Technical review: Mr. Sun, Technical Manager
Summary: Non-contact modal testing uses optical vibration measurement to obtain spatial response data without attaching sensors to every point. Reliable results depend on boundary conditions, point geometry, excitation, synchronization, signal review and the correct analysis method.
1. Define the Engineering Objective
Start with the decision the test must support: identify natural frequencies, estimate damping, visualize operating deformation, validate a finite-element model or diagnose a structural change. The objective determines the required inputs, spatial density and method.
2. Establish Boundary Conditions
Free-free, fixed, suspended and in-service boundaries produce different dynamics. Record fixtures, suspension points, preload, temperature and operating state. A modal result without its boundary conditions is difficult to reproduce or compare.
3. Plan and Register Measurement Points
The grid must represent the geometry and expected deformation. Include points near antinodes, interfaces and regions of local flexibility. Maintain a clear coordinate system and verify that the laser can reach each point with acceptable incidence angle and return.
4. Select Excitation and References
| Method | Typical input | Suitable use |
|---|---|---|
| EMA | Measured hammer or shaker force | Controlled tests requiring FRFs and input-output modal parameters |
| OMA | Operational/environmental excitation, input not measured | Structures tested under realistic operating conditions |
| ODS | Measured operating response, often with a phase reference | Visualizing deformation at selected operating frequencies |
5. Configure Acquisition
Set bandwidth, sampling frequency, record length, window, averaging, trigger and reference channels according to the lowest and highest modes of interest. Preserve synchronization when phase or transfer functions are required.
6. Review Data Quality Before Analysis
- Check optical signal level and dropouts at every point
- Review clipping, noise floor and dynamic range
- For EMA, inspect coherence and force-window settings
- Repeat selected points to assess stability
- Confirm that operating conditions remained consistent during a scan
7. Identify and Validate Modal Results
Use method-appropriate indicators such as stabilization diagrams, singular-value spectra, mode-shape consistency and MAC. Separate structural modes from harmonics, rigid-body motion, optical artifacts and operating-order content.
Recommended Report Content
A useful report includes structure and boundary conditions, hardware and calibration status, point map, acquisition settings, excitation, data-quality decisions, modal parameters, mode shapes, uncertainty or limitations, and links to source data.
Frequently Asked Questions
How many scan points are required?
There is no universal number. Point density must represent the shortest spatial wavelength and local features relevant to the modes of interest.
Can non-contact testing measure damping?
Yes, when excitation, signal-to-noise ratio, record length and identification method are suitable. The report should state the estimator and test conditions.
When should OMA be used instead of EMA?
OMA is useful when controlled excitation is impractical and the structure can be measured under representative operational excitation. EMA is preferred when measured inputs and FRFs are required.