A novel concentric circular ring structure applied in AUV’s inductive power transfer system for resisting the disturbance of ocean current

Inductive Power Transfer (IPT) has been developed rapidly recently. In an IPT system, electrical power transmission is based on electromagnetic coupling, avoids physical contacts between energy supplier and load. Autonomous underwater vehicle (AUV) is a suitable tool to acquire and provide extremely worthy information and data for human in ocean exploration, thus it is broadly utilized in ocean observatory, inshore survey, seafloor search and other underwater applications. Unfortunately, AUV cannot get sufficient power supply for long mission working time and range owing to the limited capacity of its self-carried batteries. Nowadays, with the mature development of seafloor observatory networks and underwater docking techniques, charging the AUV’s batteries at submerged docking station or energy node in deep sea observatory networks with power transfer via the seabed cable is of extreme high reliability. Traditionally, wet conductive connectors were employed, however, the seawater high pressure and conductive property usually cause difficulties to connections and charging during underwater power supply for AUV. Wet plug interfaces suffer wear, hermetic seal failure, and are inclined to arcing. Moreover, the adoption of conductive connectors leads to modification of AUV and undersea base station’s structure, thus, inducing increase in complexity and decrease in system’s reliability.

Inductive power transmission depends on loosely magnetic coupling, realizes no physical connection, furthermore, its simple configuration and well insulation performance arise good prospects; therefore, it becomes a possible substitute for conductive charging in AUV power feeding applications. However, in marine environment, the seawater stream vibration could bring disturbance to the clearance and axial relative displacements between the primary and secondary sides in IPT system, which might change the primary and secondary parameters all the time. Thus, the coupling coils’ parameters are changed, which reduces the stability of the power transmission system.

In wireless power transfer system, both in air and underwater, to guarantee that IPT system has better misalignment tolerance is a great challenge for researchers. Several analytical models have been proposed to investigate the influences of some possible disturbance including radial, axial, and angular misalignments between transmitting coil and receiving coil on system’s characteristics, such as mutual inductances, magnetic flux density distribution and magnetic field.¹ Currently, solutions which aim to enhance system’s misalignment tolerance could be classified into four kinds: compensation topology and circuit structure optimization,² coil or winding adaptive design,³ frequency control methods,⁴ and system multi-objectives optimization.⁵ 

Considering the unmanned and uncertainty of work environment, simplicity and reliability of IPT system are the most crucial design requirements for AUV power feeding applications. Misalignment tolerance improving methods mentioned above, circuit topology optimization and electromagnetic coupler design are credible approaches. This paper proposes a concentric circular ring coupling structure applied in AUV’s IPT system and studies its misalignment tolerance. The rest of this paper is organized as follows. Section II demonstrates the fixed configuration of concentric circular ring structure for AUV’s IPT system. Section III studies the design considerations for axial and radial misalignment tolerance. Experiments are given in Section IV. Finally, the conclusions are drawn in Section V.

The configuration of a novel concentric circular ring structure for AUV’s IPT system is shown in FIG. 1. In this novel structure, the IPT system is composed of two coaxial solenoids, where the transmitter side is wound on the submerged base station and the receiver coil is assembled to the AUV’s hull. In AUV docking process, the secondary winding is inserted into the primary winding. The electric power could be transferred from submerged base station to AUV.

Usually, most of IPT systems are designed to be symmetrical for primary and secondary coil. When the couplers are kept in stable gap distance and fixed perfectly, good coupling coefficient could be achieved. But when misalignment occurs or gap distance changes, the coupling coefficient will decrease drastically.⁶ The study aims at 200mm diameter AUV, considering docking error and vibration caused by ocean current, 20mm radial misalignment and 50mm axial misalignment are taken into consideration. Two circular cylinders are manufactured to model underwater base station and AUV, respectively, as FIG. 2(a) shows. The inner cylinder’s diameter is 200mm and outer cylinder’s diameter is 238mm. Considering the profile of submerged base station, three situations are evaluated when keeping the primary coil turns to be 19, while changing the number of turns of secondary coil among 14, 19, and 24. They are named as the 1st coupler, 2nd coupler and 3rd coupler, respectively. 38AWG litz wire consisting of 800 strands is utilized. Take both the primary and secondary coils’ z-direction center at same place and concentric as perfect aligned condition, as FIG. 2(b) shows, z-direction displacement represents axial misalignment and the eccentricity is considered as radial misalignment.

FEA simulation tool Comsol Multiphysics is used to explore the relationships between the coupling coefficient and several key parameters of the coils. Wayne Kerr 1J6520B precision impedance analyzer which has a precision of 0.05% is used to measure the self and mutual inductances in order to calculate the coupling coefficient. The results are demonstrated in FIG. 2(c). Keep the primary and secondary coils concentric (no radial misalignment), compared to the 2nd coupler, the 3rd prototype improves the coupling coefficient reduction from 27.8% to 23.6% when the axial misalignment varies from 0 to 50 mm. As to the 1st coupler, the coupling coefficient drops drastically by 34.7%, while the axial misalignment increases from 0 to 50 mm. The measured results agree well with the simulated results. Thus, the 3rd prototype (19:24 turns coupler) has good tolerance to axial misalignment. FIG. 2(d) depicts that when the radial misalignment ranges from 1 to 18mm, the coupling coefficient nearly remains constant. This is because radial misalignment is much smaller than the diameter of coils.

An experimental prototype is implemented, as shown in FIG. 3. The 3rd coupler (19:24 turns) is picked as inductive coils. An inverter is added in the primary side to convert the DC power into AC power. The SS compensation network is selected in system. Waterproof processing must be done to coils, a tank containing salt water is used to simulate an underwater environment.

In the experiment, the mutual inductance drops from 84.5μH to 64.5μH as the axial misalignment increases from 0 to 50mm, the load resistance is selected as 50Ω, regulate the input DC voltage to keep the output power to 100W. FIG. 4(a), (b) demonstrate the primary and secondary waveforms of voltage and current under different axial misalignments. The operating frequency for 0 and 50mm misalignment conditions is 101 kHz. FIG. 4(c) compares the efficiency under different misalignment conditions when the output power of system is 100W. When the primary and secondary coils are in concentric aligned position and the axial misalignment changes from 0 to 50 mm, the system efficiency ranges from 93.2% to 82.4%. At first, the power transmission efficiency decreases slightly with the growing axial misalignments, then the efficiency drops sharply when the misalignment increases from 20mm to 30mm, at last the efficiency reduces slowly as axial misalignment increases to 50mm. The experiment results indicate that the proposed concentric circular coil structure could transfer power for AUV with high reliability under disturbance caused by ocean current.

This paper has investigated a concentric circular ring structure, which could be applied in AUV’s IPT system. The study focuses on its misalignment tolerance for it could resist the disturbance of ocean current. Three couplers with different secondary winding turns have been developed, simulation and measurement results indicated that the 3rd coupler (19:24 turns) has better tolerance on axial misalignment. Since the radial misalignment is much smaller than the diameter of coils, radial displacement almost has no influence on coupling coefficient. The coupler has been selected and verified by experiments, the system efficiency drops from 93.2% to 82.4%, when the axial misalignment between the primary and secondary coils changes from 0 to 50 mm.

This work was supported by the Natural Science Basic Research Plan in Shaanxi Province of China under Grant 2018JM5033.