Investigation on the frequency response characteristics of an instrument for laser vibrometry and laser displacementmetry

Abstract

This paper proposes the new technique for the frequency response characterization of the laser vibrometry or the laser displacementmetry, where the optical interference is the working principle. In order to investigate the frequency response of laser vibrometers or laser displacement meters, the surface of high speed translational motion with the broad frequency bandwidth is absolutely required. That motion is materialized by the reflection of an elastic pulse propagating in a metal bar. A projectile made of aluminum is accelerated by the pressurized air and impinges on one end surface of the bar. The elastic wave is generated by the collision and propagates in the bar axis direction and reflects at the other end surface of the bar. The bar is supported by four steel bearing balls which are placed on the V-shaped grooves. The motion of the end surface can be considered to be in plain based on the numerical calculation. The surface is measured simultaneously using the reference laser interferometer developed in NRLM and a laser displacement meter based on the heterodyne technique (Hoshin Electronics HS-1100). HS-1100 uses the real time fringe counting technique. The reference interferometer, on the other hand uses a transient recorder (Tektronix RTD-710) which stores all of the interference signals during the elastic pulse reflection. The analysis and the wave form calculation based on the phase analysis is done after the experiment, leading to the broad frequency bandwidth. The comparison in the frequency domain using Fast Fourier Transform provides the frequency response characteristics of the tested laser displacement meter. It turned out that bandwidth of HS-1100 was up to 20 kHz, though the design value was 100 kHz.