Direct drive thruster: Reduction of magnetic field distortion for navigation of an underwater robot

This paper reports development of a direct drive thruster for underwater vehicles. The direct drive thruster does not use mechanical gear and magnetic coupling for torque transmission to propeller. Therefore it generates less magnetic distortion than usual electrical thrusters which use magnetic coupling. Many underwater robots use magnetic field for determining the heading of an underwater robot. The use of direct drive thruster contributes reducing the disturbance on magnetic field, and thus, reduces the uncertainty in measured heading direction. The thruster consists of motor, propeller, electrical driver, and water resistant housing. It has 1 kW power rating with brushless DC direct drive motor. It is designed to provide the maximum speed of 7 nautical miles and maximum thrust force of 25 Kg force. Since it uses direct drive motor, gear system, which is needed to reduce revolution speed and increase torque, is no longer needed. With no gear system, reduction of weight, power loss, and maintenance effort is possible. Also, it does not use magnetic coupling, and does not generate magnetic interference. The test results show power efficiency and robust drive.