Conclusion

In this study, position control of the UUV as the application of a novel optimal control was discussed. The dynamics and control of the 6-DOF UUV are presented and the new optimal control method was introduced. To the best knowledge of the authors, there is no 6-DOF underwater vehicle that can be controlled with one actuator in literature. Based on this control approach, the criteria function determined included a terminal error function. The terminal error function plays an important role in control of the UUV. After the minimization of the criteria function, the terms coming from the terminal error function appeared in the control law. After simulations, it was observed that these terms are crucial in the maneuver of the vehicle. It is understood that they dominantly, with respect to the other terms, determine the value and direction of the force applied to the UUV. It has been proved that, the control of the UUV necessitates the initial conditions as either pitch velocity or yaw velocity at time to=0. The roll velocity is produced by the rolling moment. Gyroscopic moments and centrifugal forces occur when the pitch or yaw velocity has an initial value. Existence of these forces and moments make the control possible. Even though the use of only one actuator seems to be a big challenge for a 6-DOF underwater vehicle, the UUV has succeeded to reach the target point by application of the energy-based control method.

At present, attitude control of the UUV has not been achieved. Research for controlling the attitude variables is ongoing. Even the vicinity of the target state is attained, the vehicle may still have small velocities. The body needs forces in each direction for a necessary maneuver. Since the nonholonomic constraints do not allow every possible motion, motion to minimize the position errors needs other necessary forces to be produced by only Fx. Therefore, the body moves undesired directions recurrently. If attitude control would be achieved, this hovering motion is thought to be prevented.

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