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Ground Vibration Testing

Recently, m+p international supplied a complete ground vibration test system to a reputable customer who will use the system for mobile aircraft testing.

The deal comprises the m+p SO Analyzer including the “Normal Mode Tuning” (NMT) software package, m+p VibRunner hardware with 56 input channels and 8 output channels as well as 4 modal shakers (200 N sine force). The customer will perform ground vibration testing on aircraft with a total of 8 modal shakers.

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Time history recording to throughput disk

Normal Mode Tuning uses multi-point sine excitation at a strategic choice of the exciter locations with force control and tuning of the sinusoidal inputs. The tuning objective is to excite a “pure normal mode” which means the modal frequency is strongly excited and not influenced by adjacent modes or repeated roots at an identical frequency. This method ensures the most accurate estimate of the normal mode frequency and related damping values and is not limited by FFT resolution issues.

The real-time user interface shows progress towards the optimum normal mode frequency. MIF display, Lissajous diagrams, real-time mode shape animation and Nyquist plots help the user fine-tune the excitation.  As soon as the optimum tuning is found, a high-resolution stepped sine sweep is started around the set frequency. Result data can then be processed by an appropriate tool, e.g. the „Circle Fit“ editor, to identify the normal mode frequency and related damping.

Normal Mode Tuning is an excellent method for extensive vibration ampliude testing which clearly goes beyond the possibilities of classical random excitation.

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„Normal Mode Tuning“ test set-up using two modal shakers

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