Design of a magnetic coupling for reducing maintenance in long-term-autonomous bio-inspired underwater vehicles

Abstract

Bio-inspired underwater vehicles are highly researched as a means for low-invasive monitoring. As a result, the performance of these vehicles has improved greatly. Research has thus far been directed towards the performances of the swimming motion. In order for the development of these bio-inspired underwater vehicles to continue, the increased reliability and durability should also be investigated. This research is aimed at long-term-autonomous bio-inspired underwater vehicles, specifically vehicles based on Median/Paired Fin swimming, which could provide continuous, unassisted monitoring of an area.
The goal is to investigate existing options for the propulsive mechanism and find the most suitable solution. A suitable propulsive mechanism can cope with the pressure changes that the vehicle will face due to the varying depths while requiring no maintenance for extended periods of time. This expresses itself in a mechanism that has no dynamic contact between the internal mechanics of the vehicle and the external environment because the dynamic contact is prone to leakage. One actuator and two transmissions are proposed as propulsive mechanisms in this research. The actuator is a reluctance actuator and the transmissions are a flexure joint and a magnetic coupling. The transmissions will be used in combination with an electric motor that is located inside the hull. The reluctance actuator has been found to lack torque generation. The flexure joint will be fused with both the hull and the fin and will be actuated from within the vehicle. It has the ability to either follow the large deflections that are required without encountering fatigue or to resist pressure differences, but both are required. The magnetic coupling does possess the torque, the range of motion, and the rigidity to resist the pressure, and is therefore chosen to be used as the propulsive mechanism that will be investigated further. Existing designs of the magnetic coupling do not comply with the requirements of this swimming mode. Three types of designs are therefore proposed, the coaxial, the face-to-face, and the moment-arm design. To reduce the resultant noncontributing magnetic forces on the fin, the designs have been made symmetrical in the motion plane. To reduce the effect of unstable equilibriums that result from attractive forces on both sides, each type of design has a repulsive design as well as an attractive design. In addition, the designs have magnet orientations that are inspired by Halbach arrays to increase the magnetic field where desired and reduce it elsewhere. Simulations have been performed on the designs using the magnetic finite element method. These simulations indicated that the coaxial designs do not create enough coupling torque and the moment-arm designs create too much radial forces on the fin. The face-to-face coupling designs show promise, both in coupling torque and in resultant force directions. Simulations of the double-sided, face-to-face coupling with Halbach-inspired magnet orientation have been validated and the results indicate that this new magnetic coupling could serve as a solution to reduce maintenance and increase reliability and durability in bio-inspired underwater vehicles. The use of repulsive forces in this design transfers the noncontributing resultant forces from occurring in the unpredictable external environment of the fin to the controlled internal environment of the vehicle.