Journal of Vibration and Control recent issues

Analysis of Narrowband Active Noise and Vibration Control Systems Using Parallel Adaptive Notch Filters

Yang, F., Gupta, A., Kuo, S.M. — 2008-06-20

This paper analyzes narrowband active noise and vibration control (ANVC) systems using multiple second-order adaptive filters configured in parallel form with the filtered-x least mean square algorithm. Theoretical analysis of convergence of the algorithm is based on the autocorrelation and crosscorrelation matrices of the overall parallel structure. Analysis results show that extra harmonics are induced in the residual error because of using a single error signal to update all adaptive filters with different sinusoidal reference signals. An optimized filter-adapting method which uses bandpass filters to split the error signal into multiple bandlimited error signals according to frequencies of reference signals is proposed to improve the performance of narrowband ANVC systems in steady state. Computer simulations are conducted to verify analysis results and performance of the optimized narrowband ANVC system.

Modified Fuzzy Variable Structure Control Method to the Crane System with Control Deadzone Problem

Chang, C.-Y., Hsu, K.-C., Chiang, K.-H., Huang, G.-E. — 2008-06-20

This paper proposes a modified fuzzy based variable structure control to achieve the position and swing control of the 3-D nonlinear overhead crane system. One derives the control power according to the variable structure controller and feedback signals-trolley position and payload swing angle. Compensating algorithm for the deadzone problem is provided in this paper and the heuristic sliding factors are also tuned automatically by the proposed fuzzy method to enrich the system performance without plant information of crane. Several experiments for the position and swing control of the nonlinear overhead crane system demonstrate the effectiveness of the proposed scheme.

Application of Genetic Algorithms to Observer Kalman Filter Identification

Schoen, M. P. — 2008-06-20

In this paper several applications of genetic algorithm (GA) as an aid to the system identification process are presented. First, GAs are used in a set of covariance-based optimum input signal algorithms using a proposed architecture suitable for online system identification. The optimal signals are computed recursively using a predictive filter. The efficiency of these algorithms are compared based on a set of simulations. Second, a novel input design for a two-step identification scheme is presented. Constraint systems, such as commonly found in structural and biomedical engineering applications, are considered for the input design algorithm. This paper presents a novel approach that induces a learning scheme into the input design computation and allows for considerations of the given constraints of the system. The optimization of the new input signal is accomplished using a simple elitism based genetic algorithm. Simulation results indicate the proposed piecewise adaptive input design algorithm performs well compared to the general white-noise-based estimation results. In the third portion of this paper proof is given that no dynamic controller can reduce the noise influence in linear system identification. A new selection scheme of the corresponding singular values is proposed for the eigensystem realization portion of the Observer Kalman filter IDentification algorithm in noisy systems. The selection is done using a GA. Simulation results of the proposed algorithm in comparison with the traditional used method are presented. The results indicate an improved ability to extract system models from highly noise corrupted data.

Simulation of Tool Vibration Control in Turning, Using a Self-Sensing Actuator

Freyer, B.H., Theron, N.J., Heyns, P.S. — 2008-06-20

Besides reducing the restricting effects of tool vibrations on productivity, work-piece surface finish and tool life, it is desirable to handle lack of space for sensors at the tool tip and the cost of control systems in turning processes in an effective way. This work considers these two aspects by exploiting the concept of a self-sensing actuator (SSA) in the simulation of tool vibration control. The tool holder structure, in its passive as well as active state, is modeled as a supported cantilever. A feedback filtered-x least-mean-square (LMS) algorithm is chosen to compute the control action. A known technique, which consists of pre-filtering the inputs to the LMS-algorithm maintains the stability of the control system. The self-sensing path is modeled and illustrated. It consists of the transmission of the tool tip displacement to the SSA where it is sensed by converting it into a voltage signal. A considerable reduction of 93% of the displacement r.m.s. values of the tool tip, was obtained when simulating this control system.

Parametric Identification and Health Monitoring of Complex Ground Vehicle Models

Metallidis, P., Stavrakis, I., Natsiavas, S. — 2008-06-20

A systematic methodology is applied for performing parametric identification and health monitoring in the suspension substructures of complex vehicle models. The equations of motion are derived by applying a finite element method. As a consequence, they involve quite a large number of degrees-of-freedom (DOF). In addition, they include strongly nonlinear terms. In particular, the main nonlinearities arise due to the function of the suspension dampers and springs. Moreover, the action of the bushings connecting the suspension subsystems to the vehicle body is also strongly nonlinear. Since the resulting number of DOF is large, an appropriate coordinate condensation technique is applied first. This drastically reduces the dimension of the original system and allows the application of a statisticcal system identification methodology, which is effective for dynamical systems with relatively small dimension, in order to perform parametric identification and fault detection studies in the suspension subsystems of an example vehicle model. In the second part of this study, the methodology developed is applied and yields numerical results related to parametric identification and fault detection in the suspensions of the vehicle model examined. The results are found to be sufficiently accurate even in the presence of considerable measurement noise and model errors.

Suppression of Vehicle-induced Bridge Vibration Using Tuned Mass Damper

Xiaomin Shi, , Cai, C.S. — 2008-06-20

This paper focused on a numerical model for vehicle-induced bridge vibrations controlled with a tuned mass damper (TMD) system, taking road surface conditions into account. A computer program was developed to study the possible effectiveness of the TMD for suppressing the vibration of bridges under vehicle loads. Then, a comprehensive investigation was conducted on different bridges under two vehicle load patterns, i.e. two trucks moving side by side or several trucks passing over the bridge in a traffic flow. It was found that the additional damping provided by the TMD results in a reduction of the maximum dynamic displacement for both free and forced vibrations. On the other hand, for all the bridges investigated in this study, the reduction of acceleration is also significant. More generally speaking, it can be concluded that for the same TMD installed on the same bridge, it is more effective for cases with trucks passing over the bridge in traffic flow than for cases with only one truck. Such a study is helpful in evaluating the control performance before the real control devices are designed in practice.

An Experimental Study on Semi-active Seismic Response Control of a Large-span Building on Top of Ship Lift Towers

Tu, J.-W., Qu, W.-L., Jing Chen, — 2008-06-20

A vertical ship lift under earthquake excitation may suffer from a violent whipping effect due to the sudden change of building lateral stiffness at the top of ship lift towers. This paper presents an experimental investigation to explore the validity of using a magnetorheological (MR) isolation system to prevent the whipping effect manipulated by neuro-fuzzy control algorithm. The two-story building was constructed as a ship lift model, in which the first story was much stiffer than the second story and the plates of the second story were divided into two mass pieces. MR isolation systems were used to link those two mass pieces. The dynamic characteristics of the model with rigid connection and with only seismic isolator connection were first identified. The models with those two connections and with MR isolation system connection were then tested under the scaled Three Gorge artificial ground motion and the scaled El Centro 1940 north-south ground motion. Finally, the seismic responses of the ship lift model were recalculated with the modified structural parameters through measured natural frequencies and were compared with the experimental seismic responses. The experimental results show that an MR isolation system manipulated by neuro-fuzzy control algorithm can achieve a significant reduction in seismic whipping effect on the building model. The comparison consequences indicate that the theoretical simulation results are accurately consistent with the experimental results.

Energy Recovering in Active Vibration Isolation System -- Results of Experimental Research

Kowal, J., Pluta, J., Konieczny, J., Kot, A. — 2008-06-20

Studies of systems with energy regeneration have been carried out for years, because they primarily cover the assemblies with electrodynamic actuators. This paper addresses the issue of active reduction of mechanical vibration using an electrohydraulic actuator. The testing procedure aims to assess the potential use of those assemblies in a different frequency band and force range than in electrodynamic actuators. The paper explains the operating principle of the system, and the findings of laboratory tests are presented. The tested vibration reducing system is the physical model of a 2 degree-of-freedom (DOF) suspension. An initial analysis has been conducted to explore the potential use of the energy produced by the vibration of unsprung mass in the first degree of the suspension system, for power supply to the active component incorporated in the second suspension degree.

The energy recuperated from the first suspension DOF is transferred by a dedicated hydraulic system and stored in an accumulator. Results of the experiments revealed that the mechanical parameters of the system can be selected in such a way that for specific interfering signals the accumulated energy should be at least equal to the energy used up by the system.

Experimental and Analytical Study of a Steel-concrete Bridge

Morassi, A., Tonon, S. — 2008-05-16

This article is an experimental-analytical investigation of the structural behavior of a two-span steel-concrete bridge. The use of mixed experimental data coming from both static and dynamic testing has proved crucial for the identification of an accurate finite element model of the bridge and for a full understanding of its structural behavior. In addition, the results suggest that, in order to avoid the indeterminacy that frequently affects structural identification problems, it is useful to consider all the known a piori information on the system and carefully select the parameters to be identified.

Numerical Solution of the Controlled Rayleigh Nonlinear Oscillator by the Direct Spectral Method

Navabi, M.R., Shamsi, M., Dehghan, M. — 2008-05-16

This paper presents two numerical methods for solving the controlled Rayleigh nonlinear oscillator problem. The first method is based on construction of interpolation polynomials to approximate the states and subsequent control using the roots of Legendre polynomials as collocation nodal points. In the second method, we look for a piecewise continuous approximation polynomial for each state and a piecewise constant function for the control function. We show that this method leads to better results and enables us to accurately compute switching times. These methods are easy to implement, and yield very accurate results.

Dynamics of a Laminated Composite Beam on Pasternak-Viscoelastic Foundation Subjected to a Moving Oscillator

Ahmadian, M.T., Jafari-Talookolaei, R.A., Esmailzadeh, E. — 2008-05-16

Dynamic behavior of a laminated composite beam (LCB) supported by a generalized Pasternak-type viscoelastic foundation, subjected to a moving two-degree-of-freedom (DOFs) oscillator with a constant axial velocity is studied. Analytical solution using the Galerkin method is sought and the couplings of the bending—tension, shear—tension, and bending—twist with the Poisson effect are considered. The possible separation of the moving oscillator from LCB during the course of motion is investigated by monitoring the contact force between the oscillator and LCB. The effects of the non-rigid foundation, oscillator parameters, and the load speed on the separation are also studied. It is found that the separation of the oscillator from the slender LCB will occur with a high stiffness of the oscillator and by having either a low or a high axial momentum of the oscillator. The separation can be suppressed by an elastic foundation with a relatively large stiffness. The bending moment and the beam deflection at the beam center and just below the oscillator due to the load velocity and position are examined and the corresponding velocity for the maximum values of those parameters is determined.

A Modification to Filtered-X LMS Control for Airfoil Vibration and Flutter Suppression

Carnahan, J. J., Richards, C. M. — 2008-05-16

The effects of error path model dynamics and choice of reference signal on the performance of the filtered-X least main square (LMS) algorithm for active vibration and flutter suppression is investigated using a two-degree-of-freedom aeroelastic (pitch-plunge) system. It is shown that the convergence coefficient of the filtered-X LMS algorithm must vary in order to reduce vibration over a broad airspeed range. The control reduces vibration at sub-critical airspeeds and is capable of extending the critical speed by 11%. A modification to the control scheme is presented where the error path model is removed from the feedforward loop. The modified method also reduces vibration at sub-critical airspeeds while maintaining a constant convergence coefficient, and extends the critical speed by 50%. A performance study is conducted comparing the maximum amplitude reduction, settling time, and robustness to measurement noise of all control schemes presented.

Dynamic Viscoelastic Effects on Free Vibrations of a Submerged Fluid-filled Thin Cylindrical Shell

Hasheminejad, S. M., Shahsavarifard, A., Shahsavarifard, M. — 2008-05-16

The novel features of the Havriliak-Negami model for dynamic description of viscoelastic material behavior along with the Donnell theory of shell motion are applied to study free vibrational and damping characteristics of an infinitely long viscoelastic thin circular cylindrical shell submerged in and filled with acoustic fluids. The analytical results are illustrated with numerical examples in which two fluid-filled shells of distinctive viscoelastic material properties are undergoing free vibrations in a surrounding ideal fluid medium. The natural frequencies and the associated modal loss factors as a function of the circumferential mode number at selected axial wave lengths and submergence conditions are numerically evaluated and discussed. The numerical results reveal the imperative influence of dynamic viscoelastic properties on the vibrational characteristics of the fluid-coupled system. They demonstrate an interesting correlation between the modal loss factor and the material loss factor in the vicinity of the calculated natural frequency. In particular, the modal loss factor (natural frequency) is found to be highly dependent on (fairly insensitive to) the frequency dependence of the loss factor in the viscoelastic material. A limiting case involving a thin elastic steel shell submerged in water is considered and fair agreement with a well-known solution is established.

Optimum Parametric Studies on Tuned Rotary-Mass Damper

Obata, M., Shimazaki, Y. — 2008-05-16

We suggest a new passive type vibration control system, tuned rotary-mass damper (TRMD), which consists of the rolling mass (rotary-mass) and the container (outer shell) allowing the free movement of the mass along its inner arc. This TRMD has a simple construction and is applicable to the structure. The tuning of TRMD can be made by adjusting the diameter of the rotary-mass and the curvature of inside the outer shell. To determine the optimum tuning and damping ratio for a linear system of TRMD, a design technique of tuned mass damper is applied. The frequency response analysis is also performed to the nonlinear system of TRMD. It was found that the response magnification factor of the structure-TRMD system showed a strong dependency on the external strength ratio, and can be reduced by designing TRMD against the targeted external strength ratio.

Vibration Control of Nonlinear Rotating Beam Using Piezoelectric Actuator and Sliding Mode Approach

Xue, X., Tang, J. — 2008-05-16

In this research, we develop a general methodology for the vibration control of a nonlinear rotating beam. The dynamic model of a rotating Euler-Bernoulli beam integrated with a piezoelectric actuator is formulated. An integral sliding mode approach is proposed for the vibration control of the system with nonlinear coupling effect between the hub rotation and the beam transverse vibration. The vibration control is achieved by using the piezoelectric actuator only, whereas the motor torque is treated as a time-dependent external input. In the sliding mode control design, the sliding manifold is constructed using only the partial states of the system that are associated with the beam vibration. Particularly, utilizing the internal dynamics analysis, the nonlinear coupling effect of the rigid-body rotation and the beam transverse vibration is decoupled, and the robust stability of the system is guaranteed. This integral sliding mode control is continuous in nature, which can alleviate or avoid the chattering problem. A series of simulation studies demonstrate that the proposed control method can effectively suppress the beam vibration induced by the hub rotation and the external disturbance.

Sliding Mode Control of High Order Systems using a Constant Nonlinear Sliding Surface on a Transformed Coordinate Axis

Tokat, S., Eksin, I., Guzelkaya, M. — 2008-05-16

Sliding mode control allows insensitivity to bounded parameter variations, and rejection of disturbances. However, this property is valid only in the sliding phase. Therefore, various studies have been performed with the aim of improving system performance by minimizing, or even removing, the time needed to reach the sliding phase. Sliding surface design is one method of achieving this aim. In this study, a new approach to the design of nonlinear sliding surface for high order systems, which relies on defining a new coordinate axis, is proposed. A constant and nonlinear sliding surface is presented in this newly defined coordinate axis. Next, the control law for the proposed method is derived, and it is seen that the equivalent control term is composed of the equivalent control term of the conventional sliding mode controller and an additive signal, which is a nonlinear function of the system state error vector and conventional sliding surface design parameters. In order to set these design parameters, various performance evaluations are conducted. Simulations are then performed with these design parameters, using a third order nonlinear system model. The results of the new design methodology are compared with both a conventional sliding mode controller and an alternative sliding mode controller that also uses an additive term in the control law to minimize the reaching time. It is shown that the proposed method improves the system performance in terms of reaching time, magnitude of control input, robustness to disturbances, and smoothness in error state behavior issues.

Trajectory Optimization Strategies for Supercavitating Underwater Vehicles

Ruzzene, M., Kamada, R., Bottasso, C.L., Scorcelletti, F. — 2008-05-14

Supercavitating vehicles are characterized by substantially reduced hydrodynamic drag, in comparison with fully wetted underwater vehicles. Drag is localized at the nose of the vehicle, where a cavitator generates a cavity that completely envelopes the body, at the fins, and on the vehicle after-body. This unique loading configuration, the complex and non-linear nature of the interaction forces between vehicle and cavity, the unsteady behavior of the cavity itself and memory effects associated with its formation process make the control and maneuvering of supercavitating vehicles particularly challenging. This study presents an initial effort towards the evaluation of optimal trajectories for this class of underwater vehicles. Flight trajectories and maneuvering strategies for supercavitating vehicles are obtained through the solution of an optimal control problem. Given a cost function, and general constraints and bounds on states and controls, the solution of the optimal control problem yields control time histories that maneuver the vehicle according to the desired strategy, together with the associated flight path. The optimal control problem is solved using the direct transcription method, which does not require the derivation of the equations of optimal control and leads to the solution of a discrete parameter optimization problem. Examples of maneuvers and resulting trajectories are given to demonstrate the effectiveness of the proposed methodology and the generality of the formulation.

Experimental Identification of the Nonlinear Parameters of an Industrial Translational Guide for Machine Performance Evaluation

Dhupia, J. S., Ulsoy, A. G., Katz, R., Powalka, B. — 2008-05-14

Prediction of machine dynamics at the design stage is a challenge due to lack of adequate methods for identifying and handling the nonlinearities in the machine joints, which appear as the nonlinear restoring force function of relative displacement and relative velocity across the joint. This paper discusses identification of such a nonlinear restoring force function for an industrial translational guide for use with the Nonlinear Receptance Coupling Approach (NLRCA) to evaluate machine dynamic characteristics. Translational guides are among the most commonly used joints in machine tools. Both parametric and nonparametric techniques have been employed to identify the nonlinearities. A novel parametric model based on Hertzian contact mechanics has been derived for the translational guide. A nonparametric method based on two-dimensional Chebyshev polynomials is also used. The models derived from the two techniques, i.e., parametric and nonparametric, are fitted to the experimental data derived from static and dynamic tests to get the restoring force as a function of relative displacement and relative velocity across the joint. The nonlinear representation obtained from both techniques is later converted into the describing function representation which is needed for evaluation of machine dynamic characteristics using the NLRCA. The describing function representations obtained from the two approaches are compared. The design of experiments for evaluating the nonlinearities in such industrial machine tool joints is a challenge, requiring careful alignment and calibration, because they are typically very stiff. This constrains the dynamic experiments to be carried out at high frequencies (e.g. 2000—7000 Hz) where the experimental readings are very sensitive to errors in geometry and calibration.

Damping by Parametric Stiffness Excitation: Resonance and Anti-Resonance

Dohnal, F. — 2008-05-14

Investigations are presented into the stability of vibration suppression employing variable-stiffness actuators. Systems with an arbitrary number of degrees of freedom are considered, and are subject to both parametric excitation and self-excitation. Analytical conditions of stability and instability are derived by applying a singular perturbation technique. These conditions enable a stability classification that naturally leads to the description parametric anti-resonance. The influence of the symmetry property of the parametric excitation matrix on the location of the parametric anti-resonance is discussed. Additionally, the influence of parametric resonance and anti-resonance on the eigenvalues corresponding to the slow motion of a generic system are analysed. A geometric interpretation is presented, enabling deeper insight into the mechanism of vibration suppression, and leading to the interpretation of coupling modes using parametric anti-resonance and amplification of system damping. The basic results obtained can be used for design of a control strategy for variable-stiffness actuators.

Active Vibration Control of Structures Subject to Parameter Uncertainties and Actuator Delay

Haiping Du, , Nong Zhang, — 2008-05-14

This article presents a robust H controller design approach for active vibration attenuation of structures considering parameter uncertainties and time delay in control input. The parameter uncertainties dealt with belong to the polytopic type, and are assumed to be variations of the structural stiffnesses and damping coefficients. The time delay is an uncertain time-invariant with known constant bound. In terms of the feasibility of certain delay-dependent matrix inequalities, both state feedback and static output feedback controllers can be designed. To overcome the bilinear matrix inequality problems involved in the delay-dependent conditions, a genetic algorithm is used to find feasible solutions, after which use is made of the solvability of linear matrix inequalities. The performance of the presented approach is demonstrated by numerical simulations on the vibration control of a building structure subjected to seismic excitation. It is confirmed that the controllers designed by this method can effectively attenuate the structural vibration and ensure system stability even when there are parameter uncertainties and actuator delay.

Study on Stability Improvement of Suspension Bridge with High-Sided Vehicles under Wind using Tuned-Liquid-Damper

Chen, S.R., Chang, C.C., Cai, C.S. — 2008-05-14

Smooth and safe traffic on the highway system is crucial for a modern society. As the backbone of a highway system, key long-span suspension bridges as well as moving high-sided vehicles on the bridges are vulnerable to strong wind. Excessive vibration of bridges and vehicles may cause the bridge aerodynamic instability and vehicle accidents. When wind is strong, the excessive torsional response of the bridge also contributes to the discomfort of drivers or the occurrence of overturning accidents of vehicles moving on it. Potentials of adopting the tuned-liquid-damper (TLD) system as an enhancement measure for the stability of suspension bridges with high-sided vehicles are evaluated. Firstly, a general shallow water sloshing model in rectangular containers under off-axle rotation, lateral and vertical excitations is developed. The analytical platform of the general 3-D bridge—vehicle—TLD system under wind and road roughness excitations is introduced. Secondly, a parametric study of the off-axle TLDs in controlling torsional motion of a simple single torsional mode bridge model under broadband white noise excitation is conducted to investigate the effectiveness and mechanism of the TLD on torsional motion. Finally, a numerical analysis of TLDs installed on a real suspension bridge—vehicle—wind system in time history is carried out considering interactions between the multimode bridge structure, multiple vehicles and the wind excitation. The results suggest that the off-axle TLD system can effectively suppress the torsional response of the suspension bridge through acting as a pendulum damper in addition to a typical sloshing damper, but has little direct suppression effect on the vibrations of vehicles.

Scalable and Invertible PMNN Model for MagnetoRheological Fluid Dampers

Cao, M., Wang, K.W., Lee, K. Y. — 2008-05-14

To advance the state of the art of physical-principle-enhanced hybrid artificial neural network (ANN) modeling, network configurations with parallel modules (PMNN) reflecting the structural information of the physical principles have been developed (Cao, 2001). In this paper, the PMNN configuration is applied to develop a scalable and invertible dynamic magneto-rheological (MR) fluid damper model. To advance the state of the art and address issues of the current ANN-based MR damper models found in the open literature, two ANN-based MR damper models are developed in this study. The first one is a conventional first-principle-enhanced hybrid neural network model (defined as the baseline model in the current study) that improves upon the previous ANN-based MR damper models by introducing feedback loops to represent the dynamic behaviors of the MR damper. A PMNN-based MR damper model is then derived to further improve the control-input-output scalability and realize the invertible model concept. "Input-output scalability" refers to the model's capability to accurately estimate the system response with input profiles significantly different from the training data. "Invertible model" means that the resultant forward model can be directly transformed into an inverse model through a simple algebraic operation. The network training/testing results indicate that while both models provide satisfactory performance, the PMNN model outperforms the baseline model by showing superior control-input-output scalability. The candidacy of PMNN as a control-oriented actuator modeling tool is further strengthened by the fact that it is invertible, in other words, the inverse model with desired force as input and the control signal—voltage as output can be easily established by algebraically manipulating the forward model. This study indicates that PMNN, as a scalable and invertible dynamic modeling tool, is feasible for developing system-design-oriented models of vibration control purposes.

Design of Mechanical Band-Pass Filters for Energy Scavenging: Multi-Degree-of-Freedom Models

Shahruz, S.M. — 2008-05-14

In this article, a model of a mechanical band-pass filter to be used in energy scavengers is studied. The filter is an ensemble of cantilever beams where at the tip of each beam a mass, known as the proof mass, is mounted. A beam with a proof mass at its tip is called a beam-mass system. The dynamics of a beam-mass system can be represented by its corresponding generalized single-degree-of-freedom (SDoF) system. Using this model, dimensions of the beams and masses of the proof masses of beam-mass systems can be determined, so that an ensemble of such systems would function as a band-pass filter. To verify the adequacy of the SDoF model in representing the dynamics of the infinite-dimensional beam-mass system, a two-degree-of-freedom (TDoF) model of this system is developed. This model affirms that, as far as the energy scavenging is concerned, the SDoF model adequately represents the dynamics of the infinite-dimensional beam-mass system. Consequently, in designing mechanical band-pass filters, it is sufficient to use the SDoF models of the beam-mass systems.

Dynamic Modeling of a Novel Microfluidic Channel Angular Accelerometer

Jurgens Wolfaardt, H., Stephan Heyns, P. — 2008-03-26

The angular accelerometer is a versatile inertial instrument, with applications ranging from vehicle stabilization to navigation and satellite pointing. A novel angular accelerometer is proposed, which is able to improve on contemporary angular accelerometers and micro-electromechanical system gyroscopes. The sensor consists of micro-machined spiral channels, fabricated on multiple wafers and used to construct a spiral-helix fluid column that generates high pressure during angular acceleration round the sensitive axis. The two ends of the fluid column are joined at a central measurement chamber, where a diaphragm-based pressure transducer produces a signal proportional to the angular acceleration applied. This article presents the dynamics of the sensor, and then investigates its potential. A discrete multiple-degree-of-freedom model simulates pressure generation and propagation, and was verified experimentally. Channel flow is simulated by means of a model derived from Szymanski's theory of unsteady laminar flow. The pressure transducer diaphragm model is based on linear flat plate theory. The sensor theory is synthesized in a linear sensor model and the dynamic response optimized by means of the Kuhn-Tucker method. A simulation study demonstrated that a sensor with a resolution of 15µ rad/s² and a bandwidth of 50 Hz can be packaged with a diameter of 22 mm and a height of 22 mm, when referenced against a noise level of 1µV/Hz.

Early Detection of Pitting Damage in Gears using Mean Frequency of Scalogram

Ozturk, H., Sabuncu, M., Yesilyurt, I. — 2008-03-26

A local gear-tooth defect such as a fatigue crack, pit or chip weakens the tooth and causes transient events when that faulty tooth is in mesh. The magnitude and duration of these events depend mainly upon the severity of the defect and the contact ratio of the gear pair. If the tooth fault severity is small and the contact ratio is high, the resulting transient may not show distinctively in the vibration signal; time-frequency analysis can be used to reveal such events. This paper presents the use of a scalogram and its mean frequency variation for the detection and monitoring of pitting faults in gears. Real gear vibrations are obtained from a test rig utilising a two-stage industrial gearbox. Pits representing differing degrees of fault severity are simulated on a few tooth surfaces. Classical processing schemes in the time and frequency domain are firstly employed to obtain general characteristics of the gear vibration. The continuous wavelet transform is then used to obtain a scalogram and its mean frequency variation. It has been found that the presence of a pitting fault cannot be clearly revealed by the time and frequency domain representations unless the fault severity is high. In contrast, the scalogram (and especially its mean frequency variation) provides early indications of presence and progression of pitting faults in gears even when the fault severity is considerably smaller.

Vibratory Characteristics of Euler-Bernoulli Beams with an Arbitrary Number of Cracks Subjected to Axial Load

Aydin, K. — 2008-03-26

A simple and efficient analytical approach for determining the vibrational frequencies and mode shape functions of beams with an arbitrary number of non-breathing cracks when subjected to axial loading is presented here. The local compliance induced by a crack is described using the rotational spring model. A set of boundary conditions are used as initial parameters to define the mode shape of the segment of the beam before the first crack. Using this, the remaining set of boundary conditions and the recurrence formula developed in the study, the mode shape function of vibration of a beam containing multiple cracks can be easily determined. Five different end conditions are considered: Pinned-pinned, clamped-pinned, clamped-free, clamped-clamped, and spring-spring (with concentrated masses). Three crack depths and seven axial force levels are used to represent service load conditions. A parametric study, to investigate the effects of crack and axial load on the vibrational properties of cracked beams, is carried out for each support condition case. The influence of cracks on the buckling load of a beam is also studied. Some of the results obtained are checked against published values, and a good agreement can be seen. The study concludes that the crack location, crack severity and axial force level all strongly affect the eigenfrequencies.

Modal Identification, Model Updating and Nonlinear Analysis of a Reinforced Concrete Bridge

El-Borgi, S., Neifar, M., Cherif, F., Choura, S., Smaoui, H. — 2008-03-26

This article proposes a rational methodology for the structural assessment of a reinforced concrete bridge in Tunisia. This methodology is based on ambient vibration measurement of the bridge, identification of its modal signature, finite element model updating and nonlinear analysis. The selected case study is an eight-span bridge with a continuous slab. Each span is simply supported at rubber bearings and has a length of 25 m. Because of the repetitive geometry of the bridge, ambient vibration tests were conducted on one span using a data acquisition system with nine force-balance accelerometers placed at selected locations. The Enhanced Frequency Domain Decomposition (EFDD) technique was applied to extract the dynamic characteristics of the bridge. A 3-D finite element model was developed and updated to obtain reasonable correlation between experimental and numerical modal properties. The parameter selected for the updating is the modulus of elasticity of the concrete in each concrete element of the finite element model. A decrease of its value indicates the possibility of damage or stiffness reduction. We demonstrate that using the EFDD technique in combination with model updating and nonlinear analysis provides valuable information for the evaluation of the bridge structural condition.

Robust Synchronization of a Class of Nonlinear Systems: Applications to Chaotic Coupled Electromechanical Systems

Bowong, S., Xiaohua Xia, — 2008-03-26

This article treats the robust synchronization problem of a class of nonlinear systems from a control theoretical point of view. Because of the tremendous complexity of nonlinear systems, the problem is restricted to chaotic electromechanical devices. The results are discussed in the context of complete synchronization. A new dynamic output feedback is applied to perform synchronization in spite of master/slave mismatches. The main idea is to construct an augmented dynamical system from the synchronization error system, which is itself uncertain. The advantage of this method over the existing results is that the synchronization time is explicitly computed. Numerical simulations are provided to verify the operation of the proposed algorithm.

Time-Optimal De-tumbling Control of a Rigid Spacecraft

Yang, C.-C., Wu, C.-J. — 2008-03-26

The problem of time-optimal de-tumbling control (TODTC) of a rigid spacecraft moving between two attitudes is studied in this article. Unlike conventional approaches, which involve solving a set of differential equations, a novel numerical method is introduced. In the proposed method, by fixing the count of control steps and treating the sampling period as a variable, the TODTC problem is formulated as a nonlinear programming (NLP) problem by utilizing an iterative procedure. Generating initial feasible solutions systematically is also discussed, since these are usually needed in solving a NLP problem. In this manner, the optimization process of the NLP problem can be started from many different points when searching for the optimal solution. Simulation results are included, to show the feasibility of the proposed method.

Vibration Characteristics of a Rotor System in Contact with a Backup Bearing: Cases with Various Failure Patterns of the Active Magnetic Bearing

Ishida, Y., Inoue, T. — 2008-03-26

When electric power for an active magnetic bearing (AMB) is shut down during operation, the backup bearing plays an important role in stopping the rotor system safely. Recently, a flexible high-speed rotor supported by an AMB, operating above the first and second critical speeds, has been investigated. In this context, the vibration characteristics of the flexible high-speed rotor when interacting with the backup bearing system become important. In this article, the mathematical models for the contact and friction forces which exist during contact between the rotor and the backup bearing are considered, the rotation of the inner ring of the backup bearing is modelled, and the vibration characteristics of the rotor system are studied. By comparing the results of numerical simulations and experiments, the validity of the mathematical model of the backup bearing is confirmed. Moreover, the vibration characteristics of the rotor systems when various types of failures occur in the AMB are clarified both theoretically and experimentally.

Active Control of Flow-Induced Vibrations via Feedback Decoupling

Yigit, F. — 2008-03-26

This paper deals with the flutter instability characteristics of a cantilever pipe conveying fluid flow, and explores the applicability of an active nodal vibration control for suppressing the associated structural vibration. The Euler-Bernoulli theory is used to represent pipe bending. The finite element method is used to discretize the governing equation. The control law is based on full state feedback and pole assignment, and requires as many actuators as the number of nodal degrees-of-freedom. Considering that this is not practical, a reduced order model with less number of elements is used to design the controller, and the resulting control input is applied to the "full" or the "truth" model. In this case, since the feedback simplification will not be complete, some performance degradation is to be expected. It is however demonstrated that the proposed control strategy can ensure closed loop stability for a wide range of flow velocity even if the critical flow velocity is exceeded. The effectiveness of the proposed method, in damping out the pipe vibrations, is also demonstrated clearly by comparing its results with those obtained by a direct velocity feedback control which is equivalent to add an external viscous damper to the pipe. The proposed control is essentially a model-based controller, and hence suffers from modeling errors and uncertainties in model parameters. Therefore, the robustness of the control is also investigated. It is shown that the proposed controller significantly reduces sensitivity of the uncontrolled system to flow conditions. It works effectively to suppress the vibrations of a fluid conveying cantilever pipe due to any disturbance.