Ramin Sedaghati

A new dynamic hysteresis model for magnetorheological dampers

Semi-actively controlled magnetorheological (MR) fluid dampers offer rapid variation in damping properties in a reliable fail-safe manner using very low power requirements. Their characteristics make them ideal for semi-active control in structures and vehicle applications in order to efficiently suppress vibration. To take advantage of their exceptional characteristics, a high fidelity model is required for control design and analysis. Perfect understanding of the dynamic characteristics of such dampers is necessary when implementing MR struts in applications. Different models have been proposed to simulate the hysteresis phenomenon of MR dampers. The Bouc–Wen model has been extensively used to simulate the hysteresis behavior of MR dampers. However, considerable differences still exist between the simulation and experimental results. Moreover, the characteristic parameters in the traditional Bouc–Wen model are not functions of the frequency, amplitude and current excitations; therefore, the estimated parameters can characterize the behavior of the tested MR damper under specific excitation conditions and must be re-evaluated if a different combination of excitation parameters is desired. This can be extremely cumbersome and computationally expensive. In this work, a new hysteresis model based on the Bouc–Wen model has been developed to better characterize the hysteresis phenomenon of the MR damper. The proposed model incorporates the frequency, amplitude and current excitation as variables and thus enables us to predict efficiently and accurately the hysteresis force for changing excitation conditions. The proposed modified Bouc–Wen model has been validated against the experimental results through graphical and quantitative analysis in time, displacement and velocity domains and an excellent correlation has been found.

Structural Optimization with Frequency Constraints Using the Finite Element Force Method

A structural optimization algorithm is developed to minimize the weight of structures with truss and beam-type members under single- or multiple-frequency constraints. The cross-sectional areas of the structural members are considered as the design variables. The algorithm proposed combines the finite element technique based on the integrated force method with the mathematical programming technique. The equilibrium matrix is generated automatically using the finite element analysis, and the compatibility matrix is obtained directly using the displacement-deformation relations and the single value decomposition technique. When combining the equilibrium and the compatibility matrices with the force-displacement relations, the frequency eigenvalue equations are obtained with element forces as variables. Three structures, composed of truss and frame-type members, are studied to illustrate the procedure, and the results are compared with the literature. It is shown that, in structural problems with multiple frequency constraints, the analysis procedure (force or displacement method) significantly affects the final optimum design. The structural optimization based on the force method results in a lighter design. The proposed structural optimization method is efficient to analyze and optimize both truss and beam-type structures.

Modelling the hysteresis phenomenon of magnetorheological dampers

Recently, magnetorheological (MR) dampers have emerged as a potential technology to implement semi-active control in structures and vehicle applications in order to efficiently suppress vibration. Perfect understanding about the dynamic characteristics of such dampers is necessary when implementing MR struts in application. One of the important factors to successfully attain desirable control performance is to have a damping force model which can accurately capture the inherent hysteresis behavior of MR dampers. Different models have been proposed to simulate the hysteresis phenomenon in such a kind of damper. The Bouc–Wen model has been extensively used to simulate the hysteresis behavior of MR dampers. However, considerable differences still exist between the simulation and experimental results. In this work, a methodology to find the characteristic parameters of the Bouc–Wen model in the attempt to better characterize the hysteresis phenomenon of MR dampers has been proposed. The methodology takes into consideration the effect of each individual term of the Bouc–Wen model over the hysteretic loop to estimate the appropriate values of the parameters. The Bouc–Wen model in which the new established characteristic parameters have been used has been validated against experimental data and an excellent agreement has been shown between the simulation and experimental results. Moreover, the findings pointed towards the fact that linear or exponential relationships exist between the estimated parameters and the current excitation. Considering this, a new model based on the Bouc–Wen model has been proposed in which the excitation current has been incorporated as a variable. This proposed modified Bouc–Wen model has also been validated against the experimental results and a good correlation has been found.

Vibration analysis of a multi-layer beam containing magnetorheological fluid

Magnetorheological (MR) materials exhibit rapid variations in their rheological properties when subjected to varying magnetic field and thus offer superior potential for applications in smart structures requiring high bandwidth. MR sandwich structures can apply distributed control force to yield variations in stiffness and damping properties of the structure in response to the intensity of the applied magnetic field and could thus provide vibration suppression over a broad range of external excitation frequencies. This study investigates the properties of a multi-layered beam with MR fluid as a sandwich layer between the two layers of the continuous elastic structure. The governing equations of a multi-layer MR beam are formulated in the finite element form and using the Ritz method. A free oscillation experiment is performed to estimate the relationship between the magnetic field and the complex shear modulus of the MR materials in the pre-yield regime. The validity of the finite element and Ritz formulations developed is examined by comparing the results from the two models with those from the experimental investigation. Various parametric studies have been performed in terms of variations of the natural frequencies and loss factor as functions of the applied magnetic field and thickness of the MR fluid layer for various boundary conditions. The forced vibration responses of the MR sandwich beam are also evaluated under harmonic force excitation. The results illustrate that the natural frequencies could be increased by increasing the magnetic field while the magnitudes of the peak deflections could be considerably decreased, which demonstrates the vibration suppression capability of the MR sandwich beam.

Passive Vibration Control of Beams Subjected to Random Excitations with Peaked PSD

Vibration suppression in beams subjected to random excitations with peaked Power Spectral Densities (PSDs) is studied in this paper. An optimal Tuned Mass Damper (TMD) system is used to suppress the undesirable vibration. The Timoshenko beam theory is applied to the beam model and the governing equations of motion are solved using the Galerkin method. Using the Sequential Quadratic Programming (SQP) method, the problem is solved to obtain the optimum values of the design variables (i.e. frequency ratio and the damping ratio) of the TMD system. Subsequently, a parametric study is carried out and the effects of the input parameters, such as the mass ratio, structural damping ratio, and the peak frequency of the random excitation on the design variables were investigated. The robustness of the optimal control system is also studied. Based on the PSD of the random excitation and using a Monte Carlo simulation algorithm, a set of numerical data for the excitation force is generated in the time domain and the effectiveness of the designed TMD system is investigated.

Optimal vibration control of beams with total and partial MR-fluid treatments

This paper presents the synthesis of full state and limited state flexible mode shape (FMS) based controllers for the suppression of transient and forced vibration of a cantilever beam with full and partial magnetorheological (MR) fluid treatments. The governing equations of motion of the three layer MR sandwich beam are expressed in the state variable form comprising a function of the control magnetic field. An optimal control strategy based on the linear quadratic regulator (LQR) and a full state dynamic observer is formulated to suppress the vibration of the beam under limited magnetic field intensity. The lower flexural mode shapes of the passive beam are used to obtain estimates of the deflection states so as to formulate a limited state LQR control synthesis. The transient and forced vibration control performances of both the full state observer-based and the limited state FMS-based LQR control strategies are evaluated for the fully as well as partially treated MR-fluid sandwich beams. The results show that the full state observer-based LQR control can substantially reduce the tip deflection responses and the settling time of free vibration oscillations. The limited state LQR control based on the mode shapes effectively adapts to the deflections of the closed loop beam and thus yields vibration attenuation performance comparable to that of the full state LQR controller. The partially treated beam with MR-fluid concentration near the free end also yields vibration responses comparable to the fully treated beam, while the natural frequencies of the partially treated beams are considerably higher.

Optimum design of a multilayer beam partially treated with magnetorheological fluid

The modal damping characteristics of beams partially treated with magnetorheological (MR) fluid elements are studied using the modal strain energy approach and the finite element method. Different configurations of a sandwich beam partially treated with MR fluid are considered, including a beam with a cluster of MR fluid segments and a beam with arbitrarily located MR fluid segments. The significance of the location of the MR fluid segments on the modal damping factor is investigated under different end conditions. An optimization problem is formulated by combining finite element analysis with optimization algorithms based on sequential quadratic programming (SQP) and the genetic algorithm (GA) to identify optimal locations for MR fluid treatment to achieve maximum modal damping corresponding to the first five modes of flexural vibration, individually and simultaneously. The solutions of the optimization problem revealed that the GA converges to the global solutions rapidly compared to the SQP method, which in some modal configurations usually entraps in the local optimum. The results suggest that the optimal location of the MR fluid treatment is strongly related to the end conditions and also the mode of vibration. Furthermore, partial treatments with MR fluid can significantly alter the deflection modes of the beam. It has also been demonstrated that optimal locations of the MR fluid segments based on linear combination of the modal damping factors of the first five modes are identical to those obtained based on the first mode, irrespective of the end conditions. However, the optimal locations of the MR fluid segments, identified based on the logarithmic summation of the modal damping factors of the first five modes, would yield a more uniform shear energy distribution compared to that attained by considering individual modes or a linear summation of the individual modes.

Experimental and finite element analysis of an endoscopic tooth-like tactile sensor

This paper reports on the finite element analysis and experimental study of a prototype PVDF endoscopic tooth-like tactile sensor capable of measuring compliance of a contact object. Present days endoscopic graspers are designed tooth-like in order to grasp slippery tissues. However they are not equipped with tactile sensors to measure the compliance of tissue. The tactile sensor consists of rigid and compliant cylindrical elements. Determination of the compliance of the sensed objects is based on the relative deformation of contact object/tissue on the compliant and rigid element of the sensor. The polyvinylidene fluoride (PVDF) film sandwiched between rigid cylinder and plate and also between the two base plates has been used to measure the force applied on the rigid element and the total force applied on the sensor, respectively. Using the finite element method, the rigid and compliant elements are modeled as solid and elastic foundations, respectively. The data obtained for the force variation are plotted for the various modulus of elasticity of the sensed object. An array of the sensors was also designed in two different configurations depending on the method of measuring the total force. In one configuration, the total force is measured on the sensed object using common base plates and in the other configuration it is measured using different base plates arrangement. It has been shown that good agreement exists between the finite element results and experimental values. The sensor exhibits high force sensitivity and good linearity. Further, an array of these sensors could be miniaturized to integrate with commercial endoscope.

Optimum design of truss structures undergoing large deflections subject to a system stability constraint

A structural optimization algorithm is developed for shallow trusses undergoing large deflections subject to a system stability constraint. The method combines the non-linear buckling analysis, through displacement control technique, with the optimality criteria approach. Four examples illustrate the procedure and allow the results obtained to be compared with those in the literature. It is shown that a design based on the generalized eigenvalue problem (linear buckling) highly underestimates the optimum mass for these types of structures so a design based on the linear buckling analysis can result in catastrophic failure. In one of the design examples the stresses in the elements, in the optimum design, exceed the allowable stresses, pointing out the need for a design that accounts for both non-linear buckling and stress constraints. Copyright

Vibration Control on Smart Civil Structures: A Review

Smart civil structures are capable of partially compensating the undesirable effects due to external perturbations; they sense and react to the environment in a predictable and desirable form through the integration of several elements, such as sensors, actuators, signal processors, and power sources, working with control strategies. This article will focus on reviewing the main control techniques applied to suppress vibrations in civil structures using smart materials, remarking on the advantages and disadvantages of smart actuators and control strategies tendencies in smart civil structures.