Experimental Modal Analysis Procedure Research Articles

Machine tool multibody dynamic model updating using vision-based modal analysis

Machine tool dynamic behaviour is influenced by the structural properties of its subsystems assembled at interfaces as well as by the interface characteristics. Interfaces are commonly modelled as spring-damper connections, parameters of which are usually updated by minimizing the difference between model-predicted and measured dynamics characterized by frequency response functions. This model updating approach requires global mode shapes to be measured by roving the hammer and/or the sensor such as to localize the joint parameters to be updated. Such measurements are time consuming and fraught with errors. As a new, alternative, and simpler way to update joint parameters of a machine tool multibody dynamic model, this paper reports on the use of full-field vision-based modal analysis methods. Mode shapes thus identified agree with those estimated with the traditional experimental modal analysis procedures. The updated machine tool multibody dynamic model is a step towards realizing an accurate digital twin.

Methods to estimate subpixel level small motion from video of vibrating cutting tools

Cutting tools can vibrate with frequencies of up to a few kHz with amplitudes ranging from a few µm to hundreds of µm. To characterize these vibrations using vision-based modal analysis methods, video of vibrating tools must be acquired at high speed and resolution. High speed acquisition will ensure modes are temporally resolved without aliasing, whereas high resolution acquisition ensures that the pixel size is small enough to resolve small motion. However, since most digital cameras trade speed for resolution, estimating small motion of high frequency modes becomes difficult. To address this issue, this paper presents new methods to estimate small motion from video of vibrating cutting tools. Two methods are illustrated on a video of a slender end mill. One is hardware-based, and the other is software-based. In the hardware-based method, we use combinations of extension tubes and a reverse ring fitted with a standard lens to achieve a pixel size of as less as 1.3 µm. In the software-based method, we leverage the capability of intensity- and phase-based motion registration methods to detect small subpixel level motion, even when the pixel size is 83 µm. In both methods we use the object’s own features to detect and register motion, thus overcoming the limitation of digital image correlation schemes that need markers for target tracking. Modal parameters evaluated from motion estimated with both methods are found to agree with those extracted from twice integrated accelerations. Since methods proposed herein proffer new ways to register small oscillatory motion, we believe that our findings will further help leverage the adoption of vision-based modal analysis methods in machine tool systems as an alternative to the more traditional experimental modal analysis procedures.

Evaluating tool point dynamics using smartphone-based visual vibrometry

Modern smartphones can record high megapixel videos at high frame rates. This paper leverages these capabilities and presents a new method to evaluate dynamics of a representative grooving tool using a smartphone’s video camera. Pixels within images are treated as vibration sensors measuring response to impact-based excitation. Motion is detected and tracked using image processing techniques to give pixel-displacement time series data. Spectra of these data give natural frequencies, and damping is estimated using the logarithmic decrement method. These parameters compare well with those extracted using experimental modal analysis procedures. Unscaled tool vibration shapes are visualized using motion magnification techniques.

Vision-based modal analysis of cutting tools

This paper presents the use of vision-based methods for cutting tool motion registration and modal analysis. Motion of three illustrative tools were recorded using low- and high-speed cameras with sufficiently high resolutions. The tool’s own features are used to register motion. Pixels within images from recordings of the vibrating tools are treated as non-contact motion sensors. Comparative analysis of three different methods of motion registration are presented to evaluate their suitability for the application of interest. These include variants of expanded edge detection and tracking schemes, expanded optical flow-based schemes, and established digital image correlation methods. Performance of different methods was observed to be governed by the tool’s own features, illumination conditions, noise, and the image acquisition parameters. Extracted motion was benchmarked against twice integrated measured tool point accelerations, and motion was generally observed to compare well. Modal parameters extracted from vision-based measurements were also observed to agree with those extracted using more traditional experimental modal analysis procedures using a contact type accelerometer as the transducer. Since methods presented are generalized, they can suitably be adapted for other applications of interest.

Evaluating tool point dynamics using output-only modal analysis with mass-change methods

This paper discusses the evaluation of tool point dynamics using output-only modal analysis methods. As compared to the traditional experimental modal analysis procedures that rely on the input and the output, both being known, output-only methods discussed herein can evaluate modal parameters using only the measured response of the tool subjected to unknown impact-based excitations. Since the input is not known, the estimated shapes are unscaled. To correctly scale shapes, we use the mass-change method. We present analytical models to guide the optimal selection of the mass to be added as well as to guide its placement on the tool. Analytical models systematically characterize errors in scaling the eigenvector and errors due to uncertainties in the estimation of the natural frequencies. The model suggests placement of the mass to be added at the anti-node(s) for the mode(s) of interest. These models guide experiments on two tools – a slender boring bar, and an end mill. Tool point frequency response functions (FRFs) reconstructed with the natural frequencies, damping ratios, and scaled mode shapes evaluated with the output-only mass-change method are compared with and found to be in good agreement with FRFs obtained from the traditional experimental modal analysis procedures. Since output-only methods require one less transducer, and since these methods are robust to uncertainties and inconsistencies in the input, output-only methods promise advantages from the industrial point of view.

Modal analysis of machine tools using visual vibrometry and output-only methods

This paper presents visual vibrometry as a new video camera-based vibration measuring technique for machine tools. Pixels within images from recordings of the vibrating machine are treated as motion sensors to detect and track vibrating edges. Modal parameters are extracted from the measured pixel-displacement time series data using output-only mass-change methods. Methods are experimentally demonstrated on a small milling machine, on a slender boring bar, and on a regular end mill. Modal parameters extracted using visual vibrometry agree with those extracted using experimental modal analysis procedures. Moreover, visual vibrometry aids shape visualization and maintains advantages of other non-contact measurement techniques.

The effects of S-glass fiber hybridization on vibration-damping behavior of intraply woven carbon/aramid hybrid composites for different lay-up configurations

In this study, the vibration and damping characteristics of the hybrid composites reinforced with plain woven S-glass and intraply woven carbon/Kevlar fiber fabrics are investigated. Vibration tests on the samples have been conducted only for measuring the first mode of natural frequency with application of experimental modal analysis procedures. Symmetric and asymmetric layering structures have been prepared for the hybrid composite configurations in order to study the stacking sequence effects. Damping ratios have been evaluated from the logarithmic decrement method by using acceleration–time envelope curves. Results have shown that position and percentage of the fibers in the composites are the most effective parameters for natural frequency and damping characteristics of hybrid samples, and some samples have shown the synergistic effects leading to a significant enhancement in damping and natural frequency.

Integrated Reverse Engineering Strategy for Large-Scale Mechanical Systems: Application to a Steam Turbine Rotor

An integrated reverse engineering methodology is proposed for a large-scale fully operational steam turbine rotor, considering issues related to the development of the CAD and FE model and the application of robust and effective computational finite element model updating techniques based on experimental modal analysis procedures. First, using an integrated reverse engineering strategy, the digital shape of the three sections of a steam turbine rotor was developed and the final parametric CAD model was created. The finite element model of the turbine was developed using tetrahedral solid elements. Due to complex geometry of the structure, the developed model consists of about fifty-five million DOFs. The identification of modal characteristics of the frame is based on acceleration time histories, which are obtained through an experimental investigation of its dynamic response in a support-free state by imposing impulsive loading. Experimentally identified modal modes and modal frequencies compared to the FE model predicted ones constitute the actual measure of fit. CMA-ES optimization algorithm is then implemented in order to finely tune material parameters, such as modulus of elasticity and density, in order to best match experi-mental and numerical data. Direct comparison of the numerical and experimental data verified the reliability and accuracy of the methodology applied. The identified finite element model is repre-sentative of the initial structural condition of the turbine and is used to develop a simplified finite element model, which then used for the turbine rotordynamic analysis. Accumulated knowledge of the dynamic behavior of the specific steam turbine system, could be implemented in order to eval-uate stability or instability states, fatigue growth in the turbine blades, changes in the damping of the bearing system and perform necessary scheduled optimal and cost-effective maintenance strategies. Additionally, upon a series of scheduled experimental data collection, a permanent output-only vibration SHM system could be installed and even a proper dynamic balancing could be investigated and designed.

Reducing force transmission through exhaust mounts of a vehicle

Under the excitation of an engine, an exhaust system vibrates, and the vibrational energy is transmitted to the vehicle body through the exhaust mounts. The transmitted energy makes contribution to the vibration of the body, which in turn increases the overall vehicle noise level. Noise contributed by the exhaust can be eliminated up to a certain level by reducing the force transmitted through the exhaust mounts. In this study, an objective function, which represents the force transmitted through the exhaust mounts, is defined. The aim is to minimise the transmitted force to achieve better vibration isolation. First, structural dynamic analysis of the vehicle exhaust system is evaluated using computational and experimental procedures. The finite element model of the exhaust system is complemented and validated by following an experimental modal analysis procedure. In the correlation procedure, it is observed that boundary conditions of the exhaust system have significant effects on the results. Hence, the definitions and the assumptions of boundary conditions are studied in detail to achieve a better correlation between the computational and the experimental models. Then, the validated computational model is used for optimisation studies. Moreover, a mass is attached to the exhaust system to reduce the forces transmitted to the vehicle body. For the optimisation problem, the stiffness and the location of exhaust mounts and the weight and the location of hanging mass are defined as design variables. Significant improvements are achieved compared to the initial design.

Attenuation of sound radiation in concrete structure through the reduction of mechanical vibration

Abstract The efficiency of sound irradiance in structure has direct relation with its vibratory movement. Dynamic vibration absorbers (DVAs) are a low cost viable option for reducing vibrations in passive structures. Secondary systems attached to the primary system (structure) in order to reduce vibration. In this work it was used an experimental modal analysis procedure (EMA) for vibratory responses through impulsive excitations to determine the natural frequencies and the location of points suitable for attachment of DVAs in a concrete beam. Later it was designed and built DVAs to reduce vibration in a frequency band where the response of the human auditory system is more sensitive. The best project configuration for DVAs was evaluated for sensitivity thereof with respect to the change of the loss factor of the viscoelastic material used. Obtained reduction of more than 36% over the considered frequency band and over 70% in the region of the resonance frequency in which the DVAs were tuned.

Development of a new driving impact system to be used in experimental modal analysis (EMA) under operational condition

An automatic impact device system, which consists of a combination of computer controlled linear solenoid and force sensor was developed to replace an Integrated Circuit Piezoelectric (ICP) impact hammer in conventional Experimental Modal Analysis (EMA) procedure. A dedicated electrical circuit was constructed to transform an artificial signal generated by virtual instrumentation software into an analog output for driving and controlling the solenoid motion. Under the present configuration, the solenoid was positioned vertically above the force sensor that was mounted on a tested structure. The gap between the solenoid end-effector and the force-sensing pad was adjusted to obtain the optimum characteristics of the generated input force signal. This includes attaining the maximum amplitude from the impact along with the establishment of single impulse profile onto the tested structure under different operating conditions. For the validation purpose, the acquired input signal along with the natural frequency of the structure was systematically compared with the results generated under the conventional EMA procedure. The present results have demonstrated that the new system is capable to match the performance of the currently used impact hammer system in terms of providing reliable input force and modal parameter at static condition of the structure. Further, due to its capability to provide periodical impact onto the structure, the introduced system can also be extended to conduct EMA under operational condition which paves a new paradigm for vibration analysis of structures with running harmonics.

Numerical and Experimental Dynamic Analyses of a Postbuckled Box Beam

This paper proposes a numerical approach to estimate the dynamic behavior of a typical aeronautical aluminum box-beam structure liable to buckling. The methodology is based on a nonlinear finite element model and an experimental modal analysis procedure. The finite element model deals with the coupled nonlinear static and dynamic problems in two steps: 1) determining the static equilibrium considering geometrical nonlinearities, and 2) solving for a linear small-amplitude free-vibration problem based on the tangential stiffness matrix from the current static equilibrium. To illustrate the proposed method, a finite element model is built for a simple supported box beam under a uniaxial compression load with different degrees of eccentricity. The numerical results are correlated with an experimental modal analysis procedure in pre- and postbuckling regimes. Based on this comparison, an updated model is proposed for adjusting the shape and magnitude of the initial imperfections based on linear buckling modes. The results are presented in order to illustrate the effect of the buckling phenomenon and initial imperfections on the dynamic behavior of the box-beam structure.

Modal analysis of hydraulic pipelines

The laminar flow of a weakly compressible Newtonian fluid in a pipeline is treated by modal methods, aiming at a theoretical basis for the experimental modal analysis of hydraulic pipelines. For two points located at arbitrary positions along a pipeline, the frequency response function between flow rate excitation and pressure response is calculated in closed form, expanded into a modal series including transcendental modal transfer functions, and approximated by finite sums of rational fraction expressions. The preferred modal approximation is recognized as mobility function of a structurally damped mechanical multi-degrees-of-freedom system. Experimental modal analysis procedures for structurally and viscously damped mechanical systems are adapted for hydraulic pipelines and pipeline systems.

Methodology to design a vibration absorption footplate for motorcycle application: From phenomena investigation to prototype performance evaluation

The aim of this research is to reduce driver vibration exposure by acting on the modal response of key contact structures.The footplate is one of the components with which the driver comes into contact while riding. For this reason, footplate geometry and structural properties were investigated with a view to re-designing this component in order to reduce driver exposure.Due to the massive chassis area on which the footplate is constrained, vibrations induced by engine unbalances easily propagate in quasi-steady conditions on all surrounding components, inducing a width frequency band of load excitation. Even though the footplate geometry allows for fairly high natural frequencies, these could occur in the excitation range. Therefore, the first step of the methodology proposed entails the use of a numerical and experimental (modal analysis) procedure to identify the local vibration modes of the original components to detect if/when/how the resonances of the above mentioned components are excited.Due to an awareness of the weakness of the original solution, structural modifications, using numerical models, were studied. The footplate geometry was modified to minimize nodal displacement of the footrest beam binding. All structural modifications were designed, developed and installed on the vehicle.Finally, in order to predict modification efficiency, both the new footplate and the original one were experimentally compared. The comparison was made by means of modal investigations and by positioning the reference vehicle on a suitable roller test bench in order to simulate real working conditions.

Modal Parameter Identification using Ambient Vibration Testing with Introducing a new Software

The ambient vibration measurement is a kind of output data-only dynamic testing where the traffics and winds are used as agents responsible for natural or environmental excitation. Therefore an experimental modal analysis procedure for ambient vibration testing needs to base itself on output-only data. The modal analysis involving output-only measurements present a challenge that requires the use of special modal identification technique, which can deal with very small magnitude of ambient vibration contaminated by noise. Two complementary modal analysis methods are implemented. They are rather simple peak picking (PP) method in frequency domain and more advanced stochastic subspace identification (SSI) method in time domain. According to these methods Graphical User Interface (GUI) software developed in MATLAB. This program is called SIP and can be used in modal parameter identification of large civil structures such as bridges and towers. This paper presents the application of ambient vibration testing and experimental modal analysis on large civil engineering structures and also gives brief review of program. Also modal parameters of the Babolsar arch steel bridge are extracted using SIP and ambient vibration data from dynamic test carried out on this bridge.

Output-only modal parameter identification of civil engineering structures

The ambient vibration measurement is a kind of output data-only dynamic testing where the traffics and winds are used as agents responsible for natural or environmental excitation. Therefore an experimental modal analysis procedure for ambient vibration testing will need to base itself on output-only data. The modal analysis involving output-only measurements presents a challenge that requires the use of special modal identification technique, which can deal with very small magnitude of ambient vibration contaminated by noise. Two complementary modal analysis methods are implemented. They are rather simple peak picking (PP) method in frequency domain and more advanced stochastic subspace identification (SSI) method in time domain. This paper presents the application of ambient vibration testing and experimental modal analysis on large civil engineering structures. A 15 storey reinforced concrete shear core building and a concrete filled steel tubular arch bridge have been chosen as two case studies. The results have shown that both techniques can identify the frequencies effectively. The stochastic subspace identification technique can detect frequencies that may possibly be missed by the peak picking method and gives a more reasonable mode shapes in most cases.

Experimental and numerical investigation on structural effects of laser pulses for modal parameter measurement

In this paper it has been described part of the research devoted to the development of a complete non-intrusive experimental modal analysis procedure based on laser techniques both for excitation and for measurement. In particular, attention has been focused on the thermal effects generated by laser pulses on the excited structure. An analytical model of the energy exchange between the light pulse and the target surface is proposed together with a finite element model of thermal and mechanical behaviour of the structure under excitation. Both the models (analytical and numerical) have been experimentally validated by measuring the thermal and the vibration responses induced by the laser pulses. The experimental part of the study has been performed on a cantilever beam excited with laser pulses from an Nd:YAG source (532nm, 100mJ/pulse) using an high-speed infrared camera and a scanning laser Doppler vibrometer. Results from this work can be used to improve understanding concerning the features of laser excitation and to establish a mechanical equivalent system of forces and moments, useful in order to increase the accuracy in the measurements of modal parameters when laser pulses are used as excitation sources.

Experimental assessment of low‐aspect‐ratio, reinforced concrete shear wall stiffness

AbstractLow‐aspect‐ratio, reinforced concrete shear walls are the primary lateral‐load‐carrying element in many structures designed for protective purposes. A review of the technical literature shows that considerable uncertainty exists regarding the elastic stiffness these structures will exhibit during seismic excitation. Because of this uncertainty, current design practice often employs a stiffness reduction factor. In an attempt to develop accurate information regarding the stiffness of these structures, 13 shear wall elements were tested statically; dynamically, with simulated seismic base excitations on a shake table; and with experimental modal analysis procedures. Results of these tests show that the shear wall's stiffness can be accurately estimated with a mechanics‐of‐materials analysis that accounts for shear deformation.