Development of a horizontal component seismometer using a magnetic spring

Discoveries in seismology depend on seismological-observational networks using highly sensitive/broadband seismometers, such as the STS-I. A pendulum with a long natural period is directly associated with the high sensitivity and wideband characteristics of these seismometers. The aim of this study on a vibration detector is to realize a seismometer with a sensitivity of less than several tens of microgal and a natural period of more than 100 s, which would surpass those of the STS-I. A new principle for a vibration detector, rather than a mechanical spring, which traditional seismometers use, is needed to avoid any spring-constant drift that depends on the temperature and the elastic deformation. The principle is a magnetic spring using the interaction force between a magnetic field and a permanent magnet. The magnet, which is inserted into the inner bore of a solenoid coil, behaves like a pendulum with a magnetic restoring force. The detector comprises a cylindrically shaped permanent magnet, a solenoid coil, a position detector, and a weight with a levitation system. This detector for horizontal components, which can achieve a natural period of more than several hundred seconds, has been theoretically shown by a calculation using parameters that would be possible with commercial parts. A detector using pneumatic levitation was recently tested. The results of the test showed that the natural period could reach more than 27 s without a position-feedback control system for the weight. The basic principle of the magnetic spring, that acts as a mechanical pendulum, was proved by this test. The detector showed the possibility of realizing a long natural period of more than 100 s with a position-feedback control system for the weight. (C) 2000 American Institute of Physics. [S0034-6748(00)04212-X].