Mechatronic System and Experiments of a Spherical Underwater Robot: SUR-II

This paper describes the structural design of the SUR-II spherical underwater robot. A spherical shape was adopted due to its outstanding shock resistance and flexibility. We designed and developed vectored water-jet thrusters to implement 4-degrees-of-freedom (4-DOF) underwater motion while saving energy. Because each thruster provided 2-DOF motion, three were sufficient for 4-DOF motion. Therefore, the propulsion system was composed of three vectored water-jet thrusters mounted on an equilateral triangular support. A master-slave structure was employed for the electrical design to realize data collection and motion control. The master side was used for the sensor data collection and control algorithm, and the slave side was used to control the propulsion system. After examining the performance of a first-generation electrical system, we chose a more powerful master processor to allow for a more complicated control algorithm. A microelectromechanical system (MEMS) inertial measurement unit replaced the original gyroscope to collect the attitude angle for the three axes. A Kalman filter was used to calibrate the data output and reduce the noise of the MEMS sensor. A series of underwater motion experiments were carried out to test the performance of the spherical underwater robot; these included surge motion, yaw motion, depth control, and multiple-depth control tests. A proportional-derivative (PD) controller was used to control the direction of the vectored water-jet thrusters for underwater motion. The experimental results demonstrated that the spherical underwater robot could realize underwater motion by controlling the direction of the thrusters. However, the robot was not very stable because the change in the propulsive force was nonlinear.