Environmental Noise Cancellation for High-TC SQUID-Based Magnetocardiography Systems Using a Bistage Active Shield

An active noise cancellation method is proposed for superconducting quantum interference devices (SQUID)-based magnetocardiography systems working out of magnetically shielded rooms. Using YBCO high-T
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rf-SQUID magnetometers as magnetic field sensors, an active shielding system was implemented based on this method. This method incorporates two different shielding frequency regimes of operation simultaneously. This is because the unwanted background magnetic field signals range from very low frequencies up to high frequencies with a wide range of amplitudes at the upper and lower frequency spectra. Therefore, the shielding system is designed in a bistage configuration, and each stage covers one part of the frequency spectrum. Each shielding stage is based on a coil designed and optimized by finite element method calculations. One of the coils is used for compensating high amplitude and very low frequency (0-0.1 Hz) far-field environmental magnetic field noise, and the second one is used for relatively low amplitude and higher frequency (0.1-100 Hz) magnetic field noise. Because of these frequency and amplitude characteristics, each coil uses independent control circuit. For designing the controllers, methods based on the dynamic model approximation of the first and second orders were examined, which led to optimized proportional-integral-derivative controllers. The shielding efficiency of the proposed bistage active shielding system shows a significant improved effectiveness compared to that of the conventional one stage active shielding systems.