Remote video based vibration monitor

Abstract

The measurement of oscillatory responses of full-scale large structures to wind, waves, vehicles, earthquakes, etc, has relied in the main on the use of in situ accelerometers (Van Nunen and Persoon, 1979; Smith, 1979; Jeary and Ellis, 1979). Problems of measurement often occur with low-frequency vibrations, even of large amplitude, in which cases analytic predictions are frequently relied upon using measured loadings (Taska and Chiu, 1976; Shiraishi, 1976). Such fixed transducer investigations are relatively inflexible and often very expensive, particularly if complex modal data are required. A range of straightforward optical techniques has been available for some time (Harris and Crede, 1961) but these are generally restricted in displacement accuracy and frequency response, although some of the current laser interferometric or Doppler systems offer more promise, (Ashkenazi et al, 1979; Field, 1981). To facilitate the problems of remote vibration measurement of large structures, a video based instrument has been developed. This was designed to produce a TV image of the object under investigation, together with the time variation of displacement within a target window superimposed on the image. This combination of a remotely obtainable TV image of the overall structure (and particular point of interest) together with its vibration signal was felt to constitute a useful addition to existing measurement techniques. System conf igurat ion