Vibrometers
Introduction
A vibrometer or VLD (short form of laser Doppler vibrometer) is a measuring instrument for the quantification of mechanical oscillations in one, two and three dimensions. Lately it has been related to surface metrology of relatively medium (meso) and microscopic (micro) bodies to help do modal analysis and correlate experimental results[1] with Finite Element Analysis.
The vibrometer contains a laser, which is focused on the surface to be measured. Due to the Doppler effect, the frequency of the reflected laser light varies if the focused surface moves. This variation in frequency or Doppler shift is measured in the vibrometer with the help of an interferometer that demodulates the frequency shift and transforms this shift into a voltage signal or a digital data stream.
History
1980 to 1990
VLD Technology (LDV in English) has been used for more than 30 years. This technology was born from the need to not be able to use accelerometers in lightweight parts or systems or where large arrays of these transducers imposed restrictions on curve adjustment to complex systems given the changes in modal mass, stiffness and damping. LDV techniques historically began to be used in basic science and technical development laboratories (Universities, Institutes, etc.) and in a number of cutting-edge laboratories (IBM, Xerox) and different industries that need high precision in dynamic measurements to offer high-tech products and services. Historically the first VLDs to enter the market (before 1990) used Michelson interferometers as shown in Figure 1. This arrangement is generally called homodyne, since the two arms of the interferometer have the laser frequency as a base frequency. This simple scheme allows the interference pattern to be carried out on the detector arm (in Fig 1 it is described as screen).
This technology allows for simple measurements in which displacements are set in a relatively restricted range, frequencies are low, and acceleration levels are below 10,000 gs. During that period, invisible infrared lasers of more than 1mW were used, another laser was used as visual guidance. This was an obstacle to user safety both because of the power of the laser and because it was invisible.