Use Of Operational Modal Analysis Research Articles

Deterministic-Stochastic Subspace Identification of the Modal Parameters of a Machine Tool During Milling

Modal analysis is a standard tool for evaluating the dynamic behaviour of machine tools. Since the dynamic behaviour can differ for operating and analysis conditions, the use of operational modal analysis for machine tools has been researched over the last decade. However, the operation of a machine tool is a special case with respect to the excitation, as the excitation is both deterministic and stochastic in nature. Therefore, a perfect broadband excitation, which can be assumed to be white noise, does not occur in machine tools. This fact must be taken into account for the identification and makes the application of classical techniques of operational modal analysis to milling machines insufficient. The approach for identification of the modal parameters of a milling machine during machining, presented in the paper, is based on the deterministic-stochastic subspace identification, which can consider both the deterministic and the stochastic character of the excitation in machine tools better than the stochastic subspace identification being often used in operational modal analysis. For this purpose, the formulation of the input vector for a milling machine is crucial. The approach is developed with the help of simulations and demonstrated on a real machine tool. The results show an improvement compared to the stochastic subspace identification, which is visible in more clear stability diagrams and in a higher number of identified resonance frequencies.

Experimental Study on the Natural Dynamic Characteristics of Steel-Framed Modular Structures

Corner-supported modular structures are made of repetitive prefurnished, prefinished modular units, which are fabricated in a factory and transported to the site of a building to form a permanent building block. The modular units are then tied together through the use of so-called inter-modular connections, or inter-connections, which form a different configuration at joints compared to conventional steel structures. The presence of inter-connections in these structures, in addition to beam-to-column connections or intra-connections, may change their dynamic characteristics, including natural frequencies, mode shapes, and damping ratios. This paper aims to investigate the dynamic characteristics of a modular building through the use of operational modal analysis (OMA). A half-scaled three-storey modular structure, designed and instrumented with highly sensitive accelerometers, was experimentally tested under pure and randomly generated vibrations. The time history of the response acceleration of the structure was recorded using a data acquisition system. Different output-only techniques of OMA, based on both frequency and time domains, were employed to analyse the recorded response acceleration of the structure and extract the natural frequencies, mode shapes, and damping ratios. These techniques are peak picking (PP), enhanced frequency-domain decomposition (EFDD), and stochastic subspace identification (SSI). The outcomes in this paper can be used for further research on the development of an experimental formula for the design of multistorey modular buildings against lateral loads.

The use of operational modal analysis and mode tracking for insight into polar vessel operations

Accurate dynamic models of flexible ship structures are required to predict structural vibration caused by environmental loads. Knowledge of dynamic or modal properties are important, for example, to study fatigue damage. In this paper, operational modal analysis (OMA) is used to identify the vibration characteristics of a polar vessel from full-scale acceleration measurements. Five modes are tracked over a range of different operating and environmental conditions, including calm, open water, two ice cases and an open water storm. Compared to the calm, open water conditions, vertical bending modes were generally affected the most by the operating environment, with natural frequency and damping increasing up to 3.7 and 349 %, respectively, in ice. Severity of two-node and three-node vertical bending responses, quantified using a root-mean-square value, were found to be especially large during the open water storm case. Temporal behaviour of two-node and three-node vertical bending due to wave-slamming further contained multiple whipping responses, pointing towards possible cumulative damage over a large number of stress cycles occurring at the midship. Higher damping in ice resulted in responses with lower magnitudes and shorter durations. • Dynamic characterisation of a polar vessel through operational modal analysis. • Identification of structural damping in open water and ice. • Variations in modal parameters are tracked to understand structural interactions with waves and ice. • High energy modal responses due to whipping are caused by wave-slamming in open water.

Modal parameters identification of a rotor-journal bearing system using operational modal analysis

The paper investigates the use of operational modal analysis (OMA) techniques applied directly into a rotating machinery with hydrodynamic journal bearings. OMA has proven to be a strong tool for analyzing the dynamics of large structures, especially to its capabilities in civil engineering; however, its application in rotordynamics is still very incipient. The objective here is to make a broader analysis of how this technique can be used in the universe of rotating machines and how the results become reliable. The application of this technique in rotating machines implies some care due to the inherent operating conditions of the system, especially regarding the presence of harmonic forces. The study emphasizes resonance problems with the early natural modes of the structure and its damping. The main novelty of the paper is to present a simple procedure, tapping throughout the rotor, which made possible the application of OMA to extract the modal parameters with satisfactory results in a system in which the classical EMA steeped sine technique was insufficient to provide a good analysis of the rotor. In this type of system, the hydrodynamic bearings and misalignments caused by the coupling of shaft and motor (something quite common in real systems) may cause difficulty in extracting modal parameters depending on the technique used. However, when applying traditional OMA methods, using both the frequency domain (enhanced frequency domain decomposition) and the time domain (stochastic subspace identification), it was possible to make a good identification of the modal parameters of the rotor, with promising results to encourage a broader modal identification of rotating machinery in operating conditions.

The use of operational modal analysis in the process of modal parameters identification in a rotating machine supported by roller bearings

This paper investigates the use of conventional operational modal analysis (OMA) techniques for modal parameters identification of a rotating system supported by roller bearings. Although largely applied in civil engineering, in-depth studies on different types of systems are still limited in the literature. The novel of the paper is to address such issue by applying conventional OMA methods, such as enhanced frequency domain decomposition (EFDD) and stochastic subspace identification (SSI-data), to identify the modal parameters of a rotating machine, investigating the challenging particularities of these systems due to their inherent operating conditions, especially regarding the presence of harmonic forces, eventually closed-spaced modes, and non-proportional damping due to the bearings. The results presented in the experimental tests showed that, with the use of specific tools, in comparison with traditional experimental modal analysis (EMA), the used OMA methods have managed to successfully identify the modal parameters of a roller bearing supported rotor.

Vibration-Based Monitoring of Wind Turbines: Influence of Layout and Noise of Sensors

The reduction in operating and maintenance costs of wind farms is a fundamental element to guarantee the competitiveness and growth of the wind market. Wind turbines are highly dynamic structures prone to wear during their lifetime. Therefore, dynamic monitoring systems represent an excellent option to continuously evaluate their structural conditions. These systems allow early detection of damages, permit a proactive response, minimising downtime, and maximising productivity. In this context, the present paper describes the main results obtained with alternative instrumentation strategies tested in a 2.0 MW onshore wind turbine to reduce the costs of the monitoring equipment and at the same time ensure an adequate accuracy in structural condition evaluation. The data processing strategy encompasses the use of operational modal analysis combined with algorithms that deal with the particularities of operation of the wind turbines to continuously track the main vibration modes. After this automated online identification, the influence of the environmental and operating conditions on the tracked natural frequencies is mitigated, making the detection of abnormal variations of the natural frequencies possible, which might flag the appearance of damage. A database of continuously collected acceleration time series during one year is adopted to test the efficiency of alternative monitoring system layouts in detecting simulated damage scenarios. The tested alternative monitoring layouts present a varying number of sensors, alternative distributions in the wind turbine tower, and different sensor noise levels.

Identification of industrial robot frequency response function for robotic milling using operational modal analysis

Impact hammer experiments are typically used for identifying the Frequency Response Function (FRF) of six-degree-of-freedom (6-dof) industrial robots for machining applications. However, the modal properties of 6-dof industrial robots change as a function of robot arm configuration. Hence, describing the robot’s modal parameters within its workspace requires off-line impact hammer experiments performed at discrete robot end effector positions, which are costly and time consuming. Instead, it is more efficient to calculate the robot FRF using Operational Modal Analysis (OMA), a method that utilizes data acquired during the actual machining process. This paper presents an OMA approach to identify the robot FRF from measured milling forces and robot tool tip vibrations. Analysis of the milling process data reveal that periodic forces produced in the milling process are accompanied by background white noise that induce broadband excitation across the robot structure’s frequency spectrum. Hence, the tool tip vibration signal contains the signature of the structure’s free response that enables the use of OMA to estimate the robot’s FRF. The FRF calculated using OMA is shown to be in good agreement with results obtained from impact hammer experiments.

Identification of Noise, Vibration and Harshness Behavior of Wind Turbine Drivetrain under Different Operating Conditions

Noise, vibration and harshness (NVH) problems are critical issues to be tackled for wind turbine drivetrains. Tracking the behavior of modal parameters of the machines’ fundamental modes during operation it is of high interest to validate complex simulation models. A powerful approach for this purpose is represented by operational modal analysis (OMA). This paper describes the investigation of an automated technique for continuously tracking the modes of a rotating mechanical system running in normal operating conditions. The modal estimation procedure is based on an automatic version of the pLSCF (poly-reference Least-Square Complex Frequency-Domain) algorithm. The latter is coupled with a method that automatically tracks the modal parameters along different data sets. The use of OMA on a rotating component of the wind turbine creates the need to deal with harmonics in order to satisfy one of the assumptions of OMA. For this purpose, the use of a cepstrum editing procedure is analyzed and implemented. Modal estimates obtained from an automated analysis on stand still data and normal operating conditions data are compared, to test the added value of the cepstrum editing procedure and the robustness of the method when used on real data. To illustrate and validate the implemented methodology, data acquired during a long-term monitoring campaign of a wind turbine drivetrain are used.

Evaluation of freeze-thaw durability of pervious concrete by use of operational modal analysis

Abstract It is well-known that laboratory testing of pervious concrete's freeze-thaw performance is too harsh and does not agree well with field observations. The most commonly used laboratory freeze-thaw test method for pervious concrete is similar to that used for conventional concrete even though the void structure of the two materials is completely different. In the present study, a new freeze-thaw test method for pervious concrete is suggested and tested on one baseline mix, with three different contents of entrained air. The evaluation of freeze-thaw damage on pervious concrete beams was evaluated from the decrease in mass and from operational modal analysis which provides an accurate determination of the change in natural frequencies with freeze-thaw exposure. Operational modal analysis was also used to determine the Young's modulus, shear modulus, and Poisson's ratio of the pervious concrete mix.

Experimental study of strain prediction on wave induced structures using modal decomposition and quasi static Ritz vectors

Offshore structures are continuously subjected to dynamic loading from wind and waves which makes fatigue an important parameter for the structures expected lifetime. Monitoring the vibrations of the structure using real time operating data enables an assessment of the general health state of the structure. This paper proposes a method for an accurate full-field prediction of the strain history. Experimental mode shapes are found by the use of operational modal analysis and expanded to strain modes using a well correlated finite element model. The measured response from the structure is divided into two parts using complementary filters: Low frequency response caused by the quasi-static effect of the waves acting on the structure, and the high frequency response given by the modal properties of the structure. The high frequency response is then decomposed into modal coordinates using the experimental mode shapes. Strain histories are predicted by multiplying the modal coordinates with the expanded strain mode shapes. The low frequency response is decomposed using Ritz-vectors corresponding to the shapes that the structure vibrates with due to the wave loading. Strain Ritz-vectors are then extracted from the finite element model by applying a load corresponding to a representative wave and the strain history for the low frequency response is found by multiplying the decomposed signal with the strain Ritz-vectors. Finally the combined strain history is found by adding the strain histories from the low and high frequency responses. To validate the theory tests were performed on a scaled model of an offshore structure where the strain history was predicted using only the response from the accelerometers.

Laboratory Tests of Reinforced Concrete Beams with the Use of Operational Modal Analysis

This paper presents the potential for the use of operational modal analysis (OMA) in the testing of reinforced concrete elements. The main difficulties encountered by the author when carrying his own experiments in this regard are described. The investigations carried out as part of this research and reported here covered: the identification and selection of a proper test stand, the effect of the loading advancement degree on the eigenfrequencies and damping of reinforced concrete beams. In addition, general tips on the testing of reinforced concrete elements with the use of OMA, which can be useful for conducting other experiments of this type, are included.

Monitoring of composite structures using a network of integrated PVDF film transducers

Aiming to reduce costs, polyvinylidene difluoride (PVDF) film patches are an emerging alternative to more classic piezoelectric technologies, like ceramic patches, as transducers to measure local deformation in many structural applications. This choice is supported by advantages such as the low weight and mechanical flexibility of PVDF, making this polymer suitable for embedding inside full scale polymer based composite structures. Piezoelectric transducer patches can be used as actuators to dynamically excite full-scale composite structures, and as sensors to measure the strain. The main objective of this paper is to verify that the PVDF transducers can provide exploitable signals in the context of structural health monitoring. In order to do so, two aspects of the design of transducer network are investigated: the optimization of the sensor network, for which the effective independence method is proposed, and the use of operational modal analysis (OMA), since it is a simple method to extract the natural frequencies of a structure from a time series. The results of the analysis are compared to a reference set issued from experimental modal analysis (EMA), a simple, well-known, classic method, which is carried out using accelerometers and an impact hammer. By statistical means, it is shown that there is no significant difference between the two methods, and an optimized PVDF transducer network combined with OMA can perform the dynamic analysis of a structure as well as a classic EMA setup would do. This leads the way to the use of low-cost PVDF embedded transducer networks for robust composite material characterization.

Operational Modal Analysis for Dynamic Characterization of a Ro-Lo Ship

The dynamic characteristics of ship structures are becoming more important as the flexibility of modern ships increases, for example, to predict reliable design life. This requires an accurate dynamic model of the structure, which, because of complex vibration environment and complex boundary conditions, can only be validated by measurements. In the present paper the use of operational modal analysis (OMA) for dynamic characterization of a ship structure based on experimental data, from a full-scale measurement of a 210-m long Ro-Lo ship during sea trial, is presented. The measurements contain three different data sets obtained under different operating conditions of the ship: 10 knots cruising speed, 18 knots cruising speed, and at anchor. Natural frequencies, modal damping ratios, and mode shapes have been successfully estimated for the first 10 global modes. Damping ratios for the current ship were found within the range 0.9%–1.9% and natural frequencies were found to range from 0.8 to 4.1 Hz for the first 10 global modes of the ship at design speed (18 knots). The three different operating conditions showed, in addition, a speed dependency of the natural frequencies and damping ratios. The natural frequencies were found to be lower for the 18-knots condition compared with the two other conditions, most significantly for the vertical bending modes. Also, for the vertical bending modes, the damping ratios increased by 28%–288% when the speed increased from 10 to 18 knots. Other modes were not found to have the same strong speed dependency.

Operational Modal Analysis of Torsional Modes in Rotating Machinery

Traditional experimental modal testing techniques rely on controlled and measured excitation together with measured responses in order to identify the mode shape, natural frequency, and damping factor of each mode. Applying a controlled and measured excitation to a rotor train when in operation is logistically difficult and especially challenging in the field. Operational modal analysis (OMA) identifies the modal parameters of a system from measurement of response due to some (unknown) excitation. OMA has proven successful over the past several decades on nonrotating structures but has relatively rarely been applied to rotating machinery. Case studies are presented demonstrating the use of OMA in identifying torsional modes on an electric motor driven reciprocating compressor, on a diesel engine driven fire water pump, and on a marine propulsion system. In contrast to lateral modes, torsional modes of rotor trains are typically not speed dependent. However, phenomena exist whereby the torsional modes may be different at stand still, off-load and at different loads. The case studies provide examples of such phenomena and also of significant differences between predicted and measured behavior which suggests that improvements in industrial practice would be beneficial. Such improvements should be based on reconciliation of measured and predicted behavior and OMA offers a valuable tool to facilitate this. OMA provides a significant benefit in investigating and understanding torsional behavior in operation.

The Reliability Testing of Brick Infrastructure with Operating Modal Analysis / Badanie Niezawodności Infrastruktury Murowej Z Użyciem Operacyjnej Analizy Modalnej

Abstract Modal analysis is widely used in the removal of defects caused by vibration of infrastructure, structure modification, updating the analytical model, or the control of the state and is used to monitor the vibration of structures in the aerospace and civil engineering mechanics from early 1990 began to pay close attention to the use of operational modal analysis (OMA) in a study of the existing building structures. In this case, the vibration exciter platforms, buildings, towers, bridges, etc. to force Operating (ambient). Here we measure only the response of the force generated by the environment. OMA is also very attractive for aerospace and mechanical engineering. This article presents the results of the existing building structure (reinforced concrete wall using operational modal analysis software and used to carry out the LMS and visualization of the results of such research.