Nonlinear Suppression and Efficiency Improvement of Electromagnetic Acoustic Transducers: A Permanent Magnet Bias Design
Liu Jiefeng1, Liu Weicheng1, Gao Bing2, Xu Qianming2, Zhao Xinzhe2
1. Department of Electrical Engineering Guangxi University Nanning 530004 China; 2. State Key Laboratory of High-Efficiency and High-Quality Conversion for Electric Power Hunan University Changsha 410082 China
Abstract：Electromagnetic transducers (EMTs) are widely used in underwater acoustics and marine geophysical exploration. However, traditional EMT designs often suffer from input-output frequency-doubling effects, which lead to additional core losses, reduced energy efficiency, and shortened service life. To address this issue, this study draws inspiration from the structure of permanent-magnet motors and proposes a topologically optimized permanent-magnet configuration integrated into the excitation magnetic circuit. By adjusting the flux path and biasing the magnetic field distribution, the system achieves synchronized input-output response under rated operating conditions. A dynamic modeling method is developed to analyze the distributed permanent- magnet bias field, and zonal magnetic-circuit analysis is employed to quantitatively evaluate the influence of local bias gain on output performance. Based on the topological structure of permanent magnet motors, this study embeds a spatially optimized permanent magnet array into the EI-type excitation magnetic circuit to construct a static bias magnetic field system. This design can stably constrain the transducer's operating point within the single-quadrant magnetization range, thus fundamentally suppressing the frequency-doubling effect. To clarify the regulatory mechanism of permanent magnets on transducer performance, an electromagnetic-mechanical multi-field-coupled dynamic model is established to analyze the bias gain characteristics of permanent magnets across different regions (the coil bottom, magnetic pole air gap, and slot) and their influence on the transducer's output consistency. Although introducing permanent magnets into the traditional excitation structure can effectively optimize the magnetic field distribution, it also introduces additional problems, such as increased magnetic resistance. Accordingly, the model comprehensively incorporates key factors, including nonlinear magnetic- circuit characteristics, dynamic air-gap fluctuations, and magnetic leakage losses. The size parameters, spatial arrangement, and magnetization direction of permanent magnets are optimized using a multi-objective optimization algorithm. Two types of EMT prototypes (a traditional non-permanent-magnet structure and a permanent-magnet bias structure) were designed and developed, with comparative testing conducted in both laboratory and natural lake environments. Results demonstrate that the permanent-magnet bias structure achieves input-output synchronization under all operating conditions, with total harmonic distortion of the output waveform below 3%. Under identical input power, mechanical output displacement increases from 0.746 mm to 0.846 mm—a 13.4% gain—while dynamic driving force significantly exceeds the 133 N upper limit of the conventional structure. The following conclusions are drawn. (1) Under the permanent magnet biased configuration, significant differences exist in magnetic field gain across different spatial regions. The gain level at the coil base is the lowest; the dynamic magnetic force of the permanent-magnet gain-type structure is significantly higher than that of the traditional structure (133 N). (2) Optimizing the arrangement of permanent magnets realizes input-output synchronization characteristics, and the Total Harmonic Distortion (THD) of the output waveform is less than 3%. (3) At the equivalent power level, the maximum output displacement increases from 0.764 mm to 0.903 mm, resulting in a 5.11% improvement in mechanical efficiency in the laboratory static water environment. In the natural lake environment, the maximum output displacement increases from 0.746 mm to 0.864 mm with a 3.4% improvement in mechanical efficiency.
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