# Array Design and Transmitter Coils Selection for Underwater Wireless Power Transfer System
## 1 Introduction
A conical electromagnetic coupling mechanism with a gap of 5 mm was designed and tested in salt water. The results show that the system can transmit energy at 300 W power, and the maximum efficiency is 88%. The system is easy to connect and has high stability. In [15], a new type of magnetic coupling structure was designed. The receiving end adopts the embedded propeller mode, and the transmitting side adopts the vertical arrangement mode to form the annular magnetic field. This coil structure can optimize the magnetic field distribution and significantly reduce the interference to the internal equipment of the Autonomous Underwater Vehicle (AUV). At the same time, it has a strong anti-rotation offset performance. In [19], a new type of high-tolerance pendulum coupling mechanism suitable for underwater wireless power transfer of an AUV was proposed. The coupling mechanism not only has a compact structure, light weight, and strong anti-offset performance, but also has high transmission efficiency. The experimental results show that the system output is stable in the range of rotation and axial offset [\(-\)30 mm, 30 mm], and the efficiency is always above 95%, and the efficiency fluctuation is less than 1%. Coil array optimization refers to the use of the basic coil structure to build a multi-transmit coil array, and the anti-offset performance of the system is improved by optimizing the array shape and arrangement [20; 21; 22]. In [23], a hexagonal coil array with dynamic switching control was proposed, which enhances spatial coverage and efficiency in multi-receiver WPT systems by adaptively activating sub-arrays based on receiver positions. In [24], a scheme for optimizing the coil layout of the underwater wireless power transfer system was proposed. The transmitting end adopts the arrangement of double-layer coilsup and down to form a transmitting coil array. The receiving end consists of two coils to form a coil array, so as to ensure that the underwater equipment can effectively carry out wireless power transfer at any position in the transmitting array area, with more than 74% transmission efficiency. In [25], a kilowatt-level WPT system for an AUV was designed. The transmitter of the system adopts a three-coil structure, which can effectively deal with the offset problem of the coupling mechanism, with high position freedom and high transmission efficiency. The experimental results show that the system can transmit wireless power in a seawater environment with a high degree of freedom. The transmission power can reach 3 kW and the efficiency is 77% to 80%. In summary, the optimization of the coupling mechanism can effectively solve the problem of coil offset caused by water flow disturbance. Among them, the optimization of the coil structure will complicate the coil body structure and manufacturing process, and have higher requirements for application scenarios and equipment; the optimization method of the coil array is to expand the number of coils at the transmitting end and expand the charging range. The requirements for the coil structure at the receiving end are not high, the application range is wider, and the anti-offset effect is better. Therefore, this paper will use the coil array optimization method to improve the anti-offset performance of the system. Aiming at the problem that the water flow disturbance causes the coil offset of the receiving end to affect the transmission performance in the complex marine environment, this paper mainly studies a new array underwater wireless power transfer system. The coupling mechanism of the system adopts an array design, which expands the charging range and improves the anti-offset ability of the system. The main contributions of this paper are summarized as follows:  
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