). The time-frequency control scheme effectively restores dynamic instabilities, including repelling manifold and chaotic response, back to an attracting limit cycle or periodic motion of reduced vibration amplitude and frequency response. The force magnitude of the dynamically unstable cutting process is also reduced to the range of stable cutting.

Control of cutting vibration and machining instability : a time-frequency approach for precision, micro and nano machining

Machining instability is a topical area, and there are a wide range of publications that cover the topic. However, many of these previous studies have started by assuming that the behavior of the system can be linearised. Meanwhile, there are many recent advances in the fields of signal processing, nonlinear dynamics, and nonlinear control, all of which are relevant to the machining stability problem. This book establishes the fundamentals of cutting mechanics and machine tool dynamics in the simultaneous time-frequency domain. The new nonlinear control theory developed by the authors that facilitates simultaneous control of vibration amplitude in the time-domain and spectral response in the frequency-domain provides the foundation for the development of a controller architecture universally viable for the control of dynamic instability including bifurcation and chaos. Once parameters underlying the coupling, interaction, and evolution of different cutting states and between the tool and workpiece are established, they can then be incorporated into the architecture to create a control methodology that mitigate machining instability and enable robust, chatter-free machine tool design applicable in particular to high speed microand nano-machining.

On Controlling Milling Instability and Chatter at High Speed

Risk assessment is one of the main pillars of the framework directive and other directives in respect of health and safety. It is also the basis of an effective management of safety and health as it is essential to reduce work-related accidents and occupational diseases. To survey the hazards eventually present in the workplaces the usual procedures are i) gathering information about tasks/activities, employees, equipment, legislation and standards; ii) observation of the tasks and; iii) quantification of respective risks through the most adequate risk assessment among the methodologies available. From this preliminary evaluation of a welding plant and, from the different measurable parameters, noise was considered the most critical. This paper focus not only the usual way of risk assessment for noise but also another approach that may allow us to identify the technique with which a weld is being performed.In this paper, we present two Partial Least Squares Regression (PLSR) models for compressive and flexural strength responses of a concrete composite material reinforced with pultrusion wastes. The main objective is to characterize this cost-effective waste management solution for glass fiber reinforced polymer (GFRP) pultrusion wastes and end-of-life products that will lead, thereby, to a more sustainable composite materials industry. The experiments took into account formulations with the incorporation of three different weight contents of GFRP waste materials into polyester based mortars, as sand aggregate and filler replacements, two waste particle size grades and the incorporation of silane adhesion promoter into the polyester resin matrix in order to improve binder aggregates interfaces. The regression models were achieved for these data and two latent variables were identified as suitable, with a 95% confidence level. This technological option, for improving the quality of GFRP filled polymer mortars, is viable thus opening a door to selective recycling of GFRP waste and its use in the production of concrete-polymer based products. However, further and complementary studies will be necessary to confirm the technical and economic viability of the process.

Simultaneous Time-Frequency Control of Active Magnetic Bearing

Active magnetic bearings enable greater spindle dynamic stiffness through higher attainable bearing surface speeds. However, the active magnetic bearing system is highly nonlinear due to the interaction between electromagnetic field and rotor dynamics. Its nonlinear character becomes prominent when rotating in high speed. The operation undergoes route-to-chaos and is vulnerable to external excitation, which eventually leads to detrimental failure. A novel simultaneous time-frequency control theory is developed for controlling the active magnetic bearing at high speed. The control theory is able to tolerate the uncertainties in the system due to on-line identification and the deterioration in both time and frequency domain can be restrained.Copyright

Simultaneous Time-Frequency Synchronization of Non-Autonomous Chaotic Systems

A novel chaos control concept is presented for the synchronization of a non-autonomous chaotic circuit system in the time and frequency domains concurrently. The controller effectively eliminates the differences between two chaotic circuits in the time domain and at the same time restores the characteristics of the driving response in the frequency domain. The simultaneous time-frequency control is achieved through manipulating wavelet coefficients, thus not limited by the increasing bandwidth of the chaotic system — a fundamental restraint that deprives contemporary controller designs of validity and effectiveness. The feedforward feature of the control concept prevents errors from re-entering the control loop and inadvertently perturbing the sensitive chaotic system. Because neither closed-form nor linearization is required, the innate, genuine features of the chaotic response are faithfully retained. The on-line identification feature allows the response system to start at arbitrary initial conditions and to be driven by the sinusoidal forcing term of different amplitudes and phases requiring no knowledge of the system parameters.Copyright

Control of Friction-Induced Instability in Simultaneous Time-Frequency Domain

A flexible cantilever beam pressed against a rigid rotating disk is explored for studying self-excited friction-induced vibrations that are inherently unstable due to alternating friction conditions and decreasing dynamic friction characteristics. Because no linearization or approximation scheme is followed, the genuine characteristics of the system including stick-slip and inherent discontinuities are fully disclosed without any distortion. It is shown that the system dynamics is stable only within certain ranges of the relative velocity. With increasing relative velocity, the response loses its stability with diverging amplitude and broadening spectrum. A novel time-frequency controller is subsequently applied to negate the chaotic vibrations at high relative velocity by adjusting the applied normal force. The controller design requires no closed-form solution or transfer function, hence allowing the underlying features of the discontinuous system to be fully established and properly controlled. The inception of chaotic response at high relative velocity is effectively denied to result in a restoration of the system back to a relatively stable state of limit-cycle.Copyright

Simultaneous Time-Frequency Control of Multi-Dimensional Micro-Milling Instability

A novel control concept is presented for the online control of a high-speed micro-milling model system in the time and frequency domains concurrently. Micro-milling response at high-speed is highly sensitive to machining condition and external perturbation, easily deteriorating from bifurcation to chaos. When losing stability, milling time response is no longer periodic and the frequency response becomes broadband, rendering aberrational tool chatter and probable tool damage. The controller effectively mitigates the nonlinear vibration of the tool in the time domain and at the same time confines the frequency response from expanding and becoming chaotically broadband. The simultaneous time-frequency control is achieved through manipulating wavelet coefficients, thus not limited by the increasing bandwidth of the chaotic system — a fundamental restraint that deprives contemporary controller designs of validity and effectiveness. The feedforward feature of the control concept prevents errors from re-entering the control loop and inadvertently perturbing the sensitive micro-milling system. Because neither closed-form nor linearization is required, the innate, genuine features of the micro-milling response are faithfully retained.Copyright

Mitigation of Milling Chatter at High-Speed

A highly interrupted machining process, milling at high speed can be dynamically unstable and chattering with aberrational tool vibrations. While its associated response is still bounded in the time domain, however, milling could become unstably broadband and chaotic in the frequency domain, inadvertently causing poor tolerance, substandard surface finish and tool damage. Instantaneous frequency along with marginal spectrum is employed to investigate the route-to-chaos process of a nonlinear, time-delayed milling model. It is shown that marginal spectra are the tool of choice over Fourier spectra in identifying milling stability boundary. A novel discrete-wavelet-based adaptive controller is explored to stabilize the nonlinear response of the milling tool in the time and frequency domains simultaneously. As a powerful feature, an adaptive controller along with an adaptive filter effective for on-line system identification is implemented in the wavelet domain. By exerting proper mitigation schemes to both the time and frequency responses, the controller is demonstrated to effectively deny milling chatter and restore milling stability as a limit cycle of extremely low tool vibrations.Copyright

Control of High Speed Milling Chatter in Simultaneous Time-Frequency Domain

The dynamics governing high speed milling has been extensively explored for decades, both numerically and experimentally. The process loses its dynamic stability and becomes chaotic through either Neimark-Sacker or period doubling bifurcation. While its associated response is still bounded in the time domain, however, it could become unstably broadband in the frequency domain, thus causing the uneven cutting surface on the workpiece and tool damage. A discrete-wavelet-based feedforward adaptive controller is developed to stabilize system response in the time and frequency domains simultaneously. An adaptive controller along with an adaptive filter effective for on-line system identification is implemented in the wavelet domain. By controlling both time and frequency responses, the presented controller design is demonstrated to effectively suppress milling chatter and restore the system back to dynamic stability.Copyright