We present a low-cost, three-degree-of-freedom (3-DOF) prismatic-spherical-revolute (PSR) parallel mechanism used as a testing platform for an unmanned aerial vehicle (UAV) tethered to an unmanned surface vehicle (USV). The mechanism has three actuated linear rails kinematically linked to a platform which replicates boat motion up to 2.5 m vertical heave (sea state 4, Douglas Sea Scale). A lookup table relating relative slider heights to platform roll and pitch was developed numerically leveraging geometric constraints. A design parameter study optimized the arm length, platform size, and ball joint mounting angle relative to the overall radius to maximize the workspace. For this design, a maximum roll and pitch range from -32° to 32° and -25° to 35°, respectively, is achievable. A prototype was manufactured to carry the tethered UAV winch payload. Experimental testing confirmed the workspace and demonstrated boat motion replication, validated using an inertial measurement unit (IMU).

The quasi-static catenary curve of a semi-slack tether between an essentially stationary unmanned air vehicle (UAV) and a small unmanned surface vehicle (USV) is investigated and characterized. An empirical analysis, performed over a discretized space of vertical and horizontal separations of the two vehicles, determines an optimum cable length & tension for maximizing system robustness during the vertical heave of the USV due to high seas. Operating at this optimum condition allows for equal displacements of the USV in the up and down directions, minimizing the possibility of both fouling (with the tether touching the water) and excessive downforce on the UAV (with the tether pulled taut) during dynamic heave events. Scaling the horizontal offset, tether length, and tension by the flying height collapses all empirical results into convenient curves depending only on a nondimensional relative position parameter (Δx/Δy), accurately fit by low order polynomials. This eliminates the need for a lookup table, and decreases computation time during implementation. The heave robustness analysis results in a recommended operating relative position of Δx/Δy ≈ .46. Experimental results are presented and confirm the catenary analysis for the proposed tether.

Skid-steered, tracked, tele-operated robots are used to perform high-risk critical missions such as bomb disposal under conditions deemed too risky to send a human. Often the robots carry heavy payloads that raise their centers of mass, increasing the risk of tip-over. Since it is often not feasible to send a human to right a toppled robot, tip-over is equivalent to mission failure. Hence, an autonomous behavior to prevent robot tip-over is desired. In this research, a simplified model of mobile robot dynamics permits separation of pitch and roll stabilization. Adaptive control is used to stabilize the appropriate angle based on the normalized tip-over measure. Experimental validation of this control is successfully demonstrated on an iRobot Packbot and a Segway RMP 440.