Short-period seismometer Willmore

for vibration measurementvery high-sensitivity

The basic Willmore Adjustable-period Seismometer is a velocity sensing devise suited for field or observatory use. The mass is a permanent magnet, which is suspended on ligament springs giving functionless axial movement but restraining it from movements in other planes. When used in the vertical position, the mass is counter-balanced by springs, which are connected to the frame via a lever arrangement that imparts minimal stiffness on axial movement. These counter-balance springs are separated from the mass when the instrument is used horizontally. The output is derived from two coils fixed to the frame, one at each end of the mass. Auxiliary coils are provided for feedback and calibration. The natural period of oscillation of the mass is adjustable so that the instrument may be optimally matched to the required operation conditions. As options on the basic MK111A there are models with a built in low powered preamplifier. By including the amplifier within the seismometer housing the chance of external interference with the signal is minimised. Adjustable electronic feedback damping is included to give the user, besides the noise reduction of the technique, an instrument capable of detecting earth motion from the micro-seismic to the strong motion range. DC drift may be effectively eliminated by the servo control which also acts as a low frequency cut off. The feedback term may also be shaped in frequency response to give the characteristics of a long period instrument.