Frequently Asked Questions about Laser Doppler Vibrometer

The object being measured and Laser Doppler Vibrometer are in a non-contact relationship. This means that the object being measured and the sensor are affected by background vibrations (background noise) separately.

Although the frequency of this background vibration is low, the displacement is surprisingly large. When measuring objects with high vibration frequencies and minute displacements, the amplitude of the background vibration can consume the dynamic range of the display or analyzer, making it impossible to achieve accuracy and resolution.

■When background vibrations are strong, "speed" is effective.

If the dark vibrations are so large that the target amplitude waveform is obscured by the dark vibrations even when increasing the voltage range of the FFT analyzer or oscilloscope, we recommend observing using the "velocity" output rather than the "displacement" output. Even if you measure using "velocity," it is possible to convert it to displacement through integration.

The advantage of measuring by "velocity" is that it is less affected by background vibrations. This is because, although the displacement of background vibrations is large, their frequency is low, meaning their velocity is low. Measuring by "velocity" becomes more advantageous as the observed frequency increases.

For example, when measuring an ultrasonic horn for ultrasonic welding;

When comparing the background vibrations with the vibrations of the ultrasonic horn, there is a 500-fold difference in displacement, and the displacement of the ultrasonic horn is output superimposed on the large background vibrations. The analyzer's range is set to match these background vibrations, and it is not possible to accurately capture the minute displacements of the horn in the analyzer.

In contrast, with velocity output measurement, the output of the object being measured is greater, allowing the analyzer's range to be adjusted to match the object. Measuring by velocity first and then integrating it using an FFT analyzer to convert it to displacement results in more accurate measurements. As in this example, for objects with high vibration frequencies and minute displacements, velocity measurement is effective.