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As a space application, the brushless dc motor (BLDCM) configured in a magnetically suspended control moment gyroscope has extremely high requirements for reliability and power consumption, where accurate commutation control is the key point. However, inevitable nonideal factors, especially the nonideal asymmetric back-electromotive force (back EMF) characteristics, would dramatically reduce commutation accuracy. In order to meet the requirements, this article proposes a fast self-correction commutation method for the sensorless BLDCMs with nonideal asymmetric back EMF. First, it analyzes the influence of nonideal asymmetrical back EMF characteristics on commutation accuracy and obtains the commutation point through the zero-crossing points of line-to-line voltages. Second, a novel phase shifter is proposed for the commutation error correction based on the amplitude transformation of back EMF. Finally, based on the relationship between the voltage difference and the commutation error, a closed-loop voltage adjuster with feedforward compensation is proposed to correct the commutation error using the phase shifter. Both simulation and experimental results validated the effectiveness of the proposed method.

Most magnetic shielding rooms (MSRs) are made of highly permeable permalloy. The limitation of such MSR is its intrinsic magnetic noise, which exceeds the sensitivity of magnetic sensors, impeding the further applications of ultrasensitive magnetic sensors, such as the magnetometers operated in the spin-exchange relaxation free (SERF) regime. In this article, a cubic ferrite magnetic shield with a low magnetic noise and magnetic field is proposed. The demagnetization model and magnetic noise calculation models are given. After optimizing several different coil structures and degaussing parameters, the magnetic field and the magnetic noises inside the cubic ferrite shield were measured. Within a distance of ±10 mm near the center, the residual magnetic field is below 0.5, 1.72, and 1.57 nT at the x-, y- and z- directions. The magnetic noise measurements at the center of the cubic ferrite magnetic shield reveal an averaged magnetic noise of 2.62 fT/Hz−1/2 from 20 to 100 Hz. Compared with the noise spectrum of permalloy shield, the noise level is reduced more than 133%. The results of this scaled-down cubic ferrite magnetic shield can be applied in larger sized shielding rooms. By assembling multiple ferrite panels together, it is possible to create a larger shielding space with a low magnetic field and low magnetic noise.

Optically pumped magnetometer (OPM) has gained popularity as a viable alternative to the superconducting quantum interference device (SQUID) for biomagnetic measurements. However, the presence of amplitude and phase errors in the OPM can degrade the signal quality of, e.g., magnetoencephalography (MEG) signals. This study presents a dual-PI closed-loop control (DPCC) method aimed at suppressing amplitude and phase errors of the OPM when operated in the spin-exchange relaxation-free (SERF) regime, thereby enhancing MEG localization accuracy. The main sources of the amplitude and phase errors in the OPM are identified through a theoretical analysis of the inertial element characteristics and static residual magnetic fields. The experimental results demonstrate that the utilization of DPCC for the OPM reduces the amplitude and phase errors in static MEG measurements by 94.2% and 95.6%, respectively. At a residual magnetic field of 6.5 nT, the amplitude and phase errors are further suppressed by 97.8% and 87.2%, respectively. Furthermore, the fluctuation of the residual magnetic field is reduced by a factor of 25, significantly enhancing the robustness of the OPM. This straightforward and effective arrangement of the OPM holds promise in the field of biomagnetism and is expected to enhance the localization and calibration accuracy of OPM-based MEG (OPM-MEG) systems.

In this study, we demonstrate a three-axis synchronous sweeping field (TSSF) method for in situ magnetic field compensation under three-axis nonorthogonal conditions for optically pumped magnetometers (OPMs). The OPM operated in the spin-exchange relaxation-free (SERF) regime is gaining popularity as a promising substitute for the superconducting quantum interference device (SQUID) in biomagnetism measurements. Nevertheless, the three-axis nonorthogonality inherent in the OPM will give rise to residual magnetic field inaccuracies along the pumping axis while conducting the in situ magnetic field compensation procedure, consequently introducing supplementary OPM errors. The TSSF method can more proficiently rectify the residual magnetic field along the pumping axis and suppress the corresponding errors. In contrast to traditional methods, the TSSF method diminishes both the rotation angles of the sensitive axis and dual-axis coupling coefficients, which are introduced through compensation errors along the pumping axis, by more than 80%, and improves the measurement sensitivities by more than 30%. This effective magnetic field compensation method of the OPM holds promise for further improving the performance of OPM. It is expected to enhance the localization and calibration accuracy of multiple-channel OPM-based magnetoencephalography (OPM-MEG) systems.