جستجوی مقالات مرتبط با کلیدواژه « modal parameters » در نشریات گروه « مکانیک »

In the present research, performance of harmonic components identification and elimination techniques in practical operational modal data is investigated. Firstly, classical modal testing has been carried out on a cantilever steel beam and modal data are extracted. Four bending modes were available in the frequency band of interest. Afterwards, the same structure underwent operational modal tests with simultaneous random and harmonic excitation forces. In OMA, measurement data obtained from the operational responses are used to estimate the parameters of models that describe the system behavior. Extracted modal parameters from these data compared with baseline parameters. Extraction of modal parameters from operational data was fulfilled through different common OMA techniques. Each set of obtained results was evaluated in comparison with reference results. Accuracy and capability of each method in eliminating spurious modes caused by harmonic input components have been studied. For data analysis, in addition to developed MATLAB codes, commercial software of PULSE has been utilized in parallel as a tool for verification. The results of this survey demonstrate that the accuracy of the Stochastic Subspace Identification method is higher compared to Frequency Domain Decomposition and Enhanced Frequency Domain Decomposition methods. However, when the system has low modal damping, Frequency Domain Decomposition methods provide better estimates.

Choosing appropriate objective function is necessary for detection of damage location and severity. To this end, objective functions based on modal parameters such as mode shapes and natural frequencies are combined with other objective functions capable to detect damage location. Then the location and severity of damage is obtained using optimization techniques. In this paper a cantilever beam is simulated numerically in MATLAB software using finite element method and then single and multi-objective genetic algorithms with different objective functions based on modal parameters are used to detect damage location and severity. To compare different objective functions, the effect of coordinate incompleteness, frequency range of interest and also environmental noise on the obtained results is studied.

One of the common methods of structural damage detections in structures is investigation of changes in modal parameters before and after damage. These methods which are mostly based on optimization techniques use an objective function based on natural frequencies and mode shapes. Success in detection of location and severity of the damage in these methods is severely a function of sensitivity of the objective function to the damage. As the objective function itself is function of natural frequencies or mode shapes or both of them, it is necessary that the sensitivity of modal parameters to damage location and severity is investigated. To this end, a cantilever beam is simulated numerically in this paper. Changing the location and severity of the damage, the effect of them on natural frequencies and mode shapes is studied. The results show that by increasing damage severity, natural frequencies decrease. On the other hand, moving the crack from the fix end to the free end of the cantilever beam, changes in the natural frequencies does not obey a specific pattern and for a crack of constant severity, decrement of natural frequencies from intact beam depends on the location of crack on the beam. Also it is observed that as the cracks approaches to the fix end, decrement of natural frequencies increased. The results show that the sensitivity of mode shapes to crack is low in beam. Therefore it is recommended to use natural frequencies or both natural frequencies and mode shapes together in objective functions.

One of the common methods of structural damage detections and identification of location and severity of the damage in structures is investigation of changes in modal parameters before and after damage. These methods which are mostly based on optimization techniques use an objective function based on natural frequencies and mode shapes. This paper reviews previous researches in damage detection techniques based on modal parameters and common objective functions used in the optimization stage in the damage detection process. To this end, damage detection methods based on natural frequency, mode shape, curvature mode shape and both of mode shape and natural frequency are reviewed. In the following, due to high importance of objective function in the damage detection process, objective functions based on modal assurance criterion, flexibility matrix and natural frequency and mode shapes are compared

Identification of structural dynamic parameters has received much attention in regard to health monitoring and damage detection over recent years. Within these objectives, assessment of the dynamic behaviour and determination of structural vibration responses are related to dynamic properties, which can be defined as mass, damping and stiffness matrices. This paper presents a new approach for estimation of dynamic parameters, including mass and stiffness matrices, by a model updating technique. These matrices are estimated based on the initial dynamic properties of the analytical model and utilizing modal data such as natural frequencies and mode shapes. Modal parameters are identified by numerical simulation of modal testing or based on signal processing and data acquisition in the experimental modal analysis. In the numerical simulation, these data are determined by the solution of generalized eigenvalue problems. Furthermore, in all parts of the formulation, the damping matrix is assumed as proportional. Therefore, the results of the simulation method are calculated as real data. The proposed approach is introduced based on the general objective function that is defined by the difference between analytical and experimental models. Once the objective function is evaluated, the dynamic parameters are estimated via expanding mass and stiffness orthogonality conditions. The dynamic properties are identified at two stages. In the first stage, the complete modal parameters are used. Then, in the second stage, the mass and stiffness matrices are determined by the first mode of vibration. Measured modal data are obtained by experimental modal analysis on a three story, simple, laboratory frame. It can be noted that the algorithm of identification of dynamic parameters is sensitized to accurate and pure modal data, which are originally extracted from experimental testing or simulation techniques. Investigation of the proposed formulation is verified by a numerical solution on a simulated four story shear frame building as an experimental model. Eventually, comparison of the natural frequency between estimated and experimental models can provide reliable results to accurately identify the dynamic parameters as mass and stiffness matrices.

The increasing demand for precision manufacturing of components for computers, electronics, nuclear energy and defence applications has caused the appearance of ultra precision machining, UPM, processes, as high speed rotation with small heat generation is possible for air spindles, due to the low viscosity of the air lubricant. It also gives rise to noise-free and smooth running, and does not add to the sound and vibration levels of the machine like high-speed ball bearings. UPMs are made in order to create very fine and accurate products. The main features of an UPM can be classified as: A machine tool structure with high loop stiffness, high thermal and mechanical stability, low vibrations, and high precision axis of motion. Air spindles and drive systems are important parts of ultra precision machines, because spindle motion error has a significant impact on the surface quality and accuracy of machined components. Spindles in ultra precision machines have high motion accuracy and rotational speed, and its vibrations directly affect the quality of the work surface. In order to achieve nanometer accuracies, the low vibration of air spindles is vital. Pressurized air is injected into the gap of the spindle in order to make it operative. Injected air may create a lack of stiffness for several reasons. Some of the parameters affecting air spindle vibrations are: Rotational speed, input hole diameter, parameters of air pads, air gap pressure, and etc. In this study, the air pad depth and rotational speed are experimentally investigated. The air pad depth is considered variable. 6 levels are selected for the air pads bottom mode: flat, conical, pyramid, spherical with 2 various radii, and constant depth. Also, for rotational speed, 3 levels are selected. Totally, 18 experiments have been undertaken. For accomplishing these experiments, air spindles are made using various production processes. The VibroTest 60 is used for studying air spindle vibrations. Then, experimental results are analyzed using the DOE method. The results show that the case of air spindles with air pads in a pyramid bottom, at low speed, has minimum vibrations.

Conventional modal testing of large structures is always associated with difficulties in artificial excitation of the structure. Operational Modal Analysis (OMA) is one approach to overcome the excitation problem. In OMA only the responses are measured and the structure is excited by ambient forces. For large structures the simultaneous measurement of responses in all selected points is usually impossible due to not having enough available accelerometers or measurement channels. Therefore، the structure is tested in several steps. As a result، some reference points should be selected to correlate all of the measurements. In this article a new criterion is introduced for selection of reference points based on finite element model of structure. A beam is used for numerical validation of the method. A steel beam is also used for experimental case study. Both numerical and experimental results demonstrate the effectiveness of this criterion.

One of the drawbacks of the operational modal analysis techniques is that there is no possibility of measurement of the responses in all required points، which is attributed to the limitations of either the number of accelerometers or the number of measurement channels. To overcome this shortcoming، an experiment should be performed in different steps and relations between these steps of the experiment are determined by selection of some specific points as reference points. Existing techniques use the correlation between measurement points to determine the reference points in which an increase in the environmental noise leads to the incapability of the method. In this paper، a new index for selection of reference points is introduced which is more efficient in the noisy environments. To evaluate the proposed method numerically، a comparison has been drawn between the results of this method and correlation approaches using a FE model of a beam. To validate the method، an experiment has been conducted on a steel plate. Obtained results from numerical and experimental cases show that the proposed index is more capable in reference point selection and calculation of the modal parameters of the structures comparing to the results of correlation method.

Dynamic analysis of the civil structures such as bridges, towers and buildings is required for their design and maintenance. Modal analysis is a powerful tool to conduct some part of dynamic analysis in determination of the modal parameter in terms of natural frequencies, damping factors and mode shapes. However, excitation of these structures is usually difficult and sometimes impossible. As these structures are usually excited by ambient forces such as wind, this idea is suggested that the structure is modeled considering the natural forces as the inputs.However, the ambient forces are unknown and have a complicated nature to be measured. An alternative approach is using the operational modal analysis concepts in which only the responses are measured and the modal parameter are extracted. In this article Frequency Domain Decomposition (FDD) method is used for identification of the modal parameter of a clamped-clamped beam and the results are compared with those of the FEM. The operational modal analysis is conducted on a type of a bridge under ambient forces in a real test and the results are compared with those of the conventional Modal testing. The results confirm the method for engineering applications.