Modal analysis and model verification

Modal Analysis is the study the dynamic response of machines or structures, i.e. the way they vibrate and their frequencies, when excited by a vibrating force or impact.

ISVR Consulting has all the tools necessary for detailed modal analysis of most structures. In recent times a wide variety of structures has been successfully tested, ranging from a 6 metre steel beam with additions weighing 13 tons, to a small plastic disk of a few millimetres diameter, which is part of a Bluetooth headset. Modal analysis of existing structural components and prototypes allows potential problems to be seen, with slow-motion video animation of selected modes.

Two modes of a beam with lumped masses.

A complete set of transfer functions between the force and response of the structure is taken on a grid sufficient to give a realistic visual interpretation of the potential motion of a test piece. By leaving one or more accelerometers in place and “roving” with hammer excitation it is possible to conduct a test quickly, and with the minimum of attachments to the structure. In this way one or more rows of the frequency response matrix Rxy is measured. Alternatively, a vibration exciter, such as an electrodynamic exciter, may be mounted at a fixed position. The structure can be excited with sine, or broadband excitation, and the input force measured. One or more accelerometers are moved around the structure and the resulting frequency response functions complete columns in the frequency response matrix. This method has benefits applied to structures which may exhibit some non-linearity, as coherence measurement can also be used effectively to judge the validity of the measurements.

Measurements are made using a Brüel & Kjær “PULSE” system, with associated instrumentation according to the size and complexity of the test& object. Results of the measurements are analysed using the Modal Analysis software – ISVR Consulting currently use “ME'scope” from Vibrant Technology.

The shapes of the modes can be listed as tables, and if the point response is measured, can be scaled to unit modal mass. The mode shapes can be inspected for complexivity and Modal Assurance Criteria (MAC and CoMAC) can be determined. Most importantly the natural frequency and damping ratio associated with each mode is obtained from the analysis. These values, together with the mode shapes can be used in Finite Element Analysis to validate a theoretical model, and provide values for the damping, without which the forced response calculations are not possible. This enables the modeller to predict the levels of vibration that would be expected from forces of known strength and frequency.

Operational deflection shapes

On a structure subject to external forces which cannot be measured, frequency response functions between a fixed accelerometer and other fixed or roving accelerometers can be animated in the frequency or time domain, thus illustrating, for example, the response of a vehicle driving over a rough surface, or bumps. This would enable visualisation of the suspension behaviour above and below the natural frequency of the suspension, for instance.

ISVR Consulting also has access to a scanning laser vibrometer which can animate operating deflection shapes in real time.

For further information contact

Ken Brown
ISVR Consulting
University of Southampton
Highfield
Southampton
SO17 1BJ
UK

Telephone 023 8059 2162 (+44 23 8059 2162 from outside the UK),
or e-mail: consultancy@isvr.co.uk