Xiuqi Zhao

Ultra-weak magnetic experimental environments require high magnetic-field stability, whereas passive magnetic shielding alone cannot fully suppress geomagnetic disturbances. This study presents a high-precision triaxial active geomagnetic compensation (AGC) system that combines optimized compensation coils with adaptive closed-loop control. The parameters of square compensation coils are optimized using particle swarm optimization to improve field uniformity while satisfying practical geometric constraints. A two-stage compensation architecture is implemented, in which DC coils compensate the static geomagnetic background and AC coils suppress time-varying residual disturbances. To enhance robustness under changing environmental conditions, a fuzzy adaptive PID controller is developed to adjust the control gains online according to the field error and its rate of change. Experiments show that the DC compensation stage reduces the residual magnetic-field fluctuation to within ±30 nT. With fuzzy adaptive PID control, the peak-to-peak fluctuations along the north–south, vertical, and east–west directions are reduced to 0.91, 1.53, and 1.16 nT, respectively, representing reductions of 57.6%, 75.4%, and 41.2% compared with conventional PID control. These results demonstrate the improved accuracy and stability of the proposed AGC system for ultra-weak magnetic experiments.

Low-frequency and power-frequency magnetic field disturbance in the environment severely affects the measurement of weak magnetic signals, while traditional active magnetic compensation (AMC) methods exhibit insufficient suppression capabilities for such disturbance. To address this issue, this paper proposes a novel proportional-four-integral control (P-FIC) AMC system (AMCS). Firstly, the mathematical model of the AMCS based on the P-FIC is established, and then the structure and transfer function of the P-FIC are presented. Subsequently, the disturbance suppression capability and stability of the P-FIC system are analyzed through simulations of the sensitivity function and closed-loop transfer function. Finally, the experimental results demonstrate that, compared to the traditional control methods, the proposed P-FIC method exhibits superior performance in suppressing low-frequency and power-frequency magnetic disturbance, significantly enhancing the system’s disturbance suppression capability.

Weak magnetic fields are essential environmental conditions for sensors used in magnetic biological measurements. However, traditional demagnetization method proves insufficient in reducing the unidirectional residual field inside magnetic shielding box caused by the gap in the door seam. This paper proposes a novel demagnetization method incorporating DC bias adjustment and optimizing the demagnetization coil design to effectively suppress the residual field. Experimental results demonstrate a 34.4% reduction in the residual field compared to the traditional demagnetization method. The proposed approach is well-suited for enhancing magnetic shielding performance in applications requiring ultra-low magnetic environments.

There are two problems in the low-noise magnetic field compensation of desktop small magnetic shielding boxes (SMSBs). First, the uniformity of the magnetic field generated by the coil is reduced due to the strong coupling effect of the ferromagnetic boundary. Second, the larger coil constant requires a highly accurate current source. To improve uniformity and reduce the coil constant, an optimization design method with an ultrasmall coil constant biplane coil (BC) based on the same trend coupling effect of the ferromagnetic boundary is proposed. In this article, for the first time, the relationship between coil constant and shielding distance is analyzed by constructing the magnetic field and distance (M-D) function. The parameters of the primary coils and the secondary coils are optimized by particle swarm optimization (PSO). The combined BCs (CBCs) with an ultrasmall coil constant are designed based on the double image method and M-D function. Compared with traditional BCs, the ${B}_{x}$ and ${B}_{y}$ coil constants of CBCs are reduced by two orders of magnitude to 41.8 and 35.4 nT/A, respectively, and the $ {B}_{z} $ coil constant is reduced by three orders of magnitude to 49.9 nT/A. The uniformity of the magnetic field in the target area is improved by 4.4 times in the X-direction and 7.4 times in the Z-direction. The peak-to-peak value of the magnetic field is reduced by 24.7%, and magnetic field noise is reduced by 49% at 1 Hz in the closed loop.

The closed-loop optically pumped magnetometer (CM) is a non-cryogenic sensor with high dynamic range, wide bandwidth, and high sensitivity, and has broad application prospects in the field of cardiac magnetic field measurement. The measurement accuracy of the CM depends on the signal tracking capability of the system. Conventional CM have insufficient tracking capability for quasi-periodic signals. It limits the measurement accuracy of the CM for quasi-periodic signals such as cardiac magnetic field. Based on this problem, a novel method is proposed in this paper. The method introduces parallel multiple second-order generalized integrals (MS) in the proportional integral-cascade extended state observer (PI-CLESO) to enhance the tracking capability of the CM. In the experiment, the CM’s measurement accuracy of the cardiac magnetic field is improved by 70% by the proposed method. This method is not only applicable to the high-precision measurement of the cardiac magnetic field, but also has greater potential for application in the measurement of other quasi-periodic weak magnetic field.