Non contact measurements of structural vibrations

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Microflowns acoustic particle velocity sensors offer an alternative to both accelerometers and a scanning laser vibrometer to measure the structural vibrations normal to the surface of the testing object. The benefits become very clear especially for multi point vibration measurements on light very weight structures tested under non stationary conditions.

Close to a vibrating surface, due to the incompressibility of the air in the so called very near field region, the acoustic particle velocity is a direct measure for the normal structural velocity. 

Acceptance of this approach has been confirmed in several papers from EU companies like LMS International from Belgium and universities/institutes like INSA Lyon. It implies that there is now a possibility for non contact multipoint vibration measurements that can be applied to low weight structures most favorably.ONERA, the French aerospace institute, did successful low frequency comparisons in 2003.Leuven Measurement Systems tested successfully for higher frequencies up to 25 kHz.These tests show the broad frequency range of the sensors for vibration applications.

Accelerometers do cause a mass load effect that becomes an issue to light weight space structures such as solar panels. Obviously, scanning laser vibrometers don’t cause a mass load effect. But there are some limitations as well.

During the reverberant room testing, the acoustic load is only applied for a very limited period of time equivalent to true rocket launch conditions.In this short period of time, the number of measurement points to be scanned is limited. Furthermore, inherently, a scanning laser measures sequentially and requires a line of sight.

For in stationary problems arrays of particle velocity sensors offer a multi point solution not causing a mass load effect.

In cases where no line of sight is available, for instance noise attenuation measurements in keyholes of sandwiched solar panels, acoustic particle velocity sensors are the only credible option.