Most general waveforms used for driving a vibrator include stationary waves, periodic waves, and irregular waves. It is important to use each waveform, according to the characteristics and case.

 

 

Stationary wave

 

A stationary wave refers to a regular excitation by a sine wave. However, actually, a stationary wave generates low-speed sine wave excitation by sweeping through the slow change of frequency or discontinuous sine wave excitation.

 

(Advantages)

• The maximum possible energy can be applied to the object. (A small vibrator is adequate.)

• The amplitude, phase, duration of excitation force, and frequency variability rate can be accurately adjusted.

• The peak factor is 1.4, which is the lowest among all the waveforms.

• The S/N ratio is by far the largest.

• Nonlinearity can be characterized. In sine wave excitation, the impact of nonlinearity is highest.

 

(Disadvantages)

• Since the impact of nonlinearity is most prominent, the impact may result in distortion of the frequency response function or discontinuity if such impact appears.

• It takes too long.

 

 

Periodic wave

 

This signal mode is most compatible with FFT. This mode is most generally used since a signal generator can be installed in the FFT analyzer. Periodic waves include swept sine (chirp), periodic random, and pseudo random.

 

 

(Common advantages)

• No leakage error occurs at frequency analysis. The common advantages of these periodic waves are uniform and even distribution of frequency spectrum amplitudes, low peak factors, and strong noise-resistant response. The power spectrum densities of imported data at a time completely match. Since the power spectrum density is constant, less noise elimination averaging is required.

 

(Common disadvantages)

• Harmonic wave distortion that is generated by non-linearity such as a structural looseness and amplitude dependency cannot be removed by averaging processing since the same item appears repeatedly cyclically. A satisfactory result cannot be obtained if an object of large non-linearity is excited with a periodic wave.

 

 

Pseudo Random

Although this signal appears to be a complete random wave, the wave is a complete definite periodic wave, which is formed by overlapping of sine waves of equal amplitude at the full sampling frequency point. This is a pseudo random wave. The impact of external disturbances can be eliminated by averaging.

 

(Advantages)

• Amplitudes and frequency bands of excitation signals can be given freely and accurately.

• Amplitudes of excitation signals are constant and do not lean toward a specific frequency.

• Since all the frequencies are excited simultaneously and evenly, the response peak factor is lower than that of swept sine.

• Response leakage errors and resolution errors can be eliminated.

• Impact of external disturbance can be eliminated by averaging.

 

 

(Disadvantage)

• Easily influenced by non-linearity. The frequency is susceptible to the impact of looseness and non-linearity and the impact cannot be eliminated by averaging since they appear cyclically.

• Since all the frequency bands are excited, excitation energy per frequency is smaller than that of a stationary wave.