Frequently Asked Questions about Laser Doppler Vibrometer

The most commonly used waveforms to drive vibrators are standing waves, periodic waves, and irregular waves. It is important to choose the appropriate waveform based on its characteristics for each application.

■Standing wave

This refers to steady-state excitation using a sine wave, but in practice, it is usually either slow-sweep sine wave excitation, where the frequency is slowly changed and swept, or discontinuous-sweep sine wave excitation.

(Strong Points)

• It can impart the greatest possible energy to the object. (A small vibrator is sufficient.)
• The amplitude, phase, duration, and frequency change rate of the excitation force can be precisely adjusted.
• The crest factor is 1.4, the smallest of all waveforms.
• The signal-to-noise ratio is exceptionally high.
• It is possible to characterize the nonlinearity. Sinusoidal excitation exhibits the most pronounced nonlinear effects.

(Disadvantages)

• Nonlinear effects are most pronounced in this region, and when these effects occur, the frequency response function becomes distorted or discontinuous.
• It will take time.

■Periodic wave

This is the signal format that works best with FFT. It's the most commonly used because FFT analyzers can often be equipped with a signal generator. Swept sine (chirp), periodic random, and pseudo-random signals fall into this periodic wave category.

(Common strengths)

• There is absolutely no leakage error during frequency analysis. The common advantages of these periodic waves are that the amplitude of the frequency spectrum is uniformly distributed and flat, the crest factor is small, and the response is robust to noise. The power spectral density is perfectly uniform for each data acquisition. Because the power spectral density is constant, less averaging is required for noise reduction.

(Common disadvantages)

• Harmonic distortions and amplitude dependencies caused by nonlinearities, such as structural play, recur periodically and therefore cannot be removed by averaging. Exciting objects with large nonlinearities with periodic waves will not yield good results.

Pseudo Random

This signal may appear to be a perfectly random wave, but it is actually a superposition of sine waves of equal amplitude at all sampling frequency points within the measurement frequency range of the FFT analyzer. It is a perfectly definite and periodic wave. It is a pseudo-random wave. The effects of disturbances can be removed by averaging.

(Strong Points)

• The amplitude and frequency bandwidth of the excitation signal can be freely and accurately specified.
• The amplitude of the excitation signal is constant and not biased towards a specific frequency.
• Since all frequencies are excited simultaneously and uniformly, the crest factor of the response is smaller than the swept sine.
• It can eliminate response leakage errors and resolution errors.
• The effects of external disturbances can be removed by averaging.

(Disadvantages)

• It is susceptible to nonlinear influences. Play and nonlinear effects are easily present, and because they occur periodically, they cannot be removed by averaging. Therefore, the accuracy of mode characteristic identification deteriorates.
• Since it excites the entire frequency band, the excitation energy per frequency is smaller than that of a standing wave.