Turning of a Short-Length Cable Using Flapping Fin Propulsion

In this paper, the context of several self-propelled, short-length cables, embedded with passive sensors for environmental diagnostics and swimming efficiently in formation over long duration and in shallow water, is considered. The basic problem of this volumetric diagnostic-namely, the low-speed motion control of a short-length, neutrally buoyant cable-is examined. More specifically, the constant-rate, circular turning of a 7-m-long cable held taut in a shallow-water basin using a biorobotic propulsor that has multiple flapping fins at one end, the other end being tied to a mooring post, is examined via modeling and laboratory and basin experiments. A drag analysis is used to estimate the fastest steady turning rate achievable while holding the cable taut. An axial tension and position controller, as well as a depth controller, is developed and evaluated in a quiescent laboratory tank accounting for the cycle-averaged hydrodynamic characteristics of a rigid cylinder to which six flapping fins are attached, three at each end. A small test range of 100-m scale, containing seven floor-mounted hydrophones in a hexagonal layout, is built in a stillwater basin to track the motion of the propulsor, to which a pinger is attached. The estimated overall resolution of the acoustic tracking system is 5 cm; it is possible to detect the imprint of the environmental unsteadiness on the cable and propulsor assembly. In the basin experiment, a mean radius of turning of 8.91 m can be achieved within a standard of deviation of 0.27 m, and a uniform turn rate of 22 min for one full revolution can also be maintained, when the applied turning force is 10% of the cable tension. The basin experiment has verified the drag analysis. This paper explores the value of a flapping fin propulsor (which is inspired by large swimming animals) as an alternative to conventional rotational propulsors for the low-speed maneuvering of a short cable.