An underwater vector-drag dynamic positioning system for an underwater payload (“payload”), the system comprising:
- (a) a plurality of dual-module, single-winch, coaxial alternating vector-drag drones (“drones”) spatially distributed around the payload and positioned such that respective tether lines-of-action are oriented to generate controllable forces and moments on the payload, each drone including:
- (i) a first underwater module including a geometry-adaptive drag-inducing structure and one or more control surfaces, the geometry-adaptive structure and/or the control surfaces being configured to modulate hydrodynamic drag magnitude and direction, and an auxiliary intermittent repositioning subsystem operative during non-pull phases;
- (ii) a second underwater module disposed coaxially with the first module including a geometry-adaptive drag-inducing structure and one or more control surfaces, the geometry-adaptive structure and/or the control surfaces being configured to modulate hydrodynamic drag magnitude and direction, and an auxiliary intermittent repositioning subsystem operative during non-pull phases;
- (iii) a first tether coupled to the first module and a second tether coupled to the second module;
- (iv) a local controller;
- (b) a corresponding plurality of controllable dual-port winches mounted to the payload, each winch having a first port driving the first tether and a second port driving the second tether of a corresponding drone, each winch being dedicated to a single corresponding drone;
- (c) for each drone, a coaxial routing in which at least one of the first and second modules defines an axial through-passage along a common longitudinal axis and the tether coupled to the other module passes through the axial through-passage such that the line-of-action of the tethers is maintained substantially collinear with the common longitudinal axis, thereby reducing yawing moments and tether interference during operation;
- (d) a coordination controller, implemented centrally and/or distributed across the payload, the drones, and/or the winches, in communication with the local controllers and the winches and configured to command winch velocities, drag-geometry settings, and operation of auxiliary intermittent repositioning subsystems, including onboard propulsion where present, for the plurality of drones, and to control the spatial disposition of the drones relative to the payload;
- (e) wherein, for each drone, the local controller, together with the corresponding winch, is configured to execute alternating phases in which, in a first phase, the winch retracts the first tether while releasing the second tether and, in a second phase, the winch retracts the second tether while releasing the first tether, the module undergoing release during each phase being positioned by its auxiliary intermittent repositioning subsystem, the local controller further being configured to command dynamic adjustment of the geometry-adaptive drag-inducing structures and the control surfaces to set the hydrodynamic drag vector, in both magnitude and direction, including, for at least one of the modules, independent of control surfaces, intentionally altering the deployed drag-inducing structure's geometry asymmetrically so as to produce a lateral component of hydrodynamic drag for attitude control, such that at least one of the modules remains in a drag-generating configuration throughout the alternating phases, thereby providing a substantially continuous vector-drag output for the corresponding drone;
- and, during each release phase, the local controller further being configured to steer the repositioning module to adjust its attitude in response to changes in the attitude of the module undergoing retraction, so that it remains substantially collinear with the common longitudinal axis of the module undergoing retraction, thereby suppressing yaw moments, minimizing tether interference, and maintaining the repositioning module's tether in tension along a line-of-action substantially collinear with the common longitudinal axis throughout operation, including during hand-off intervals;
- (f) wherein modulation of pulling velocities in combination with adjustment of the geometry-adaptive drag-inducing structures and the control surfaces provides primary translational traction and attitude control via hydrodynamic counter-forces that increase approximately with the square of the magnitude of the component of the relative velocity of the respective drag-inducing structure with respect to the surrounding fluid along the retracted tether;
- and
- (g) wherein the coordination controller includes a state-estimation module configured to fuse inertial data with at least one of acoustic, electromagnetic, or tether-based measurements and to command the winches and the drones' local controllers to maintain a six-degrees-of-freedom pose of the payload within a controller-specified tolerance band around a target setpoint or along a commanded trajectory.