Technical Brief

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

[+] Author and Article Information
Hong Cheet Lim

Mechanical Engineering Department, Faculty of Engineering, University of Malaya, Malaysia

Alex Ong

Civil Engineering Department, Faculty of Engineering, University of Malaya, Malaysia

Zubaidah Ismail

Mechanical Engineering Department, Faculty of Engineering, University of Malaya, Malaysia

Shin Yee Khoo

Department of Mechanical Engineering, Faculty of Engineering, University of Malaya Kuala Lumpur, 50603 Malaysia

1Corresponding author.

ASME doi:10.1115/1.4041609 History: Received July 05, 2017; Revised September 27, 2018


As an alternative to operational modal analysis (OMA) 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 utilised 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 utilising 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°, 90°, 180° and 270°. 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.

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