The Problem: To Identify and Characterize Vibration Modes

The K100/HOL optical head using the HoloFringe300 interferometry program allows quick identification of vibration modes of a test structure as it is subjected to a vibratory excitation. The vibration modes show up as Bessel function fringes. These fringes are dark bands on the image that read like a topographical map showing the peaks and valleys of the vibration amplitude. The program controls a dual frequency generator via the PC host computer to provide excitation. The second signal can be used to produce wrapped phase fringes, which are easily converted to numerical data. No scanning of the object is required, resonant frequencies are easily located, and resonant bandwidths can also be obtained.

1a	1b
1c	1d

Figure 1. Vibration Mode Analysis of a Rectangular Plate. Fig 1a shows the vibration mode of the plate as a set of zero-order Bessel function fringes. This is what the system displays in real-time for mode identification. Fig 1b shows that fringe pattern converted to the wrapped phase function. Fig 1c shows the unwrapped phase function, otherwise known as the fringe locus function. Fig 1d shows a 3D plot of the vibration mode. Note that the plate has four holes in it that present no problem to the unwrapping program. (Full size images)