Paper 102 Rahman

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IOMAC'13 5 th International Operational Modal Analysis Conference 2013 May 13-15 Guimares - Portugal

IMPACT-SYNCHRONOUS MODAL ANALYSIS (ISMA) AN ATTEMPT TO FIND AN ALTERNATIVE

Abdul Ghaffar Abdul Rahman 1 , Zubaidah Ismail 2 , Siamak Noroozi 3 , Ong Zhi Chao 4

ABSTRACT

Modal frequencies, modal shapes and modal damping comprehensively define the dynamiccharacteristics of a structure. Prior to developing a mathematical model of the dynamic behaviour of any structure, these parameters need to be firstly determined. Currently, the two techniques used toextract these parameters are the classical Experimental Modal Analysis (EMA) and the OperationalModal Analysis (OMA). The fundamental difference in the two techniques lies in the manner bywhich the system is excited. A novel method called Impact-Synchronous Modal Analysis (ISMA)utilising the modal extraction techniques commonly used in EMA but performed in the presence of theambient forces is proposed. The differences between EMA and ISMA lie in the averaging technique

used. ISMA utilised Impact-Synchronous Time Averaging (ISTA) triggered by the impulse of theartificial force introduced by the impact hammer prior to performing the Fast Fourier Transform (FFT)instead of frequency domain averaging adopted by conventional EMA. Results showed that modal

parameters were successfully determined in the presence of periodic responses of cyclic loads andambient excitation using ISMA in both laboratory rig and industrial machinery. The well correlatedresults with classical EMA during static condition show the effectiveness of performing ISMA in the

presence of the unaccounted forces.

Keywords: Experimental Modal Analysis, modal frequency, modal damping, modal shape, Impact-Synchronous Modal Analysis, Impact-Synchronous Time Averaging

1. INTRODUCTIONThree parameters namely modal frequencies, modal shapes and modal damping comprehensivelydefine the dynamic characteristics of a structure. Prior to developing a mathematical model of thedynamic behaviour of any structure, these parameters need to be determined. Currently, the twotechniques used to extract these parameters are the classical Experimental Modal Analysis (EMA) and

1 Professor, Faculty of Mechanical Engineering, University Malaysia Pahang, [email protected] 2 Associate Professor, Civil Engineering Department, Faculty of Engineering, University of Malaya,[email protected] 3Professor, School of Design, Engineering and Computing, Bournemouth University,

[email protected] 4 Mechanical Engineering Department, Faculty of Engineering, University of Malaya, [email protected]
http://us.mg1.mail.yahoo.com/yab-fe/mu/MainView?.src=neo&themeName=blue&stab=1318914654413http://us.mg1.mail.yahoo.com/yab-fe/mu/MainView?.src=neo&themeName=blue&stab=1318914654413http://us.mg1.mail.yahoo.com/yab-fe/mu/MainView?.src=neo&themeName=blue&stab=1318914654413mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://us.mg1.mail.yahoo.com/yab-fe/mu/MainView?.src=neo&themeName=blue&stab=1318914654413


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A.G.A. Rahman,Z. Ismail,S. Noroozi, O.Z. Chao

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the Operational Modal Analysis (OMA). The fundamental difference between the two techniques liesin the manner by which the system is excited.

EMA requires the system to be in complete shutdown mode. In other words, there should be nounaccounted excitation force induced into the system. Measurable impact or random forces are used toexcite the system. Cross-s pectrum of measured inputs and systems responses produce the transfer

functions at a discrete set of geometrical positions. Various curve-fitting techniques are then used toextract the three parameters.

In practical situations where the system cannot be shut down completely or the structure is too huge toresponse to artificial excitation, OMA is sought. The operating forces coming from the machinecyclic loads or ambient forces are used as the exciters. As these quantities cannot be measured, OMAutilized only information from the measurable responses (multi-output data) and some algorithms [1-5] are used to extract the three modal parameters i.e. modal frequencies, modal shapes and modaldamping.

In OMA, structural modal parameters can be computed without knowing the input excitation to thesystem. Therefore, it is a valuable tool to analyse structures subjected to excitation generated by their own operation. Presently, OMA procedures are limited to cases where excitation to the system is white

stationary noise [6]. The main advantage of OMA is that the measured responses can be used for modal identification of structures under real operation without requiring excitation from a hammer andshaker. Therefore, the measured responses are representative of the real operating conditions of thestructure.

Although OMA holds advantage over EMA in terms of its practicality and simplicity to carry out the procedure and performing the analysis while the system is in operation, the lack of knowledge of theinput forces does affect the parameters extracted. For example, mode shapes obtained from OMAcannot be normalised accurately. This results in approximations of the mathematical models.

Over the years, as the part of the effort to improve the estimation accuracy in OMA and EMA, thefocus was mainly on the development of modal identification algorithms. Relatively less effort was

put into improving the digital signal processing aspects, especially upstream of the collected data. Inthis research, a novel method, named Impact-Synchronous Modal Analysis (ISMA) that utilisesImpact-Synchronous Time Averaging (ISTA) [7, 8] is proposed. ISMA has the advantages of theOMA and EMA combined. It carries out the analysis while the system is in operation and at the sametime is able to provide the actual input forces in the transfer functions, hence, allowing for better modal extractions. This novel technique can be applied in both rotor and structural dynamic systems toobtain the dynamic characteristics of structures without disturbing the operations. This is very crucialfor the industrial plants especially those high downtime cost rotating machinery. In petrochemical

plants, the downtime cost alone can go up to USD 6,000 to 90,000 per hour.

2. IMPACT-SYNCHRONOUS MODAL ANALYSIS (ISMA)

In Impact-Synchronous Modal Analysis (ISMA) [8], when analysis is performed while machine is inrunning condition, all the responses contributed by the unaccounted forces are filtered out in the timedomain, leaving only the responses triggered due to the impact hammer. This is achieved by utilisingthe Impact-Synchronous Time Averaging (ISTA) [7] prior to performing the Fast Fourier Transformation (FFT) operation. The process of modal parameters extraction follows theExperimental Modal Analysis (EMA) procedures.

In commonly used time domain synchronous averaging, signal acquisition from rotating machine istriggered at the same rotational position of the shaft using a tachometer for every cycle. The time

block of the averaged signal eliminates all the non-synchronous and random components, leaving behind only components that are integer multiples of the running speed.

In ISMA, the same and simple averaging concept is used but only to achieve the reverse i.e. to filter out all the speed synchronous and random signatures. In this case, data acquisition is triggered by theimpact signature. The periodic signatures and their harmonics are no more in the same phase position


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5th International Operational Modal Analysis Conference, Guimares 13-15 May 2013

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for every time block acquired. Equation (1) shows that averaging process will slowly diminish thesenon- synchronous components hence leaving behind only the structures response to impulses whichare synchronous to the repetitive impact force.

1

0)(

1)(

N

r orT t x N

t y (1)

where: )(t y is the averaged vibration signal in time domain, N is the total number of impacts, )(t x is

the vibration signal in time domain, r is number of impact and oT is the time interval betweenimpacts.

Cross spectrum of the averaged time block of impulse responses and the averaged time block of impact signatures is used to generate the transfer function.

In discussions on limitations of Impact-Synchronous Modal Analysis (ISMA), it is noticed that ISMAgives more prominent results with higher number of impacts. However, responses from unaccountedforces that contain even the same frequency as that contained in the impulse response could not bediminished if the impact frequency is consistent with respect to the impact signature although highnumber of impacts is taken. Information of the cyclic force is also important. ISMA has limitation inits application on high speed machines and large structures where the impact response may be toosmall as compared to the operating cyclic loads while excessive impacts may result in non-linearity.Finally, in signal processing aspect, application of exponential window is necessary in ISMA toattenuate the response signals generated by cyclic loads. Therefore, parameters like number of impacts, impact frequency, impact force level and exponential window are important in ISMA in order to improve the technique.

3. LABORATORY TESTIn this research, a motor-driven test rig consists of a motor coupled with rotor shaft system as shownin Figure 1 was used to study the effectiveness of Impact-Synchronous Modal Analysis (ISMA) duringoperating condition compared to Experimental Modal Analysis (EMA). A total of 20 points wereselected on the structural plate of the test rig.

Figure 1 Measurement Points and Locations of Motor-driven Test Rig

Figure 2 shows the instrumentation set-up. The measured input is force from the impact hammer andthe measured output is acceleration from the accelerometer. Data were obtained by using a DASYLab-

National Instruments data acquisition system together with an impact hammer and tri-axialaccelerometer. Impact hammer was connected to channel 1 of the National Instrument (NI) dynamicanalyser and a tri-axial accelerometer was connected to channel 2, 3 and 4 respectively.


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Figure 2 Instrumentation Set-up

The experiment was carried out by fixing the impact hammer and roving the tri-axial accelerometer.The sampling rate used was 2048 samples/sec with block size of 4096. This yields frequencyresolutions of 0.5 Hz and 2 seconds of time record length to capture every response signal. Fiveaverages or impacts were taken at each measurement point for non-rotating condition using EMA andone hundred averages were collected during rotating condition using ISMA. The signals were

processed by a self-developed virtual instruments application programme in DASYLab using differentaveraging techniques to generate FRFs and Coherence Functions. The modal extraction techniquesapplied to EMA could also be applied in ISMA. Figure 3 shows a three-dimensional structural modelof the test rig which was drawn with coordinate points and connected by straight lines using a modalanalysis software called MEScope. The displayed point numbers show the measurement locations asin the actual base plate. This model was used to display the mode shapes of the structural plate of testrig from the acquired data. In addition to that, the software was also used to perform post-processingof the acquired data and curve-fitting for the extraction of modal frequency, modal damping andmodal shape. List of instrumentation are displayed in Table 1.

Figure 3 Structural Wire-mesh Model of Motor-driven Test Rig

Table 1 List of Instrumentation

INSTRUMENTS

PCB Impact Hammer, Model 086C03

IMI Tri-axial Accelerometer, Model 604B31

NI USB Dynamic Signal Acquisition Module, Model NI-USB 9234

DASYLab v10.0

MEScope v4.0

Table 2 shows the summary of the correlation results of first three natural modes obtained betweenEMA and ISMA. EMA was performed during stationary condition and modal extraction results areused as a benchmark to correlate with Impact-Synchronous Modal Analysis (ISMA) results (Table 3)which was performed during operating condition at 20 Hz. The dynamic characteristics result obtained

by ISMA shows good correlation with the benchmarked EMAs result. Differences of naturalfrequency for the first three modes are less than 10%. The mode shapes correlation using MAC values


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range from 0.958 for mode 1 to 0.809 for mode 3 also show close correlation of the mode shapevectors.

Table 2 Summary of Natural Frequencies and Mode Shapes Comparison between EMA and ISMA

Mode Natural Frequency[Hz] by EMA Natural Frequency

[Hz] by ISMAPercentage of

Difference [%] MAC

1 9.92 9.56 3.63 0.9582 15.6 15.2 2.56 0.8993 24.9 23.4 6.02 0.809

Table 3 Dynamic Characteristics of Motor-driven Test Rig during Operating Condition at 20 Hz using ISMA

Mode 1

(Knock in Vertical Direction)

Natural Frequency [Hz] 9.56

Damping [Hz] 0.886

Mode 2



Damping [Hz] 0.678

Mode 3



Damping [Hz] 1.29


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4. CASED STUDIEDISMA has been tested in real industry application; performed on a diesel fuel pump package at anoffshore platform in Malaysia to investigate the high vibration problem of the package. Theinstrumentation and procedures used in real industry application was the same as used in laboratorycondition. The only difference is the impact hammer was replaced with a larger size model of hammer

(PCB Impact Hammer, Model 086D20) was used in the test.ISMA was used to determine the dynamic characteristics, namely natural frequencies, mode shapes,and damping of pump. Two identical units i.e. A and B were sat on the same skid. Pump A was shutdown while the adjacent standby unit B was in operation. This will avoid any disturbance on the dailyoperational processes at platform. Measurements were carried out in April 2012. Vibrationmeasurements were taken on motor, motor support, pump, pump support, skid, pipes, etc. as shown inFigure 4. Initially, measurement points were identified and marked. All the points were then linked toobtain a wire- mesh model representing the machine in MEScope software as shown in Figures 5.

Figure 4 Diesel Fuel Pump Package

Figure 5 Measurement Points and Locations of Diesel Fuel Pump Package

ISMA revealed that several natural frequencies were close to and coincide with operating runningspeed of 49.5 Hz at motor and pump sides of unit A. (Table 4). This explained the high vibrationwhich was recorded in these components. Generally, unit A was operating in resonance situation. Thiswas the primary cause of high vibrations at motor and pump. As a long term solution, it was

recommended to perform Structural Dynamic Modification (SDM) which requires Finite ElementAnalysis (FEA) for verification prior to fabrication.

Unit B

Unit A


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Table 4 Dynamic Characteristics of Diesel Fuel Pump using ISMA

Mode 1

(Knock in Horizontal Direction)


Damping [Hz] 0.882

Mode 2



Damping [Hz] 1.340

Mode 3

(Knock in Axial Direction)


Damping [Hz] 0.445

Mode 4



Damping [Hz] 0.310


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Currently, the machines are still requested to shutdown before modal analysis is performed. However,in these plants where downtime cost is very high, there are always some in operation and standby unitslocated adjacent to the unit under analysis. The cyclic load and ambient excitation from those runningunits are transferred to the static unit. Hence, vibration generated by the adjacent running units willcontaminate the impulse response when modal analysis is performed on the stationary unit. WithISMA applied, unaccounted forces are diminished and prominent FRF is obtained through manuallyoperated impact hammer. Impact hammer is limited to low impact numbers due to large number of measurement points on the machines and time constraint. However, random impacts with low number of averages have successfully determined dynamic characteristics of structure under analysis. Withinformation of the machinerys characteristics, further analyses are performed to solve the v ibration

problems.

5. CONCLUSIONSThis study has demonstrated the effectiveness of using Impact-Synchronous Modal Analysis (ISMA)in the determination of dynamic characteristics of a system while in operating condition. The wellcorrelated results with classical EMA during static condition show that ISMA could be performed inthe presence of the unaccounted forces. It is also noticed that modal parameters were successfullydetermined using ISMA in industrial machinery.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the financial support and advice given by Postgraduate ResearchFund (PV086-2011A), Advanced Shock and Vibration Research (ASVR) Group of University of Malaya, Advanced Structural Integrity Vibration Research (ASIVR) of Universiti Malaysia Pahangand other project collaborators.

REFERENCES

[1] Zhang, L.M., R. Brincker, and P. Andersen (2001) Modal Indicators for Operational ModalIdentification. In: Proceedings of the 19th International Modal Analysis Conference , Orlando,Florida, USA, 746-752.

[2] Brincker, R., L.M. Zhang, and P. Anderson (2000) Modal Identification from AmbientResponse using Frequency Domain Decomposition. In: Proceedings of the 18th International

Modal Analysis Conference , San Antonio, Texas, USA, 625-630.

[3] Schwarz, B. and M.H. Richardson (2001) Modal Parameter Estimation from AmbientResponse Data. In: Proceedings of the 19th International Modal Analysis Conference ,Orlando, Florida, USA, 1017-1022.

[4] Schwarz, B. and M.H. Richardson (2007) Using a De-Convolution Window for Operating

Modal Analysis. In: Proceedings of the 2nd International Operational Modal AnalysisConference , Orlando, Florida, USA, 1-7.

[5] Whelan, M.J., et al. (2011) Operational modal analysis of a multi-span skew bridge using real-time wireless sensor networks. Journal of Vibration and Control 17(13): 1952-1963.

[6] Mohanty, P. and D.J. Rixen (2004) A modified Ibrahim time domain algorithm for operationalmodal analysis including harmonic excitation. Journal of Sound and Vibration 275(1-2): 375-390.

[7] Rahman, A.G.A., Z.C. Ong, and Z. Ismail (2011) Effectiveness of Impact-Synchronous TimeAveraging in determination of dynamic characteristics of a rotor dynamic system.

Measurement 44(1): 34-45.

[8] Rahman, A.G.A., Z.C. Ong, and Z. Ismail (2011) Enhancement of coherence functions usingtime signals in Modal Analysis. Measurement 44(10): 2112-2123.

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Paper 102 Rahman