Abstract
This paper demonstrates an amazing ability of a developed seismometer-type absolute displacement sensor aimed at detecting earthquake waves with a large magnitude and long period. Since the measuring range of the displacement sensor is higher than the natural frequency of itself, it is difficult to detect a low frequency vibration below 1 Hz using conventional seismic-type displacement sensors. Therefore, for detecting earthquake waves with a long period, acceleration sensors are used commonly. However for obtaining absolute displacements by the acceleration sensor, it is necessary to integrate twice of the acceleration signal. Then the drift problem guided to signal saturation will be happen essentially in the acceleration process by unknown signals. This seismometer-type absolute displacement sensor is consisted of a sensor body with a moving mass and its supporting springs, feedback control circuits and a third order phase lag compensator. The natural frequency of the sensor body located at 6.8Hz originally is reduced to 0.23 Hz using by acceleration feedback mainly and then moving stroke of the moving mass is compressed to 1mm to 1/630mm. Moreover the new natural frequency of 0.23Hz developed by feedback control is sifted to 0.065Hz by the third order phase lag compensator. Therefore the sensor has a measuring range from 0.08Hz to 10Hz, in addition to no drift problems by the use of feedback control way. In this way, the seismometer-type absolute displacement sensor with the amazing ability has been realized to measure the long period over 10 second of earthquake waves. In addition, the large magnitude to measure 630mm is expected, nevertheless a small size with moving stroke of 1mm. So far as we know, such a sensor for measuring absolute displacement is very novel in the world. In this paper the ability of the sensor will be shown by simulation and experiment.
