When a device under test (DUT) is excited at its natural frequency (also called resonant frequency), the resulting vibration bends the device into different shapes. These shapes, called modes, are a tool for analyzing vibration, and engineers use them to determine the resonance of complex structures. Resonant frequencies are essential knowledge because they result in structural fatigue or damage if unaccounted for.
In the ObserVIEW Modal Testing software, you can capture the response of a DUT at different points to determine the modal shapes. After the data is acquired, you can analyze it in the software or export it to a UFF file and open it in an external program. One applicable program is MEscope.
MEscope by Vibrant Technology, Inc. is a series of visual engineering software packages. It contains many tools for a variety of engineering tasks. With recorded modal data, you can perform modal parameter estimation, view or animate rigid body modes, synthesize and analyze data with an array of functionalities, and much more.
Recording Modal Data in ObserVIEW
To perform experimental modal testing, the engineer first determines where on the DUT to strike with an impact hammer. In modal testing, the degrees-of-freedom (DOF) value indicates a contact point on the structure in reference to the transducer. The engineer can also determine the direction of the impact (from the x, y, or z-axis).
The Modal Testing software collects a user-defined number of responses from each DOF. It employs automatic double-hit detection, which occurs when there are multiple impulses after a hit due to structure rebound. If a second peak in the input is a certain percentage of the main peak value, it is considered a double hit and rejected automatically. The engineer defines the percentage value per their requirements.
The transducer sends the output to the data recording hardware. All VR hardware devices have recording capabilities and are compatible with ObserVIEW.
The software then calculates the average response at each DOF and generates a smooth transfer function (also called a frequency response function). The transfer function is calculated from the input signal and its corresponding output. ObserVIEW uses the fast Fourier transform to move the data from the time domain to frequency. For a linear and time-invariant system, the transfer function describes the ratio of the two signals over a defined frequency range.
Modal Analysis in MEscope
The collection of transfer functions can be exported to a modal analysis software such as MEscope. MEscope performs curve fitting to estimate the frequency, damping, and shape for each identified mode. This estimation also produces the magnitude and phase of the resonance’s strength (a value called the “residue”). The engineer can then view/interact with three-dimensional models of each mode.
MEscope also offers an array of advanced functionalities for modal analysis.
Analysis in ObserVIEW
Within ObserVIEW, the engineer can view the transfer function for each DOF. The transfer function helps to identify a resonance or an anti-resonance in a system. It displays the area around any frequencies where there is a sudden increase or decrease in spectral power. In general, the transfer function cannot be known but can be estimated.
The engineer can also add a Nyquist plot to view transfer function data as a scatter plot. The Nyquist plot is often used to assess the stability of a system with feedback.
If you have the ObserVIEW software and perform modal analysis, the Modal Testing add-on is a convenient choice. Features of the software include:
- Manage table of hits per location and review hits
- Average multiple data recordings
- Calculate decay rate from transient ring-down events
- Manage the recording channel using a large, interactive display
- View transfer function as a Nyquist plot
- And more!
Learn more about the add-on on the software page and contact your sales representative to get started.
Modal Testing Software