Introduction

Laser vibrometers offer a highly advantageous solution for modal analysis due to their non-contact measurement capabilities. Traditional vibration measurement techniques often rely on contact sensors such as accelerometers, which can influence the dynamics of lightweight or sensitive structures by adding mass or altering boundary conditions. In contrast, laser vibrometers measure surface vibrations remotely using laser beams, thereby eliminating any mechanical interaction with the test object. This ensures that the structural integrity and dynamic behavior remain unaltered, leading to more accurate and representative modal data.

Another significant benefit of laser vibrometers is their ability to measure vibrations over a wide frequency range with high spatial resolution. Laser vibrometers can be easily focused on specific points of interest on the structure, enabling detailed modal shape mapping across complex geometries. When used in scanning systems, laser vibrometers can automatically acquire vibration data at multiple points across a surface, greatly facilitating experimental modal analysis (EMA) for large or intricately shaped components. This capability makes them especially valuable in aerospace, automotive, and micro-electromechanical systems (MEMS) applications where precise modal characterization is critical.

Moreover, laser vibrometers offer practical advantages in terms of setup and safety. Since the measurements are performed remotely, they can be conducted on components in harsh or inaccessible environments, including rotating machinery or high-temperature systems. This reduces the risk to operators and allows measurements in scenarios where traditional sensors would fail or degrade quickly. Additionally, the ease of alignment and operation of modern laser vibrometers contributes to reduced test times and increased productivity in laboratory and field settings.

The Luma-600 Laser Vibrometer from Crystal Instruments represents a significant advancement in non-contact vibration measurement technology, particularly suited for modal analysis applications.

Non-Contact Measurement Advantages

Laser vibrometers, such as the Luma-600, utilize the laser Doppler effect to measure vibrations without physical contact. This non-intrusive approach eliminates the mass loading effect associated with traditional contact sensors like accelerometers, ensuring that the natural dynamic behavior of the structure remains unaltered during testing. This is particularly beneficial when analyzing lightweight or delicate structures where added mass could significantly affect the results.

High Precision and Spatial Resolution

The Luma-600 is designed to offer high-precision measurements across a broad frequency range, making it suitable for capturing detailed modal data. Its capability to focus on specific points allows for high spatial resolution in vibration measurements, facilitating the accurate mapping of mode shapes across complex geometries. This level of detail is crucial in identifying and analyzing the vibrational characteristics of structures in various engineering applications.

Versatility and Ease of Use

Modern laser vibrometers like the Luma-600 are engineered for versatility, capable of operating effectively in diverse environments, including those that are harsh or difficult to access. Their non-contact nature allows for safe and efficient measurements on components in motion or at elevated temperatures. Additionally, the user-friendly interface and streamlined setup process reduce testing time and enhance productivity, making them valuable tools in both laboratory and field settings.

Modal Analysis Setup

A canterleved plate bolted down on a solid base will be tested to find out its modal parameters. The complete setup is illustrated as following photo.

The canterleved plate is meshed into 5x8 number of cells, with total of 54 measurement points. Points 46 through 54, located along the bottom edge of the plate, were excluded from measurement. The out of plane motion along the +X direction will be measured. The mesh model is shown as follows.

Figure 2. Mesh model of the canterleve plate

The plate structure is excited using a modal shaker connected via a stinger, with an impedance head mounted at the driving point to measure the input force, and acceleration response.

Figure 3. Modal shaker and impedance head connected through stinger

The response velocity is measured using the Luma-600 laser vibrometer. The laser vibrometer is roving through all of the measurement DOFs.

The test parameters in EDM Modal were configured as follows:

Frequency range: 720 Hz
Block size: 4096
Number of average: 8
Excitation type: Burst random, with 0.1 Vrms, 80% burst rate

Testing Procedure

Velocity measurements is taken at degrees of freedom from +1X through +45X sequentially. A test plan is used to manage the testing process, with points +46X through +54X excluded. The following screenshot illustrates measurement at point +41X.

Figure 5. Modal testing screenshot at test entry No. 41

The frequency response functions (FRFs) from all measurement entries are shown below.

Figure 6. FRFs from all the measurement entries

Modal results

Modal parameter identification is carried out on the FRF data set. Poly-X curve fitter is selected to identify the modes.

The first bending mode is shown as follows,

The second mode is the torsional mode,

While the 3rd mode is the 2nd bending mode,

Summary

Modal analysis of a cantilevered plate was successfully performed using the Luma-600 laser vibrometer. The results align with expectations and confirm the effectiveness of the Luma-600 as a non-contact vibration measurement tool for accurate and efficient modal testing. Its precision, ease of use, and non-intrusive nature make it an invaluable asset in modern modal analysis workflows.

Modal Analysis using Luma-600 Laser Vibrometer