Introduction to Modal Analysis Techniques

This is a summary/abstract for the full version of this article that will appear on the Prosig Noise & Vibration Measurement Blog in the coming weeks.

Modal analysis is essential for understanding the dynamic behaviour of structures and mechanical systems. This technique lets engineers ascertain vital characteristics such as natural frequencies, mode shapes, and damping ratios. In real-world engineering, modal analysis is invaluable for troubleshooting, validating designs, and optimizing system performance. This summary explains the core types of modal analysis—Experimental Modal Analysis (EMA) and Operational Modal Analysis (OMA)—along with their applications, benefits, and limitations.

Experimental Modal Analysis (EMA)

Key Features and Tests

EMA involves applying a known force to the system under study. The force can be applied using an impact hammer or a shaker, and the resultant vibrations are recorded. EMA provides accurate results in a controlled environment, making it ideal for smaller structures or individual components.

Equipment and Procedures

For hammer tests, the required equipment includes a calibrated hammer, an accelerometer, and a data acquisition system. Shaker tests necessitate an electrodynamic shaker, accelerometers, and the same data acquisition system, often coupled with an amplifier and signal generator.

Case Studies

EMA has proven its utility in various sectors like automotive and aerospace. For instance, it helped automotive engineers solve excessive vibrations in a new suspension system. In aerospace, shaker tests assisted in evaluating a wing component under varied load conditions.

Advantages and Limitations

EMA is easy to set up and effectively identifies issues in simpler structures. However, complex geometries may require more advanced techniques and expertise to interpret results accurately.

Operational Modal Analysis (OMA)

Key Features and Tests

Unlike EMA, OMA doesn’t require artificial excitation. It is well-suited for large, inaccessible, or complex structures like bridges, wind turbines, and buildings. OMA utilizes natural ambient forces, such as wind or operational machinery, to excite the system.

Equipment and Procedures

OMA employs multiple accelerometers, a data acquisition system, and specialized signal processing software for analysis. Engineers mount accelerometers at critical points on the structure, record vibrations during operation or ambient conditions, and analyze the data to identify modal parameters.

Case Studies

OMA is increasingly used in renewable energy sectors, civil engineering, and infrastructure monitoring. For example, it is invaluable in monitoring the structural health of wind turbines and buildings, especially after events like earthquakes.

Advantages and Limitations

OMA offers the advantage of testing under real-world conditions, but the complexity of the unmeasured ambient excitations can make data interpretation challenging. Although often less expensive, extended data collection periods may sometimes be required.

Comparing OMA and EMA

EMA offers a controlled environment but is often limited to smaller systems. OMA provides more flexibility for real-world, large-scale structures but may involve more complex data interpretation.

Conclusion

Both EMA and OMA serve crucial roles in modal analysis, each with its own set of advantages and limitations. By understanding these, engineers can make informed choices about the most suitable approach for their specific requirements, contributing significantly to the optimization and safety of various systems.

[This is a summary/abstract of the full article that will appear on the Prosig Noise & Vibration Measurement Blog. Keep an eye on the blog or sign up for our email newsletter and make sure you don't miss it]