Top 10 Tips for Modal Testing and Analysis

1) Define the objective(s) of the test, taking into account both the technical requirements and the availability of resources.

· Quick scan of a given frequency range to identify the main resonances

· Visualisation of the operating deflection shapes

· Identification of the modes in terms of both natural frequencies, mode shapes & damping

· Validation of a finite element model

· Use of measured data for further vibration analysis

· Assessment of non-linear behaviour

2) Select an appropriate excitation type and signal. Hammer excitation has the advantage of being easy top use while sine testing with a shaker may be required to acquire high quality response data.

3) If you have a finite element model of the structure, use program MODPLAN for test planning. It will provide guidance for the selection of suspension, excitation and response locations. Use program MESHGEN to generate the measurement grid.

4) Perform an AutoMAC for the selected response co-ordinates to ensure that spatial aliasing will not occur. This is particularly important if the results of the modal analysis are going to be used for FE model validation.

5) Calibrate your response transducers by measuring the frequency response function of a known mass. If the test is interrupted for any significant amount of time, check the calibration before continuing.

6) Conduct a preliminary test before starting to acquire your production data.

· Experiment with a number of suspension schemes and try several excitation locations.

· If possible, perform linearity checks using sine excitation at several force levels.

· Perform reciprocity checks by swapping the response and excitation locations.

· Check your force and response signals in the time domain and inspect the window settings.

· If appropriate, include an oscilloscope in the test setup.

· Select the frequency resolution such that there are enough data points around each resonance for modal analysis.

7) Use program MODENT for modal analysis. Identify the modes of the point FRF using SDOF curve-fitting techniques such as circle-fit and line-fit. Establish if there are any close modes. Such information may prove to be very useful when using global curve-fitters, both for defining the analysis range and for checking the results.

8) Animate the modes during the modal analysis and look for signs of something having gone wrong. For instance, a stationary point may indicate faulty cabling/transducer. A point that is moving with a much larger amplitude than the rest may indicate a calibration error. Inspect the amount of complexity of the identified mode shapes.

9) Use program MODENT to check the orthogonality between the identified mode shapes and the original FRF data via the reciprocal modal vector technique.

10) Use program MODESH for ODS animations of the measured FRFs. Compare these with mode shape animations. If you have a finite element model of the structure, use MODESH to correlate the FE predictions and the measurements.