Unwanted Vibrations Research Articles

Effect of shock subs on self-excited vibrations in drilling systems

The present paper studies the effect of an axial elastic tool (known as a shock sub), mounted downhole in the drill-string, on the occurrence of axial and torsional self-excited vibrations. In particular, we evaluate the feasibility of stabilizing the axial dynamics, dominated by a bilateral (feedback) coupling between the bit-rock interaction and the drill-string wave-equations, through the insertion of a passive down-hole tool. We consider the problem of unwanted drill-string vibrations and explain how these vibrations relate to the so-called axial instability using a distributed parameter (infinite dimensional) model. The equations describing the feedback system causing this instability are derived and then extended to accommodate for the inclusion of the effect of the shock sub. Conditions for the design parameters of the shock sub needed to avoid axial instability are then derived and their practical feasibility are considered.

Vertical vibration attenuation for truck’s suspension seat by adjusting air spring stiffness

Our study deals with the vibration isolation at the suspension seat to reduce the human body’s harmfulness from the oscillation. The unwanted vibration transmitted from unpredicted road roughness via chassis throughout to the driver in widely range of excited frequency. The passive suspension cannot overcome the problem, but the active suspension is too complicate and expensive for this system. Consequently, we propose the idea concerning an adjustment of the air spring stiffness in order to change the characteristic of the system. The air spring stiffness was initially identified via the vibration investigation and measurement. After that, the method of stiffness adjustment is proposed. The result show that the weight root mean square vertical acceleration must follow the comfort range following ISO 2631-1.

Structural load alleviation using distributed delay shaper: Application to flexible aircraft

Lightweight flexible aircraft suffers from unwanted oscillatory vibrations during aircraft manoeuvres. A recently developed distributed-delay signal (DZV) shaper is therefore proposed to be applied as a feedforward controller to alleviate the manoeuvre loads, as an alternative to traditional structural filters used routinely in this context. Structural filters are essentially linear low-pass filters with bandwidth below the significant flexible modes, applied to control signals generated either by the pilot’s direct input or by the flight control system. It has been showed that if instead a properly tuned signal shaper is used, better performance can be achieved: first, the target modes are significantly attenuated while the responsiveness of the aircraft is less compromised and secondly, the oscillatory nature of the vibrations are reduced. The high fidelity simulation results on a full scaled dynamic model of a highly flexible blended wing–body (BWB) aircraft show that in comparison to traditional structural filters, signal shapers significantly reduce the wing root loading (forces and moments) which provides potential structural benefits.

Dynamic Mechanical Properties of Magnetorheological Elastomers: Experiment and Modelling

As a kind of smart material, magnetorheological elastomer (MRE) is composed of magnetizable particles dispersed in an elastomer matrix. Because adjusting an external magnetic field can continuously, rapidly and reversibly control the dynamic properties, there has been increasing research on MRE for mitigation of unwanted vibrations. In this paper, the dynamic mechanical analysis tests were performed to investigate the influence of strain, frequency and magnetic field on the controllable dynamic properties. The storage modulus and loss modulus of MRE were analyzed with frequencies from 1 to 50 Hz, strain amplitudes from 1 to 6% and magnetic field intensities from 0 to 500 mT. The results show that the storage modulus decreases with increasing strain amplitude, increases with frequency and magnetic strength, and remains constant when the magnetic saturation occurs. Furthermore, the dependence of loss modulus on frequency lies on the matrix material of MRE. Based on the experimental results, a fractional derivative model was developed to describe the viscoelastic properties and the controllable dynamic properties. The comprehensive study of mechanical property characterisation is a guarantee for constitutive models to accurately describe the dynamic behaviour of MRE, which is an essential step towards the application for vibration control.

Vibration attenuation of rotor-bearing systems using smart electro-rheological elastomer supports

Nowadays, capability of safe operation sufficiently away from the critical speeds is one of the most important design requirements of rotating machineries. The focus of this paper is the application of smart electro-rheological (ER) elastomers to rotor dynamics field to reduce the vibration level of a rotor system. A Jeffcott rotor, supported via two bearings at both ends augmented with ER elastomers, is considered. A finite element approach, based on the Rayleigh beam theory, is used to model the dynamics of the system, and the proposed model accounts for the rotary inertia, gyroscopic effects and shaft’s internal damping. The ER elastomer supports are simulated with four-parameter viscoelastic model. The simulation results reveal that the use of ER elastomer in the conventional bearing supports leads to downshifting of the critical speeds and a considerable reduction in its corresponding vibration amplitude. Also, the stability limit speed of the system is improved by employing the ER elastomer technology. To extend the stability region of the rotor system to higher operating rotational speeds, a simple on–off control strategy is employed. The proposed control scheme determines the required real-time voltage to be applied at ER elastomers and guarantees low vibration amplitude over a wide frequency range. The novel idea of using ER elastomers for vibration suppression of rotor systems can be fairly extended to other applications which suffer from unwanted high amplitude vibrations.

Smooth online time-varying input shaping with fractional delay FIR filtering

Time-Varying Input Shaping has been proven effective in suppressing unwanted mechanical vibrations in industrial robots. However, no appropriate method exists for robots with online trajectory generation and position control with feed-forward torque. Based on detailed analysis, a new method is proposed, which employ Discrete Time Time-Varying Input Shaping Technology with an alternative discrete time convolver named the Output Side Algorithm FIR filter combined with numerical differentiation. Second, the proposed method is extended to be seen as a Fractional Delay FIR filter, which can be extended to higher order for additional effectiveness, and hereby forming a second new method. Numerical simulations are conducted on a simple time-varying dynamic system with parameters based on a UR10e robot. The simulations show very good vibration suppression with the two new methods, which have great potential application to the vibration control of practical robotic systems.

Vibration Controllability of Sandwich Structures with Smart Materials of Electrorheological Fluids and Magnetorheological Materials: A Review

Vibration control is very significant issue in various engineering fields such as flexible structures, rotor systems, cable and bridge, and vehicle suspension. So far, three different recipes to suppress or control unwanted vibrations are used: passive, semi-active and active. As well known, the passive method has several limitations, such as the lack of real-time avoidance of the time-varying resonances. On the one hand, active vibration control method is very effective, but it is not attractive in terms of cost due to the use of several actuators and sensors. Therefore, recently semi-active vibration control method is popularly used in many practical environments. This article reviews vibration control of flexible structures using the semi-active method associated with smart materials of electrorheological fluids, magnetorheological fluids and magnetorheological elastomers. Modal characteristics of beam, shell and plate incorporating the core (or layer) of smart materials are deeply investigated and discussed in terms of field-dependent controllability. The field-dependent natural frequency and damping property of the sandwich beam type, plate type and shell type are experimentally identified. Subsequently, an appropriate control scheme based on the field-dependent modal properties is formulated to avoid the resonance behavior. In addition, several sandwich beams which are partially filled and fully filled with the magnetorheological fluid are investigated to understand the effectiveness of the modal property change. It has shown that both damping and stiffness properties of the sandwich structures can be effectively controlled by several ways: the change of the field intensity, the location of cores zones, the partial and full treatment and boundary conditions of the structures. In addition, it has identified that mode shapes of the sandwich plates featuring electrorheological core can be partially and fully controlled by applying the input field to an appropriate zone. Smart flexible structures associated with the field-responsive materials can be effectively used for vibration control due to its controllability of the stiffness and damping as well. However, to successfully implement in real environment, a more sophisticated analytical model considering the microscopic aspects of the particle motions needs to developed. Moreover, the field-dependent bucking problem and acoustic characteristics of smart structures subjected to external disturbances need to be explored.

Graphical Analysis Springs Dampers System on the A Freight Cars

In mechanical system where spring and damper are used often create unwanted vibration, due to external forces. Unwanted vibration may cause further noise and other damages such as engine break, etc. To reach system stability, we model of coupling series of freight car and analyze them using root loci and plot waves. From the result we can analyze what is the maximum stability by altering spring and damper constant. It is found when the spring and damper constant increase, the stability will be fast.

Relaying the High-Frequency Contents of Tactile Feedback to Robotic Prosthesis Users: Design, Filtering, Implementation, and Validation

It is known that high-frequency tactile information conveys useful cues to discriminate important contact properties for manipulation, such as first contact and roughness. Despite this, no practical system, implementing a modality matching paradigm, has been developed so far to convey this information to users of upper-limb prostheses. The main obstacle to this implementation is the presence of unwanted vibrations generated by the artificial limb mechanics, which are not related to any haptic exploration task. In this letter, we describe the design of a digital system that can record accelerations from the fingers of an artificial hand and reproduce them on the user's skin through voice-coil actuators. Particular attention has been devoted to the design of the filter, needed to cancel all those vibrations measured by the sensors that do not convey information on meaningful contact events. The performance of the newly designed filter is also compared with the state of the art. Exploratory experiments with prosthesis users have identified some applications where this kind of feedback could lead to sensory-motor performance enhancement. Results show that the proposed system improves the perception of object-salient features such as first-contact events, roughness, and shape.

Dynamic Modelling of a Flexible Beam Structure Using Feedforward Neural Networks for Active Vibration Control

Active vibration control (AVC) techniques show promising results to reduce unwanted vibration level of flexible structures at any desired location. In this paper, the application of non-parametric identification method using feedforward neural networks (FNNs) to model a flexible beam structure for AVC system is presented. An experimental study was carried out to collect input-output dataset of a flexible beam system. The flexible beam was excited using a pseudo-random binary sequence (PRBS) force signal before acquiring the dynamic response of the system. A non-parametric modelling approach of the system was proposed based on feed-forward neural networks (FNNs) while its weight and bias parameters were optimised using chaotic-enhanced stochastic fractal search (SFS) algorithm. The performance of modified SFS algorithm to train a nonlinear auto-regressive exogenous model (NARX) structure FNNs-based model of the system was then compared with its predecessor and with several well-known metaheuristic algorithms. Correlation tests were used to validate the obtained model. Based on the proposed method, a small mean squared error value has been achieved in the validation phase. Considering both convergence rate and result accuracy simultaneously, the chaotic modified SFS algorithm performs significantly better than other training algorithms. In conclusion, the development of a non-parametric model of the flexible beam structure was conducted and validated for future investigations on active vibration control techniques.

Experimental Investigation of Damping Effect in Semi-active Magnetorheological Fluid Sandwich Beam Under Non-Homogeneous Magnetic Field

Despite the increased awareness of vibration control, vibration still causes many problems, such as performance degradation of the equipment and stability violation of flexible structures. In general, the vibration in machines can be caused due to the misalignment, malfunction of mounting parts, changes in temperature, and unbalanced rotating parts. Moreover, the fluctuation in magnetic force due alternating current in transformers and magnetically actuated device can cause unwanted vibrations. The development of sandwich-like structural system with combination of control capabilities plays a very important role to control the unwanted vibrations and to avoid resonance phenomenon due to external disturbances in the system. This problem can be resolved by applying a new sandwich structure incorporating smart fluids such as magnetorheological fluid. Here in this research work, design and fabrication of a sandwich beam consisting of three layers are undertaken; two outer layers of aluminium and one core layer of MR fluid. The fabricated sandwich beam is tested to effectively achieve vibration control under various magnetic field conditions. The research work results show that as the magnetic field intensity increases, the natural frequency of the beam increases, while the peak amplitude, loss factor, and quality factor are decreased. The principal criterion on the vibration control undertaken in this research work is the increment of the natural frequency as the field intensity increases. Therefore, the resonance behaviour of flexible structure, which can be occurred by external disturbances possessing several frequency spectrum, can be avoided by applying an appropriate magnetic field intensity. This directly means that the natural frequency of the smart sandwich beam can be adaptively controlled by using MR fluid which provides high damping and stiffening effect by means of the semi-active control which does not require any external input power.

Validation of a Modeling Tool for In-Plane Longitudinal Resonators with Zigzag Topology

Metastructures are viewed as a promising means for suppressing elastic waves and unwanted vibrations. Metastructures comprise of elementary cells with embedded resonators, which act as vibration absorbers. Design and frequency tuning of individual resonators inside the metastructure allows to achieve effective suppression of vibrations over a relatively wide frequency bandwidth, which makes metastructures superior compared to conventional passive vibration absorbers. This paper describes numerical and experimental validation of a modeling tool for design of planar resonators with elastic elements arranged in a zigzag configuration for suppression of longitudinal vibrations. Zigzag topology is advantageous due to its ability to provide higher compliance within a limited space, so as to achieve low resonant frequencies. Natural frequencies predicted by the proposed model agree well with predictions provided by detailed finite element models and experimental measurements.

Nonlinear optimal control for reducing vibrations in civil structures using smart devices

The frequently excessive vibrations presented in civil structures during seismic events or service conditions may result in users\' discomfort, or worst, in structures failure, producing economic and even human casualties. This work contributes in proposing the synthesis of a nonlinear optimal control strategy for semiactive structural control, with the main characteristic that the synthesis considers both the structure model and the semiactive actuator nonlinear dynamics, which produces a nonlinear system that requires a nonlinear controller design. The aim is to reduce the unwanted vibrations in the response of civil structures, by means of intelligent fluid semiactive actuator such as the Magnetorheological Damper (MRD), which is a device with a low level of power consumption. The civil structures for which the proposed control methodology can be applied are those admitting a state-dependent coefficient factorized representation model, such as buildings, bridges, among others. A scaled model of a three storey building is analyzed as a case study, whose dynamical response involves displacement, velocity and acceleration of each one of the storeys, subjected to the North-South component of the September 19th., 2017, Puebla-Morelos (7.1M), Mexico earthquake. The investigation rests on comparing the structural response over time for two different conditions: with no control device installed and with one MRD installed between the first floor and the ground, where a nonlinear optimal signal for the MRD input voltage is determined. Simulation results are presented to show the effectiveness of the proposed controller for reducing the building\'s dynamical response.

Study on the Dynamic Performance of a Modified Tuned Liquid Column Damper

The main objective of the present study is to reduce unwanted vibrations of mechanical systems through a sealed tuned liquid column damper (STLCD). A nonlinear mathematical model including nonlinear damping and air stiffness of the STLCD is considered to study the dynamic behaviour of a structure- STLCD coupled system. A parametric study to investigate the influence of the main parameters of the STLCD such as length ratio and mass ratio on the dynamic behaviour of the main system subjected to ground excitation is presented. The air volume at rest is considered as an additional design parameter. The study showed that the efficiency and the range of application of the STLCD to control the system resonance vibration are better than the traditional one. In addition, it has been noticed that the gain in vibration mitigation from the STLCD with a bigger mass higher than 7% ratio is generally modest. Also the same trend is observed when increasing its horizontal length ratio over 0.8. These enhance the actual implementation of the STLCD for small horizontal space applications. Equations for the optimum value of the initial air column height ratio and its corresponding performance index for the main structure-STLCD system as a function of both mass ratio and relative length were proposed.

Active control of three-phase CNT/resin/fiber piezoelectric polymeric nanocomposite porous sandwich microbeam based on sinusoidal shear deformation theory

Vibration control in mechanical equipments is an important problem where unwanted vibrations are vanish or at least diminished. In this paper, free vibration active control of the porous sandwich piezoelectric polymeric nanocomposite microbeam with microsensor and microactuater layers are investigated. The aim of this research is to reduce amplitude of vibration in micro beam based on linear quadratic regulator (LQR). Modified couple stress theory (MCST) according to sinusoidal shear deformation theory is presented. The porous sandwich microbeam is rested on elastic foundation. The core and face sheet are made of porous and three-phase carbon nanotubes/resin/fiber nanocomposite materials. The equations of motion are extracted by Hamilton's principle and then Navier's type solution are employed for solving them. The governing equations of motion are written in space state form and linear quadratic regulator (LQR) is used for active control approach. The various parameters are conducted to investigate on the frequency response function (FRF) of the sandwich microbeam for vibration active control. The results indicate that the higher length scale to the thickness, the face sheet thickness to total thickness and the considering microsensor and microactutor significantly affect LQR and uncontrolled FRF. Also, the porosity coefficient increasing, Skempton coefficient and Winkler spring constant shift the frequency response to higher frequencies. The obtained results can be useful for micro-electro-mechanical (MEMS) and nano-electro-mechanical (NEMS) systems.

ON AN INTEGRATED DYNAMIC CHARACTERIZATION OF VISCOELASTIC MATERIALS BY FRACTIONAL DERIVATIVE AND GHM MODELS

Abstract The passive vibration control of mechanical systems under unwanted vibrations can be accomplished in a very effective way by using devices incorporating viscoelastic materials. The design of such devices requires a broad knowledge of the dynamic properties of the employed viscoelastic material, usually supplied by adequate mathematical models. Among the available mathematical models, the fractional derivative (FD) model and the Golla-Hughes-McTavish (GHM) model, along with either the Williams-Landel-Ferry (WLF) equation or the Arrhenius equation, are now very prominent. The current work investigates the use of these models in a wide and integrated dynamic characterization of a typical and thermorheologically simple viscoelastic material. It focuses on experimental data collected from 0.1 to 100 Hz and -40 °C to 50 °C, which are simultaneously manipulated to raise both the frequency and the temperature dependencies of the material. In fitting the models, a hybrid approach - combining techniques of genetic algorithms and nonlinear optimization - is adopted. The ensuing results are evaluated by means of objective function values, comparative experimental-predicted data plots, and the Akaike’s Information Criterion (AIC). It is shown that the four-parameter fractional derivative model presents excellent curve fitting results. As for the GHM model, its modified version is the most adequate, although a higher number of terms is required for a satisfactory goodness-of-fit. None the less the fractional derivative model stands out.

An analytical study of wheel regeneration in surface grinding

Machining vibrations have been a topic of active research since the beginning of the 20th century, because of their harmful potential to deteriorate the surface finish of the workpiece, reduce the lifetime of the machine tool and limit the overall productivity of manufacturing operations. Grinding, often being a finishing operation responsible for the final quality of the workpiece, can be especially affected by unwanted process vibrations. Machining vibrations can arise from a number of sources, but they are usually classified as forced and self-excited vibrations. Forced vibrations, such as those resulting from runout or unbalance, are generally easier to avoid. However, self-excited vibrations, also known as chatter, originate in the machining process itself and are typically more difficult to predict and suppress. When it comes to grinding operations, chatter can occur as a result of uneven surface regeneration on both the wheel and the workpiece. In this paper we perform an analytical study of grinding chatter, to explore the intricate nature of wheel regeneration (i.e. chatter arising from uneven wear on the wheel) in surface grinding. Whilst there has been a great deal of previous research into grinding chatter, the present study explores a contrarian approach whereby a circumferential variation of the specific grinding energy occurs during the onset of instability. The specific energy is a fundamental quantity in grinding that relates the necessary grinding power to the prescribed material removal rate. It is also related to wheel wear and thus to grinding forces, since a worn wheel requires more grinding power and produces greater grinding forces to sustain the same material removal rate than a sharp one. Therefore, by characterising the specific energy variation around the circumference of the grinding wheel as a function of wheel vibration, we derive a stability model and discuss its practical potential.

Noise and vibration suppression of electric power steering systems: a survey

Electric power steering (EPS) systems have become the ideal steering system for most vehicles because of the generated torque assist mechanism making steering easier for drivers. However, EPS systems generate unwanted noise and vibration that can adversely affect the comfort and health of drivers. Numerous studies have been conducted to examine the source of vibrations. Various isolation techniques including passive, active, or semi active have been proposed for achieving best noise and vibration attenuation. Discrete and distributed mathematical models have been employed to predict the dynamic response of EPS systems. This paper presents a state-of-the-art review of mathematical modelling, vibration isolation methods, and optimisation techniques used for improving the design of EPS systems.

A Nonlinear Tire Model to Describe an Unwanted Flat Vibrations of the Wheels

A simplified nonlinear mathematical tire model has been created. The model takes into account the radial and torsional stiffness of the tire, as well as the effect of radial deformation on torsional rigidity and, conversely, the effect of torsional deformation on radial rigidity. The results will be used to study the conditions for the occurrence of undesirable oscillations of vehicle wheels.

Noise and vibration suppression of electric power steering systems: a survey

Electric power steering (EPS) systems have become the ideal steering system for most vehicles because of the generated torque assist mechanism making steering easier for drivers. However, EPS systems generate unwanted noise and vibration that can adversely affect the comfort and health of drivers. Numerous studies have been conducted to examine the source of vibrations. Various isolation techniques including passive, active, or semi active have been proposed for achieving best noise and vibration attenuation. Discrete and distributed mathematical models have been employed to predict the dynamic response of EPS systems. This paper presents a state-of-the-art review of mathematical modelling, vibration isolation methods, and optimisation techniques used for improving the design of EPS systems.