Impact-Synchronous Modal Analysis (ISMA)

Impact Synchronous Modal Analysis (ISMA)

Impact Synchronous Modal Analysis (ISMA) is a technique used to identify the modal parameters (natural frequencies, damping ratios, and mode shapes) of a structure by subjecting it to controlled impacts or excitations. This method is particularly useful when traditional modal analysis techniques, such as Experimental Modal Analysis (EMA), are not feasible or when the structure’s response needs to be measured under operational conditions.


Motivation of This Research

Conduct modal analysis without disturbing operating machines

ISMA allows for the modal analysis of structures under operational or in-service conditions. Unlike traditional modal analysis techniques that require the structure to be in a quiescent state, ISMA enables modal parameter identification while the structure is in its working environment. This capability is particularly useful for assessing the dynamic behavior of structures that cannot be easily brought to a static condition or when studying the effects of dynamic loads during normal operation.

Excitation Control

ISMA provides control over the excitation applied to the structure. By using a controlled impact source, such as an impact hammer, the excitation can be precisely localized and controlled. This enables the selection of specific vibration modes for analysis, facilitates the separation of mode shapes, and enhances the accuracy of modal parameter identification.

Dynamic Load Identification

ISMA can also be used for identifying dynamic loads acting on a structure. By analyzing the structural response to impact excitations, it is possible to estimate the forces or loads causing the measured vibrations. This capability is valuable for applications such as structural health monitoring, fault detection, or troubleshooting, as it helps identify and quantify external loads or excitations affecting the structure.

Limited Accessibility

ISMA can be applied to structures with limited access points or complex geometries where traditional modal testing approaches may be challenging or impractical. By using impact excitation and strategically placed sensors, ISMA can capture the structural response at multiple locations, even in difficult-to-reach areas. This makes it a valuable tool for analyzing structures such as large bridges, offshore platforms, or complex machinery.

Real-time Analysis

ISMA offers the advantage of real-time analysis. By synchronizing the impact events with the data acquisition system, it is possible to perform the modal analysis simultaneously during the testing process. This immediate feedback allows engineers to adjust the measurement setup, evaluate the quality of the acquired data, and make informed decisions on the spot, reducing the time required for subsequent analysis and interpretation.

Main Features of ISMA


Impact-Synchronous Time Averaging method to denoise the upstream transfer function

Phase Synchronisation

Allows few numbers of impacts to achieve denoising effect

In-operation modal analysis

Allow measurement without disturbing operating structures

Applied in Damage Diagnosis

Has shown to be accurate in damage diagnosis

Applied in research and industrial projects

Has been applied to perform modal testing for in-operation pumps 

> 0 %
FRAC and CSAC scores
0 %
Accuracy in detecting damage presence
0 %
Average suppresssion rate on dominant periodic response due to cyclic load

Adaptive phase control impact device (APCID)

The Adaptive Phase Control Impact Device (APCID) is an innovative tool developed for Impact Synchronous Modal Analysis (ISMA). It is designed to improve the accuracy and efficiency of impact testing by precisely controlling the phase of the impact force applied to the structure.

The APCID system consists of an impact hammer equipped with sensors and a feedback control mechanism. It continuously monitors the structure’s response to the applied impacts and adjusts the phase of the impact force in real-time. By analyzing the measured response signals, the APCID system can determine the optimal timing to trigger the impact, ensuring that the force is applied at the desired phase for each mode of vibration.

By providing adaptive phase control, the APCID system improves the accuracy and consistency of impact testing in ISMA. It reduces the impact force variations and minimizes errors caused by phase discrepancies. This precise control over the impact force phase enhances the reliability and accuracy of modal parameter identification, leading to more accurate structural dynamic characterization.

Improved Modal Parameter Accuracy

By controlling the phase of the impact force, the APCID system minimizes variations in modal parameters, such as natural frequencies, damping ratios, and mode shapes. This enables more accurate identification of the structural dynamic characteristics.

Reduced Operator Dependency

The automated nature of the APCID system reduces the reliance on operator skills and experience, minimizing operator-induced errors and inconsistencies. This enhances the reproducibility and reliability of the impact testing process.

Time Efficiency

The APCID system optimizes the impact testing process by automatically adjusting the impact force phase in real-time. This saves time and increases the efficiency of data collection, allowing for quicker modal analysis and reduced testing duration.

Compatibility with Various Structures

The APCID system can be used with a wide range of structures, including complex and large-scale systems. Its adaptive phase control capability ensures accurate impact testing regardless of the structural geometry or dynamic characteristics.

Research Outputs

  1. Siow, P. Y., Ong, Z. C., Khoo, S. Y., & Lim, K. S. (in press, 2023). Noise robustness of an operational modal-based structural damage-detection scheme using Impact-Synchronous Modal Analysis. Journal of Zhejiang University-Science A. 
  2. Bin Zahid, F., Ong, Z. C., Khoo, S. Y., & Salleh, M. F. M. (2021). Inertial sensor based human behavior recognition in modal testing using machine learning approach. Measurement Science and Technology, 32(11), 18.
  3. Chen, S., Ong, Z. C., Lam, W. H., Lim, K.-S., & Lai, K. W. (2020). Operational Damage Identification Scheme Utilizing De-Noised Frequency Response Functions and Artificial Neural Network. Journal of Nondestructive Evaluation, 39(3), 66.
  4. Bin Zahid, F., Ong, Z. C., & Khoo, S. Y. (2020). A review of operational modal analysis techniques for in-service modal identification. Journal of the Brazilian Society of Mechanical Sciences and Engineering, 42(8), 18.
  5. Ong, Z. C., Lim, H. C., Brandt, A., Ismail, Z., & Khoo, S. Y. (2019). An inconsistent phase selection assessment for harmonic peaks elimination in operational modal testing. Archive of Applied Mechanics, 89(12), 2415-2430.
  6. Lim, H. C., Ong, Z. C., Ismail, Z., & Khoo, S. Y. (2019). A Performance Study of Controlled Impact Timing on Harmonics Reduction in Operational Modal Testing. Journal of Dynamic Systems Measurement and Control-Transactions of the Asme, 141(3).
  7. Ong, Z. C., Yap, E. T., Ismail, Z., & Khoo, S. Y. (2018). Assessment on Structural Integrity of In-service Machine Using De-noised Vibrational Modal Data and Artificial Neural Network. MATEC Web of Conferences, 237.
  8. Ong, Z. C., Lim, H. C., & Brandt, A. (2018). Automated impact device with non-synchronous impacts: a practical solution for modal testing during operation. Journal of Zhejiang University-Science A, 19(6), 452-460.
  9. Lim, H. C., Ong, Z. C., & Brandt, A. (2018). Implementation of phase controlled impact device for enhancement of frequency response function in operational modal testing. Journal of the Franklin Institute, 355(1), 291-313.
  10. Ong, Z. C., Lim, H. C., Khoo, S. Y., Ismail, Z., Kong, K. K., & Rahman, A. G. A. (2017). Assessment of the phase synchronization effect in modal testing during operation. Journal of Zhejiang University-Science A, 18(2), 92-105.
  11. Ong, Z. C., Lim, H. C., Khoo, S. Y., Rahman, A. G. A., & Ismail, Z. (2016). An experimental investigation on the effects of exponential window and impact force level on harmonic reduction in impact-synchronous modal analysis. Journal of Mechanical Science and Technology, 30(8), 3523-3532.
See also

Applications and Enhancements of ISMA

Force Identification

Semi-Automated Impact Device

Structural Damage Diagnosis