Application Note Normal Mode Tuning Introduction Modal testing is performed to characterise a structure’s dynamic properties in terms of resonant frequency, damping and modeshape. ■■Resonant frequencies amplify input force so they need to be controlled to ensure vibration problems do not result. For example, a serious vibration problem will result from helicopter blade pass frequencies coinciding with structural modes. ■■The damping quantifies how much dynamic amplification will occur. This can be expressed in different ways including damping ratio, loss factor and Q. ■■ The modeshape describes the amplitude and phase at each measurement point on the structure. A bi-product is the familiar computer animation of deflections, as shown below: These properties are measured by ‘exciting’ the structure with external forces. m+p international support all the established methods, as follows: ■■Hammer excitation ■■Step relaxation ■■Single-point random Application Note ■■Multi-point random ■■Single-point sine ■■M

Background Each method has a different application depending on the objective of the test and the time available. For example, impact testing with a hammer, which is quick and easy, is ideal for studying a car’s exhaust system’s vibration problems but is wholly unsuited to the safety requirements of the aircraft industry where the objective is more likely to be correlation of an FE model and clearance to fly. For the more onerous requirements of the aircraft and space industry m+p international have implemented ‘Normal Mode Tuning’. This method uses Multi-point sine excitation but with an added...

INTERNATIONAL Normal Mode Tuning The following screenshot shows m+p’s Normal Mode Tuning user interface. A great deal of emphasis has been placed on interactivity to maximise the potential of SO Analyzer’s real-time displays. In this example the MIF has been inverted (1-MIF) so that a value greater than 0.9 is the objective. GtITIMG SIARTtD - User - m+p mtemanonalSO Arvnvre EC»t Vww Analysis AcQUSOon Windows Help- Frot«a n Jbl & . . o*spfcv n jU Dhl j, v ~ = ! * Si-!*! Analyzer U Source Control Prooo sting Normal Mode Tuning

The following features can be seen in the above plot: ■■The MIF (right-hand chart) is plotted real-time to determine the purity of the mode. ■■The frequency can be adjusted to maximise the MIF. ■■The force and phasing can be adjusted real-time. In this example Force Channel 1 = 2 Newtons @ 0 degrees and Force Channel 2 = 2 Newtons @ 180 degrees. ■■The forces are under closed-loop control. ■■Once the frequency and forcing has been optimised the frequency can be stepped through the resonance in an upward or downward direction. ■■The centre frequency whilst stepping through the frequency range will...

INTERNATIONAL The following plot shows an example of circle-fitting in a modern context. Summary In summary, m+p international have taken an important yet very traditional test approach, Normal Mode Tuning, and brought it into the 21st century with real-time interactivity and real-time displays to improve data quality and consistency whilst reducing test time. Germany m+p international Mess- und Rechnertechnik GmbH Phone: (+49) (0)511 856030 Fax: (+49) (0)511 8560310 [email protected] United Kingdom m+p international (UK) Ltd Phone: (+44) (0)1420 521222 Fax: (+44) (0)1420 521223 [email protected]...