Frequently Asked Questions about Laser Doppler Vibrometer

To extract dynamic characteristics from an object, it is necessary to observe the state of the object's system in terms of vibration frequency. The input for vibration is often "excitation force," and the response is frequently measured using "displacement," "velocity," or "acceleration," although "sound," "strain," and "stress" are also sometimes used. The transfer function in this case is called the frequency response function. The three most commonly used indicators are listed below, along with mechanical impedance, and in international standards, mobility is the standard for frequency response functions (ISO 7626).

■ Bode plot

A Bode plot uses frequency as the common horizontal axis, with amplitude and phase arranged vertically on the vertical axis. Bode plots are the most widely used because they make it easy to identify the location and magnitude of resonant peaks, and allow for the distinction between amplitude and phase.

■ Nyquist plot

This method illustrates the relationship between the real and imaginary parts on a complex plane, with the real part on the horizontal axis and the imaginary part on the vertical axis. A key feature is that only the vicinity of important resonance points is magnified and displayed. On the other hand, since the frequency is not explicitly shown, it is often displayed on a curve. The CF-5200 series can search this complex plane and display the frequency. Furthermore, a three-dimensional representation with the frequency axis on the Z-axis is also possible.

■ Co-quad plot (co.quad.plot) *

This method involves arranging two graphs vertically, with frequency on the horizontal axis and the real and imaginary parts on the vertical axis. This graph has advantages such as clearly showing the phase change near the resonance point, and the resonance peak appearing as a steeper peak in the imaginary part than in a Boad plot, making it easier to identify. However, it cannot use a logarithmic scale on the vertical axis, making accurate display difficult in cases of slight damping. It is not widely used for reasons such as the fact that the magnitude and phase of vibration are not directly shown in the graph.

*Also known as a Re-Im diagram.