An Electromagnetic Device for Lengthening the Natural Period of Torsional Vibration Pickup

A simple electromagnetic device is developed for lengthening the natural period of torsional vibration pickup, utilizing a conventional eddy-current damper which is one of the essential elements of the seismic pickup of long natural period. A rectangular coil of small number of turn, connected to a DC source, is bonded on a surface of the plate conductor of the damper in such a way that two facing sides of the coil intersect respectively leakage fluxes existing symmetrically on both sides of the permanent magnet of the damper. By proper dimensioning of the coil, each of two facing sides travels inside a range within which the leakage-flux density varies with respect to the travel of coil as linearly as possible; thus a controllable negative restoring torque is electrodynamically generated to lengthen the natural period, and the resulting natural period can be pasily controlled by adjusting the coil current. The principle of operation of the device and limiting factors for lengthening the natural period are discussed in detail. Under a laboratory condition, the natural period Of 1 s can be lengthened to as long as 7 s.