Frequently Asked Questions about Laser Doppler Vibrometer

The waveforms used to drive the vibrator are mainly steady waves, periodic waves, and irregular waves. It is important to differentiate and use each waveform according to the case.

Stationary wave

It refers to steady-state excitation by a sinusoidal wave (sine wave), but in practice, it is common to have either a low-speed sweeping sine wave excitation that slowly changes frequency or a discontinuous sweeping sine wave excitation.

Strengths

• It can provide the greatest energy to the target object. (A small vibrator is sufficient.)
• It is possible to accurately adjust the amplitude, phase, duration, and frequency change rate of the vibration force.
• The wave height ratio is the smallest at 1.4 among all waveforms.
• The S/N ratio is exceptionally large.
• Nonlinearity can be characterized. Sinusoidal excitation shows the most significant effects of nonlinearity.

disadvantages

• The effects of nonlinearity become most apparent, causing the frequency response function to become distorted or discontinuous.
• It takes time.

Cyclic wave

These are the signal types that are most compatible with FFT. Since FFT analyzers can be equipped with signal generators, these are the most commonly used. Swept sine (chirp), periodic random, and pseudo-random fall under this category of periodic waves.

common strengths

• There is no leakage error at all during frequency analysis. The common advantages of these periodic waves are that the amplitude of the frequency spectrum is uniformly distributed and flat, the wave height ratio is small, and the response is strong against noise. The power spectral density for each data capture is completely uniform. Since the power spectral density is constant, less averaging is needed for noise removal.

common shortcomings

• Harmonic distortion and amplitude dependence caused by nonlinearities, such as the rattling of structures, cannot be removed through averaging processes because the same phenomena repeat periodically. Applying periodic waves to large nonlinear objects does not yield good results.

Pseudo Random

This signal appears to be a completely random wave at first glance, but it is actually a superposition of sine waves with equal amplitudes at all sampling frequency points within the measurement frequency of the FFT analyzer, making it a completely deterministic wave and a periodic wave. It is a pseudo-random wave. The effects of disturbances can be eliminated through averaging.

Strengths

• It is possible to freely and accurately provide the amplitude and frequency bandwidth of the vibration signal.
• The amplitude of the excitation signal is constant and does not bias towards a specific frequency.
• Since all frequencies are excited simultaneously and evenly, the response amplitude is smaller than that of a swept sine.
• It is possible to eliminate both response leakage errors and resolution errors.
• The effects of disturbances can be eliminated through averaging.

disadvantages

• It is easily affected by nonlinear influences. Irregularities and nonlinear effects tend to occur, and since they are periodic, they cannot be removed by averaging. Therefore, the accuracy of identifying mode characteristics deteriorates.
• Since the full frequency band is excited, the energy per frequency is smaller than that of a standing wave.