Structural design and workspace analysis of a winch-integrated underwater cable-driven robot based on variable thrust
[Objective]Underwater multiple-degree-of-freedom robots possess a broad range of application potentials in diverse fields such as marine resource exploration,scientific investigation,and engineering construction and maintenance.However,during the execution of large-scale,long-distance underwater tasks,the conventional rigid serial and parallel manipulators frequently face the challenge of inadequate working range.Cable-driven parallel robots offer advantages such as large workspace,small inertia,and strong load capacity.However,prevalent cable-driven parallel robots for underwater applications are typically passively tensioned by gravity or buoyancy with their drive units(winches)mounted on a static platform,which constraints their motion ability and reconfigurability.Hence,a winch-integrated underwater cable-driven parallel robot based on a variable thrust mechanism is presented.The proposed robot adopts a hybrid drive form of six cables and a propeller.The thrust generated by the propeller,equivalent to cable tension,is adjustable in terms of magnitude and direction.[Methods]First,the overall mechanical structure of the robot is examined,and its kinematic and static models are established.On the basis of analyzing the judgment criterion of the wrench-feasible workspace(WFW),the wrench-feasibility testing problem under variable thrust is transformed into a constrained quadratic programming problem through the linear approximation method,and a new WFW calculation method is obtained.Then,a set of structural and force parameters of the robot are provided to evaluate and compare the WFWs of the robot with varied moving platform orientations and external forces under constant-and variable-direction thrusts.In addition,a large-span spiral trajectory is selected,a two-norm force index is implemented to optimize the thrust and cable tensions,and then the changes of all driving forces during the quasi-static motion of the robot on the trajectory are assessed under the two different thrust strategies.[Results]Calculation and analysis reveal that under constant-direction thrust,the WFW of the robot appears columnar.Although the moving platform can extend over 10 meters in the Z direction,its motion ranges in the X and Y directions are small,and the WFW is influenced by the orientation of the moving platform and the external forces,which suggest that the robot is susceptible to out-of-control phenomenon.By contrast,under variable-direction thrust,the WFW becomes a cone-shaped space;compared with the condition of constant-direction thrust,the X and Y direction motion ranges of the moving platform increase,and the volume of the robot workspace remarkably improves.The simulation for spiral trajectory motion also reveals that under constant-direction thrust,the cable tensions vary substantially,which facilitates exceeding the limit and causes problems such as slack.The change of thrust direction can considerably alleviate the variability of the tensions,and guarantee that they remain within feasible limits,hence expanding the robot's range of motion.[Conclusions]Results reveal the remarkable improvement outcome of the variable thrust mechanism on the WFW of the robot,which solves the problem of inadequate working range of the existing underwater multiple-degree-of-freedom robots.This paper can provide a reference for further studies on the design and analysis of underwater cable-driven parallel robots.