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In other words, the result of modal analysis are these frequencies at which the amplitude increases to infinity. The Modal Assurance Criterion Analysis (MAC) analysis is used to determine the similarity of two-mode shapes. For example, if an object is excited at a location where some vibration mode patterns always have minimum vibration amplitude, then these modes will not absorb energy and be excited. In this case, the measured reference DOF data will not include information needed to separate the modes. Solved c). Give two examples to show why modal analysis is - Chegg For instance, instead of fixing the DUT to the shaker armature, a modal shaker is attached to the DUT via a connection rod called a "stinger.". To understand this process more physically, imagine that structure is in a constant state of motion sub-atomically. This decision isa balance between the desired quality of the results and the required test time. For example, regarding the maximum excitation force level, the frequency range, and not least, the way that the shaker is mounted to the DUT. In other words, the resonant frequency of the 6 to 15-story structures coincided almost exactly with the frequency of the earthquake. 9.3. If the number of response sensors is limited then "roving" measurement can be used. For example, it can be of great interest to know how the launchpad structure, which holds the spacecraft until it launches, vibrateswhen excited by forces similar to those experienced under a lift-off. Many different types of modal curve fitting algorithms have been developed for this purpose. Modal analysisis widely accepted in a broad range of applications in the automotive, civil engineering, aerospace, power generation, and musical instruments industries, among countless others. It is a measure of the similarity between estimated mode shape vectors from the same parameter estimation, same data set. Global curve fitting is based on multiple DOF data. For many years the majority of practical applications for modal analysis have been in the fields of aerospace and defence, automotive, mechanical, and civil engineering, but with the rapid development of new technology, it has become more and more normal to include modal analysis across many fields and for interdisciplinary applications. With no averaging\(\Upsilon^2(f)\)will be 1. If some of these resonance frequencies will get excited when a car owner is driving, then it might be critical to change the dynamical properties of the frame, such that the structural resonances are sufficiently damped or shifted to a frequency range that isn't being excited. Engineers tend to learn from such examples (at least in the short term) and more modern suspension bridges take account of the potential influence of wind through the shape of the deck, which might be designed in aerodynamic terms to pull the deck down against the support of the structure rather than allow it to lift. Global Frequency & Damping Estimates From Frequency Response Measurements, by Mark H. Richardson Structural Measurement Systems, Inc. San Jose, California, 9.2. This is done by curve fitting to the modal measurements and extracting the parameters from the determined analytical representation of the data. . For EMA testing, depending on the test situation it might be better to use one or multiple modal exciters and one or multiple response sensors. Such estimators can be used when analyzing local modes of the DUT. Modal Testing can be performed with either applied artificial excitation sources, to get the test object to vibrate, or by having the test object running under operational conditions, in-situ testing, where in-situ vibrations will be present. Modal Analysis and Testing for Marine Structural Dynamics - LinkedIn However, low values might be accepted at anti-resonance frequencies and low-frequency stiff body vibrations. In such cases, the AutoMAC plot will more or less only indicate a value around 1 at the diagonal because it only correlates with itself. Rotating components like gears and bearingsof the DUT often rotate at different orders or fractions of the measured speeds(s). When the FRF magnitude is at a local maximum/peak, and the phase turns90 degrees at this point, it usually indicates a resonance. Inertial shakers are used for structures requiring excitation in lower frequency bands. Your hub for everything you need to know about simulation and the world of CAE. Modal Testing and Modal AnalysisStructural DynamicsStructural Dynamics Analysis and TestingNVH TestingNVH Testing (Noise, Vibration, and Harshness)AutomotiveAutomotive Test and Measurement SolutionsMonitoringMonitoring Solutions. What do all the different techniques mean? The same applies to bridges where flutter effects can occur at certain wind speeds, which can force the structure into oscillation. By knowing . 8.1. 2.0 Theory - University of Texas at Austin CrossMAC determines the consistency, or linearity, between estimated mode shape vectors from different estimations, different data sets. 7.1. For this reason, the Burj Khalifa does not use a damper system. To get an overview of which modal hammer is best suited for a certain job, the table below can be used as a guideline. Using multiple shakers on complex structures gives a more realistic force excitation and better investigation of all mode shapes. After the extraction, the signal steps to the next frequency, and the structure settlesagain. The basic idea of a modal analysis in electrodynamics is the same as in mechanics. FRF\(H_{2}\)is the best estimator at resonance peaks of the\(H\)function: Note that for\(G_{BA}(F)\)it is the output signal b that is conjugated. Permanent Magnet shakers are general-purpose types that allow the DUT to befixeddirectly to the shaker armature, and the vibrating surface area can be enlarged by using a head expander to accommodate larger objects. ODS is used successfully for machine conditioning monitoring and in civil engineering applications e.g. The latter is used in Correlation Analysis. Atresonance frequencieswith critically low damping, an object can react/vibrate strongly from even small amounts of input force or energy. Classically this was done with a SIMO (single-input, multiple-output) approach, that is, one excitation point, and then the response is measured at many other points. When performing modal testing the DUT (Device Under Test) must be able to vibrate dynamically in ways that will reveal all and correct natural frequencies and mode shapes of the structure. So, in this example, each element in the \(H_1\) matrix has the unit \(m/s^2/N\), which you can think of as a response per excitation. Ordinary coherence between two signals\(a(t)\)and\(b(t)\), defined by: with 1 indicating a perfectly linear relationship between the two signals, and 0 indicating no correlation. in a multi-shaker MIMO test should be low (close to 0) since it is desired to have completely uncorrelated reference signals. Below 0.7 is considered to indicate a bad correlation. In the previous chapter, you read about the various levels of analysis that a manager carries out in order to understand their firm's competitive environment. Modal Test and Analysis Applications. The residues are related to the mode shapes by: For MDOF Modal Parameter Models the residues are related to the FRFs in the mobility matrix by: FRF Synthesis is used as a validation tool by comparing the FRFs from the estimated modal model (the synthesized FRFs) with the real measured FRF data. For a single reference modal test (where it is sufficient to extract the modal model using only one reference DOF) a modal shaker can also be used. In the aerospace industry, there is hard competition in fuel economy, and a lot of effort is invested in the development of new lightweight materials and components with extreme strength. In many software applications, dynamic resampling is used for Swept Sine and Stepped Sine, where the FFT time block has a dynamic time length depending on the sinusoidal frequency. Testing in the lab has the advantage of a higher signal-to-noise ratio (SNR) and the ability to easily change the test setup. This is also referred to as a Roving Response Test. Modern day experimental modal analysis systems are composed of 1) sensors such as transducers (typically accelerometers, load cells), or non contact via a Laser vibrometer, or stereophotogrammetric cameras 2) data acquisition system and an analog-to-digital converter front end (to digitize analog instrumentation signals) and 3) host PC (personal computer) to view the data and analyze it. Spectral ODS testing uses the processed frequency data of the DOFs to extract the amplitude and the phase related to the reference DOF(s). Noise in vehicles is another important aspect of the automotive industry. The modalanalysis allows machines and structures to be tested, optimized, and validated. See our complete guide on choosing the right DAQ system for your measurement application. If a different support configuration is introduced then the best practice is to make an evaluation of how this affects the flexible modes. A car frame will have resonance frequencies that might be lightly damped. By . 00:00 00:11 Have you ever wondered why strong bass music is so exciting? These sub-tests add up to a full measurement of all DOF locations. Polyreference curve fitters are e.g. This technicality arises because we apply no load in a modal analysis, only restraints. With sine testing, all applied energy can be focused on exciting one specific mode at a time. When the optimal hammerhas been selected, additionalproperties should be considered: Frequency response- frequency range, selecting optimal impact tip to use, Hammer dimension and weight- practical considerations, Measurement range- providing sufficient impact force level. A harder tip material provides a larger frequency range, but it can also make it more difficult to avoid double hits. Modal analysis is used to determine the dynamic characteristics of a system like natural frequencies, damping ratios and mode shapes. Intro to Modal Analysis | Ansys Innovation Courses by Pete Avitabile. MDOF LSCE, LSCF, and RFP are some of the commonly used curve fitter methods. What's the difference between local and global curve fitting? Pseudo-random signals are ergodic stationary signalschosen to have a repeating time block length equal to the used FFT time block length (\(T\)) and to only consist of energy content at integer multiples of the FFT frequency lines (\(\Delta f\)). The modal analysis allows more realistic lift-off simulations to be tested. Other aerodynamic loading issues are dealt with by minimizing the area of the structure projected to the oncoming wind and to reduce wind generated oscillations of, for example, the hangers in suspension bridges. Each has its own advantages and disadvantages. The lateral flexibility also protects both the DUT and the modal shaker from critical forces. An example of this iscomplex structures composed of several different parts with different structural properties. What is modal analysis? Tech., B.A., 3rd edition from 1987, page 226 -252. A real-life example can be seen in todays skyscrapers like Taipei 101 in Taipei City, Taiwan, or Burj Khalifa in Dubai (UAE). Two explanations were offered for the earthquakes impact: the duration of the shaking and the resulting resonance with the lakebed frequency. 2.1.1 Introduction to Modal Analysis. When performing global curve fitting all measurements used must contain global modes. The Coherence value between two reference DOFs e.g. How To Choose the Right Data Acquisition System For Your Measurement Application? Normal Mode Indicator Functions (NMIF), also referred to as Ordinary or Original Mode Indicator Functions are better at identifying closely spaced modes than PMIF. The CMIF functions have peaks at resonances - indicating poles of the DUT. Sine sweeps, random noise, and other excitation types from a modal shaker/exciter are often the best solution for larger complex structures, where more in-depth analysis is required, including non-linearity studies, lowCrest Factor, and high Signal/Noise ratio. A bridge can start to vibrate for many reasons: wind loading and vehicle or even foot traffic. Modal shakers are available with a range of specifications, as shown in the table below: The stinger is a thin flexible rod that improves the accuracy of the modal test by mainly transmitting force in the axial direction to the force sensor or impedance head. However what can be done is a check to see if enough has been requested A single shaker may not effectively excite all the modes of a structure.[1]. Modal Analysis | What Is Modal Analysis? | PTC Having acquired the Modal Test data, the next step is to identify the modal parameters by using Parameter Estimation techniques, including Mode Indication Functions (MIFs) and Curve Fitters. If a Finite Element Model (FEM) is available this can also be used to determine good excitation locations. But, again this will always depend on the agreed accuracy of the specific application since some flexible modes will be affected to some extent. In some test cases, it is sufficient to extract the modal model from measurement data with only one reference DOF. What is the difference between all the mode indicator functions? Check outDewesoft's modern,high-quality digital data acquisition systems. Although modal analysis is usually carried out by computers, it is possible to hand-calculate the period of vibration of any high-rise building through idealization as a fixed-ended cantilever with lumped masses. Polyreference curve fitting is capable of separating closely coupled modes with repeated roots. NMIF has values in the interval [0, 1,] with mode shapes indicated by peaks on the curve. An important advantage of analytical tools is that they can reduce or even eliminate the need for expertise with complex and highly specialized analysis methods. What Is Modal Analysis and Why Is It Necessary? (With PDF) Looking atCoherencefunctions is a primary validation tool when performing Modal Testing, but dont be misled by a coherence value of 1 after a single measurement has been taken. This is done for all order components in the rotation-related spectral data, and the deflections can be animated over an order axis. The maximum vibration occurs at the natural. An example of repeated roots is when having certain symmetrical structures. Different input excitation types can be selected for EMA testing. The one-sided spectrum form of the cross-spectrum is often termed \(G_{AB}(f)\) and is defined by: Having two input signals, \(a_1(t)\) and \(a_2(t)\), and two output signals, \(b_1(t)\) and \(b_2(t)\), then FRF H1 can be derived by: is the transposed matrix of cofactors, where the cofactor of \(G_{A_iA_j}\) in \(G_{AA}\) is: where \(G_{A_iA_j}\) is the minor matrix of \(G_{AA}\). 9.4. This section will cover enough information to get a basic understanding of modal testing and how to acquire data for the subsequent modal analysis. MDOF fit should be used on structures with closely spaced modes and with heavily damped modes which cause overlaps between modes. After the modal parameter estimation process is completed, the next step is to validate the modal model. For complex structures, multiple shakers might be required if no excitation locations (referenceDOFs) can be found where all the modes have sufficiently high participation for proper modal model extraction.

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