Experimental Study of the Active Compensation to a Full-Size Separate-Shell Magnetic Shield

Shielding efficiency of a full-size separate-shell magnetic shield equipped with active compensation coils is presented. Its size is 135 cm in height, 100 cm in width, and 240 cm in depth. With respect to active compensation two control schemes, i.e., feedback and feedforward, are compared. The shielding efficiency is estimated from the measurements of the attenuation of environmental magnetic fields in two axial directions (horizontal and vertical). In the case of feedback, a reference sensor that measures the incoming magnetic fields is positioned inside the shield, whereas in the case of feedforward the reference sensor is positioned outside the shell. In both axial directions, the environmental magnetic noise can be reduced to less than 500 pT: the noise floor measured by a fluxgate magnetometer is approximately 10 pT/¿Hz in the frequency range from 10 to 100 Hz.