The modal testing experiment presented here is performed by one branch of SIEMENS Medical using ModalVIEW software and National Instruments DSA hardware as the modal testing and analysis tool. The analysis result provides instructive guidance on the design of their products.

Experiment scenario

The testing object is a steel frame for magnet coil. There are some challenges to perform modal testing of the structure:

The structure is symmetrical. Vibrations from multiple directions need to be measured to investigate complex mode shapes. There are closely spaced and conjugate modes existing. The structure is large and heavy. The vibration signal of measured points far away from excitation point is weak. The dynamic range of measured signals is large.

To identify closed spaced and conjugate modes of this symmetrical structure, accurate modal analysis method of poly-reference, multiple DOF, global fitting is required. ModalVIEW modal analysis software provides advanced modal analysis method based on mode stabilization diagram to extract modes from poly-reference FRF measurements, which meet testing requirements. National Instruments 24-bit DSA hardware PXI-4498 is used to guarantee the accurate measurement of weak vibration signal of measured points far away from excitation point. The scalability of NI DSA hardware also guarantees the investment of whole modal testing system.  

System configuration

In this modal testing experiment, the following software and hardware are used：

1．ModalVIEW1.0 Software.

2．NI DSA System (Low noise chassis PXI-1042, Dual-core controller PXI-8106, 16 channel DSA module PXI-4498)

3．Hammer (Lance LC1302) and Charge amplifier (Lance LC0106).

4．Tri-axial accelerometer (Kistler 8690C50).

 Analysis results

Modal analysis for the structure is finished with ModalVIEW’s advanced mode estimation method based on mode stabilization diagram。The following figure shows the clear mode stabilization diagram from 48*3 measured FRF data.

The closely spaced modes can be selected from stabilization diagram. The real modes are kept with mathematical or faked modes being discarded. Part of the picked meaningful modes are listed in following table :

The following figure displays the plot of synthesized FRFs from estimated modes (red line) and measured original FRFs. The synthesized FRFs should match original FRFs well, which indicates that estimated modes are accurate enough.  

One of the mode shapes is displayed as following :

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