DYNAMICAL CANCELLATION OF THE ENVIRONMENTAL MAGNETIC DISTURBING FIELDS

A cubic Helmholtz-coil triaxial system, with a mean side of 2 m and a volume of uniform field of 0.2 m(3), allows the compensation of both t he main ambient magnetic field and the urban magnetic noise, in the fr equency band of 0-10 Hz. The main ambient magnetic held can be cancell ed, or controlled, by a stabilised power supply. The alternating distu rbing fields are compensated by a triaxial magnetometric system. The t riaxial sensor unit is placed near the homogenous volume of the Helmho ltz system. The feed-back loops of the magnetometer channels are close d using special windings of the triaxial Helmholtz system. This config uration enables one to cancel out the simulated disturbances at a rati o of 1 x 10(2)-5 x 10(2) and the industrial noise al a ratio of 1 x 10 (2). Functional studies of the magnetometric sensors and magnetometric devices can be performed in the homogenous volume created by the tria xial Helmholtz system. A system of cylindrical multilayer shielding ca n be placed in the volume of the dynamically compensated field; thus t he ratio of magnetic held cancellation becomes 10(4). (C) 1997 Elsevie r Science S.A.