| Abstract: | We developed an automatic measurement system for flow velocity and direction in natural rivers using an autonomously controlled unmanned surface vehicle (USV). Oncoming mainstream velocity was measured by the propulsion force required for the USV in order to preserve the position at a measurement point. To conduct such a field mission, the system runs by changing four characteristic control stages: 1) calculation of the tentative propulsion force, 2) navigation to the target point, 3) velocity measurement by staying at the target, and 4) detection of flow direction by flowing downstream. More than 20 indoor tests were conducted under several hydraulic conditions by varying streamwise velocity, and the calibration formula was obtained by interrelating the oncoming velocity magnitude with the propulsion force required to remain at the target. The attitude control was provided with side thrusters to improve the yaw stability of the USV in the oncoming current. The adjunctive work of the side thrusters was very effective. Field tests were conducted to examine the reliability and accuracy of the present automatic flow measurements in a river. Both local velocity and direction in the river flow were measured well by the USV. Error analysis was conducted by comparing with the existing velocimetry results, and the USV was found to possess a sufficient ability to meet practical performance for the flow measurement in a calm river with about 1 m/s velocity. |
