Modeling of underwater manipulator hydrodynamics with application to the coordinated control of an arm/vehicle system.

摘要

For users of unmanned underwater vehicles, manipulators have become a valuable tool in performing a wide variety of tasks. The addition of manipulators to an underwater vehicle can pose significant control challenges due to the hydrodynamic interactions between the arm and the vehicle: When the arm is moved while the vehicle is hovering in open water, the large hydrodynamic forces acting on the arm can cause the vehicle to "swim" away from its desired station.; To compensate for this dynamic coupling, the nature of the hydrodynamic forces acting on the manipulator must be well understood. This dissertation describes efforts to characterize the fundamental hydrodynamics of a single-link arm undergoing typical robotic slews. The product of this characterization is a new accurate real-time-implementable model of the hydrodynamic forces and torques acting on a circular cylinder (length/diameter = 9.1) swinging rapidly about one end through moderate angles ({dollar}<{dollar}120 degrees) in a start-stop fashion. This research represents the first experimental investigation of the hydrodynamic forces acting on underwater manipulators.; As an example application of the new hydrodynamic model, the model was used to predict the arm/vehicle interaction forces for a system consisting of a free-swimming vehicle with a movable single-link arm. With this model of the arm/vehicle interaction forces, a coordinated arm/vehicle control strategy was developed. To demonstrate the effectiveness of this controller, experiments were conducted using the OTTER vehicle at the Monterey Bay Aquarium Research Institute (MBARI). Using this method, vehicle station-keeping capability was greatly enhanced: Errors at the manipulator end-point were reduced by a factor of 2.5 and arm end-point settling times were reduced by a factor of three when compared to results using only position feedback for controlling the vehicle subsystem. These dramatic performance improvements were obtained with only a five percent increase in total applied thrust.