Impact-synchronous Modal Analysis Research Articles

Noise robustness of an operational modal-based structural damage-detection scheme using impact-synchronous modal analysis

Noise robustness of an operational modal-based structural damage-detection scheme using impact-synchronous modal analysis

Implementation of BCI based semi-automated impact device for performing Impact Synchronous Modal Analysis

Current Impact Synchronous Modal Analysis (ISMA), an operational modal analysis technique incorporated with either manual impact hammer or Automated Phase Controlled Impact Device (APCID) faces challenges on its effectiveness and practicality in real industrial applications. This study utilizes a Brain Computer Interface (BCI) based portable semi-automated impact device with the aim to solve the current issues of ISMA and complement both the manual impact hammer and APCID. A low cost, portable and wireless Electroencephalogram (EEG) device is used with machine learning to predict impact time before the impact using manual impacts and integrated with APCID control to compensate for the behaviour randomness. This semi-automated impact device, also known as EEG-ISMA, provides an accurate, practical, time- and cost-effective solution in performing ISMA during operation with low mean error in impact time prediction, superior cyclic load component suppression capability, accurate modal parameters extraction and shorter time required.

Implementation of Bci Based Semi-Automated Impact Device for Performing Impact Synchronous Modal Analysis

Implementation of Bci Based Semi-Automated Impact Device for Performing Impact Synchronous Modal Analysis

Inertial sensor based human behavior recognition in modal testing using machine learning approach

Adaptive phase control impact device (APCID) was developed for performing in-service modal analysis using impact synchronous modal analysis. However, this device is large and heavy, making it unsuitable for real world applications. This automated impact device can be replaced with human hand but the randomness in human behavior can reduce the accuracy of APCID control scheme. To replace APCID with a smart semi-automated device while still using APCID control scheme, machine learning models are presented in this paper to recognize human behavior by classifying 13 different impact types and predicting impact time using the impact classification. The impact classification model gave classification accuracy of over 96% with 130 real time impacts. With successful classification of different impact types, randomness in human behavior can be reduced by two to three times by associating a range of impact time with each impact type. However, the impact time ranges may differ person to person. To address this issue and to further reduce variations in impact time, a time prediction machine learning model was developed to make compensations in the control scheme of APCID by predicting impact time. The model gave reasonable accuracy with mean prediction errors of 5.2% in real time testing compared to measured time for 100 impacts.

Operational Damage Identification Scheme Utilizing De-Noised Frequency Response Functions and Artificial Neural Network

A damage identification scheme combining impact-synchronous modal analysis (ISMA) and artificial neural network is developed in this study. The ISMA de-noising method makes it feasible to detect and classify the damage states with high accuracy when the machine is under operation. The feed-forward backprop network was utilized in this study. The input feature vector of the network consisted of the FRF changes in a selected vibrational mode frequency interval at several measurement points. The scheme was tested on a rectangular Perspex plate. It is proved that the trained network can successfully identify damage locations with the testing data collected by ISMA, which allows the damage detection to be carried out without shutting down the tested machine. For the plate structure in this study, an overall accuracy reached 100% when all five measurement points were used. With the input features optimized by mode shape assessment, 100% accuracy was also achieved with only two measurement points.

A review of operational modal analysis techniques for in-service modal identification

Vibrations are the root cause of many mechanical and civil structure failures. Dynamic characteristics of a structure must be extracted to better understand structural vibrational problems. Modal analysis is used to determine the dynamic characteristics of a system like natural frequencies, damping ratios and mode shapes. Some of the applications of modal analysis include damage detection, design of a structure/machine for dynamic loading conditions and structural health monitoring. The techniques used for modal analysis are experimental modal analysis (EMA), operational modal analysis (OMA) and a less known technique called impact synchronous modal analysis (ISMA), which is a new development. EMA is performed in simulated controlled environment, while OMA and ISMA are performed when the system is in operation. Although EMA is the oldest modal analysis technique, there is an increasing interest in operational modal analysis techniques in recent years. In this paper, operational modal analysis techniques OMA and ISMA are reviewed with their development over the years and their pros and cons discussed.

An inconsistent phase selection assessment for harmonic peaks elimination in operational modal testing

An improvement on modal analysis technique is always exacerbated by the limitation on both operational modal analysis (OMA) and experimental modal analysis (EMA). In a recent year, a novel method was introduced named impact-synchronous modal analysis (ISMA) which represents a magnificent achievement in this field. The efficiency of this method as a viable option for EMA and OMA is proven in previous research. However, a quick and straightforward real-time ISMA method is desired as the current procedure is labour-intensive and time-consuming due to the lack of control on the impact timing with respect to phase angle of the disturbances. Thus, the aim of this paper is to identify the significance of phase difference information between acceleration response and cyclic load component in eliminating the disturbances through impact-synchronous time averaging. The paper presented a phase selection assessment, and the results showed that a few averages, (i.e. four averages) are sufficient to filter out the disturbances by 72–80% of dominant periodic response due to cyclic load and over 50% reduction for second harmonic, when the phase angles with respect to the impact are inconsistent for each impact applied. A better modal identification result is obtained through a straightforward way of eliminating the periodic response. Thus, the estimated frequency response function is strongly enhanced and good correlation is observed between modal extraction data and benchmark EMA result.

A Performance Study of Controlled Impact Timing on Harmonics Reduction in Operational Modal Testing

As an alternative to operational modal analysis and classical experimental modal analysis (EMA), a novel method was introduced previously, namely impact-synchronous modal analysis (ISMA). The effectiveness ISMA on rotor and structural dynamic systems has been proven in previous literature. More recently, an automated impact device (AID) was introduced which utilized tachometer pulse as initiation signal and its effectiveness on ISMA was proven. An attempt to further enhance this device in term of equipment and cost is then proposed by replacing the tachometer with the in-use tri-axial accelerometer through utilizing the filtered response of cyclic load component as an initiation signal to control the impact device, which is also the primary aim for this study. Prior to modal testing, accuracy of this device is illustrated at desired phase angles of 0 deg, 90 deg, 180 deg, and 270 deg. Subsequently, frequency response function (FRF) estimations obtained for ISMA using enhanced AID has demonstrated the suppression capabilities of this device on disturbances, i.e., reduction of 93.58% at 30 Hz and 57.78% at 60 Hz, resulting in a high correlation for signature assurance criterion (SAC) and cross signature assurance criterion (CSAC). Modal parameters extracted from the EMA and ISMA using impact device are presented and compared, for the first three natural modes of the test rig. It is found that natural frequencies are deviating by less than 6%, whereas modal assurance criterion (MAC) values between the mode shapes of the two tests are found to be above 0.9.

Automated impact device with non-synchronous impacts: a practical solution for modal testing during operation

Previous study has shown that synchronization of phases between impacts and the cyclic load component should be avoided to improve the effectiveness of operational modal testing, i.e. impact-synchronous modal analysis in obtaining a cleaner frequency response function (FRF) estimation with fewer number of averages. However, avoiding the phase synchronization effect is rarely achievable with the current manual impact hammer because of the lack of control of the impact timing. We investigate how to improve FRF estimation in the presence of harmonic disturbances, such as those present in operating rotating machines. An auto impact device is therefore introduced to replace the manual impact hammer. This device ensures that impact intervals can be applied at non-synchronous instances with respect to the harmonic disturbance. We demonstrate that this new device is a viable option for operational modal testing. It allows significant improvement in FRF estimation and shows good correlation of modal extraction data with benchmark experimental modal analysis results.

Assessment on Structural Integrity of In-service Machine Using De-noised Vibrational Modal Data and Artificial Neural Network

The recent oil price drop creates a demand for swift action within oil and gas industry to shift focus from increasing daily production rates, to optimizing existing assets in achieving growth. Industrial machinery, one of the industry’s key asset many times failed due to high amplitude vibration that contributes to accelerated wear and tear and subsequently results in high cycle fatigue failure. As such there is a need to develop a structural integrity assessment for in–service machinery for continuous and safe operation. Vibration–based method such as Experimental Modal Analysis (EMA) is widely used for damage detection on civil and piping system under stationary environment. However, in industrial applications, system shutdown is very costly. EMA is also undesirable in this case due to the dominant ambient and system disturbances on the in–service system. An alternative method called Impact-Synchronous Modal Analysis (ISMA) is developed to perform modal analysis under noisy environment. Applying the ISMA technique in de-noising the non–synchronous disturbances at upstream could generate a cleaner and static–like modal data downstream for analysis. Artificial Neuron Networks (ANN) is then applied extensively in structural damage identification purposes based on changes in modal data due to its excellent pattern recognition ability. By leveraging on the latest technologies, i.e. ISMA and ANN as proposed, it allows real–time monitoring of assets, in this case, the machines, as well as the ability to transform continuous streams of data into useful information to predict damages.

Implementation of phase controlled impact device for enhancement of frequency response function in operational modal testing

This paper studies how to enhance frequency response function (FRF) estimation in the presence of harmonic disturbances during operational modal testing. A novel technique which utilizes impact-synchronous time averaging (ISTA) called impact-synchronous modal analysis (ISMA) was introduced where modal analysis can be performed in the presence of ambient forces. The phase angle information of the harmonic signal at the impact events is shown to be a key factor in enhancing the effectiveness of this technique. However, lack of knowledge and control of impact with respect to the phase angle of the disturbances using conventional impact hammer in ISMA has limited the effectiveness and practicality of this novel technique. A portable and automated phase controlled impact device is introduced in the effort to eliminate non-synchronous components with minimal possible impacts applied. This device makes use of the feeding phase angle information of responses from the cyclic load back to the device and imparts the impact at the correct time/phase which is always non-synchronous with respect to the phase of response from cyclic load. A reduced number of averages thereby expedite the overall modal testing procedure, an improved of FRF estimation and a good correlation of modal extraction data with benchmark data shown in this study has highlighted the advantages of ISTA using the proposed device.

Assessment of the phase synchronization effect in modal testing during operation

The impact-synchronous modal analysis (ISMA), which uses impact-synchronous time averaging (ISTA), allows modal testing to be performed during operation. ISTA is effective in filtering out the non-synchronous cyclic load component, its harmonics, and noises. However, it was found that at operating speeds that coincide with the natural modes, ISMA would require a high number of impacts to determine the dynamic characteristics of the system. This finding has subsequently reduced the effectiveness and practicality of ISMA. Preservation of signatures during ISTA depends on the consistency of their phase angles on every time block but not necessarily on their frequencies. Thus, the effect of phase angles with respect to impact is seen to be a very important parameter when performing ISMA on structures with dominant periodic responses due to cyclic load and ambient excitation. The responses from unaccounted forces that contain even the same frequency as that contained in the response due to impact are diminished with the least number of impacts when the phase of the periodic responses is not consistent with the impact signature for every impact applied. The assessment showed that a small number of averages are sufficient to eliminate the non-synchronous components with 98.48% improvement on simulation and 95.22% improvement on experimental modal testing when the phase angles with respect to impact are not consistent for every impact applied.

An experimental investigation on the effects of exponential window and impact force level on harmonic reduction in impact-synchronous modal analysis

A novel method called Impact-synchronous modal analysis (ISMA) was proposed previously which allows modal testing to be performed during operation. This technique focuses on signal processing of the upstream data to provide cleaner Frequency response function (FRF) estimation prior to modal extraction. Two important parameters, i.e., windowing function and impact force level were identified and their effect on the effectiveness of this technique were experimentally investigated. When performing modal testing during running condition, the cyclic loads signals are dominant in the measured response for the entire time history. Exponential window is effectively in minimizing leakage and attenuating signals of non-synchronous running speed, its harmonics and noises to zero at the end of each time record window block. Besides, with the information of the calculated cyclic force, suitable amount of impact force to be applied on the system could be decided prior to performing ISMA. Maximum allowable impact force could be determined from nonlinearity test using coherence function. By applying higher impact forces than the cyclic loads along with an ideal decay rate in ISMA, harmonic reduction is significantly achieved in FRF estimation. Subsequently, the dynamic characteristics of the system are successfully extracted from a cleaner FRF and the results obtained are comparable with Experimental modal analysis (EMA).

Investigation of Impact Profile and Isolation Effect in Automated Impact Device Design and Control for Operational Modal Analysis

A novel modal analysis technique called impact-synchronous modal analysis (ISMA) was introduced in previous research. With the utilization of impact-synchronous time averaging (ISTA), this modal analysis can be performed in presence of ambient forces whereas the conventional analysis method requires machines to be totally shut down. However, lack of information of phase angles with respect to impact in ISMA has caused it to be labor-intensive and time-consuming. An automated impact device (AID) is introduced in this study in the effort to replace the manually operated impact hammer and prepare it to be used in the current practice of ISMA on the purpose of enhancing its effectiveness and practicability. Impact profile and isolation effect are noted to be the contributing parameters in this study. This paper devoted on calibrating and controlling of the AID which gives the desired impact profiles as compared to the manual impact hammer. The AID is found effective in the determination of dynamic characteristics when the device is isolated from the boundary condition of the test structure.

Enhancement of Impact-synchronous Modal Analysis with number of averages

A new method, namely Impact-synchronous Modal Analysis (ISMA), utilizing the modal extraction technique commonly used in Experimental Modal Analysis performed in the presence of the ambient forces, is proposed. In ISMA, the extraction is performed while the machine is running, utilized Impact-synchronous Time Averaging prior to performing the Fast Fourier Transform. The number of averages had a very important effect when applying ISMA on structures with dominant periodic responses of cyclic loads and ambient excitation. With a sufficient number of impacts, all the unaccounted forces were diminished, leaving only the response due to the impacts. This study demonstrated the effectiveness of averages taken in the determination of dynamic characteristics of a machine while in different rotating speeds. At low operating speeds that coincided with the lower natural modes, ISMA with a high number of impacts determined the dynamic characteristics of the system successfully. Meanwhile, at operating speeds that were away from any natural modes, ISMA with a moderate number of averages taken was sufficient to extract the modal parameters. Finally for high-speed machines, ISMA with a high number of impacts taken has limitations in extracting natural modes close to the operating speed.

Enhancement of coherence functions using time signals in Modal Analysis

Experimental Modal Analysis (EMA) and Operational Modal Analysis (OMA) are two widely used techniques in the identification of modal parameters. EMA is synonymous with a laboratory environment requiring complete system shutdown while OMA is implemented in a real environment where the ambient forces cannot be isolated. A new method, namely Impact-Synchronous Modal Analysis (ISMA) utilising the modal extraction techniques commonly used in EMA but performed in the presence of the ambient forces, is proposed. Transfer functions, from where the modal parameters are extracted, are obtained from Fourier transform of cross and auto correlation functions. These functions are estimated quantities and their outcomes are dependable on the averaging techniques used. The coherence functions are commonly used to measure the acceptability of the estimations. Impact-Synchronous Time Averaging is compared against Spectral Averaging while performing Modal Analysis in a situation containing ambient and operating forces. Results showed that while the transfer functions obtained from both the averaging techniques were of similar quality, the Impact-Synchronous Time Averaging indicated better coherence than the Spectral Averaging.