A Novel Noncontact Failure Detection Method for Vehicle-Mounted HTS Bulks Based on Null-Flux Coils: Scheme, Theory, and Experiment

High-temperature superconducting (HTS) pinning maglev technology is considered promising for high-speed transportation due to its self-stabilizing characteristics, and it has been widely studied worldwide. In high-speed engineering applications, safety is a primary concern, largely dependent on the reliable flux-pinning capability of the onboard superconductors. This paper proposes a non-contact, non-destructive, and easy-to-implement real-time monitoring method for assessing the flux-pinning state of HTS bulks onboard maglev vehicles, based on the anti-symmetric property of null-flux coils. The configuration, working principle, and mathematical model of the method are established, along with a coil geometry optimization strategy and a tailored low-frequency, narrow-band filtering circuit. An experimental setup is then built based on the optimized parameters, and a series of tests are performed to the method’s feasibility. Furthermore, the effectiveness of the resonance-frequency-based filtering circuit in enhancing signal extraction is analyzed, and theoretical predictions of the relationship between the voltage signal and factors such as displacement, working height, and degradation level are validated experimentally. This study provides a comprehensive investigation from concept to validation and demonstrates strong potential for real-time safety monitoring in HTS maglev systems. It can also offer useful insights for other superconducting or maglev monitoring applications.