Wrench Generation of Cable-Driven Parallel Robots with On-Board Thrusters: Feasibility Analysis & Available Wrench Set

Recently, propeller-based rotors have been incorporated aboard cable-driven parallel robots (CDPRs) in order to combine the advantages of cables and thrusters. A study on the capability of the resulting hybrid robot in wrench generation is proposed. Beyond the well-established scenario of fixed-axis thrusters (FTs), the novelty of reconfigurable thrusters (RTs), with relative (controllable) degrees of freedom with respect to the platform, is also included. Initially, the analysis deduces some necessary and sufficient feasibility conditions of the robot to withstand a desired set of external wrenches, in a prescribed pose of the platform; then, a geometric characterisation of the overall available wrench set (AWS) that can be generated by the end-effector is derived. The description of the AWS is paramount in planning and control of prescribed tasks, as well as for synthesis and design purposes. Since the AWS generally exhibits curve-shaped boundaries, a conservative polyhedral approximation is proposed for the sake of linearity in algebraic representation. Several scenarios are demonstrated both for planar and spatial robots: specifically, the AWSs of the platform being acted upon by no thrusters, fixed thrusters, and reconfigurable thrusters are derived and reciprocally compared.