Modal testing aids vibration troubleshooting

A Sound and Vibration Technology product story

Edited by the Engineeringtalk editorial team May 26, 2004

Modal testing involves artificially exciting a structure to measure its natural frequencies, mode shapes, damping properties, modal mass and, sometimes, inertia properties.

Modal testing involves artificially exciting a structure to measure its natural frequencies, mode shapes, damping properties, modal mass and, sometimes, inertia properties.

It is often undertaken to correlate finite element (FE) models either visually or using more rigorous techniques such as modal assurance criteria and orthogonality.

Once the FE model has been proven to accurately represent the physical hardware, it can be used to predict a revised design or dynamic behaviour under difficult to reproduce operating conditions, a satellite launch for example.

Another major application of modal testing is the troubleshooting of noise and vibration problems.

The mode shapes indicate the points of maximum deflection plus controlling masses and stiffnesses.

This information alone is often sufficient to enable an experienced engineer to modify the structure to solve the problem with, for example, added stiffness, reduced mass, alternative attachment locations or a tuned absorber.

Sometimes a modal test is performed simply for qualification, to determine whether the structure has achieved the natural frequency or damping targets.

For example, is the resonance of a car's engine bracket higher than the excitation forces to which it will be exposed? Failing this target could result in poor refinement (NVH) and possibly reduced time to failure.

Modal testing is a very elegant way of measuring inertia properties and, as an alternative to a trifilar pendulum, has many advantages.

For example, a Challenger Main Battle Tank has been tested in its normal attitude without standing it on its nose.

Also, the results are used to populate an over-determined matrix, thereby producing confidence factors as testing progresses.

There are many ways of exciting the structure from a simple impact with an instrumented hammer through to multiple exciters inputting random or sinusoidal energy.

For ground vibration testing (GVT) of aircraft structures the sinusoidal force level and phasing can be tuned to produce a pure, normal mode thereby mimicking the modes predicted by an FE modal.

Sound and Vibration Technology (SVT) has applied these techniques to a variety of structures including aircraft, cars, trucks, buses, buildings, wind turbines, microscopes, photocopiers, medical devices, vacuum pumps and printing presses.

The company has over 25 years of experience applying modal testing and correlation techniques.

These services can be provided around the world, wherever the structure is that requires testing.

In addition, they can provide the software tools, training, technology transfer and hotline support required to introduce these methods into their customers' processes.