A strategy for robust precision control of an endbody being towed by an orbiting UAV

Abstract

This paper presents a strategy for end-body positioning maneuvers using a towed cablebody system where a fixed wing Unmanned Aerial Vehicle (UAV) is stabilized in a circular orbit. High precision maneuvers such as object pickup/dropoff are typically performed by rotorcraft UAVs, but a successful fixed-wing concept would greatly increase the possible range for this type of operation and enable missions into more remote locations. Circularly towed cable-body systems have been shown capable, both analytically and experimentally, of maintaining stable configurations with the towed endbody maintaining a small motion respective to a point on the ground. However, no known efforts consider small to medium scale UAV operations for object pickup/dropoff/manipulation. A viable concept must be able to perform well when subjected to likely disturbances such as wind that causes the center of orbit for the towed endbody to be offset downwind of the UAV orbit. It is a primary goal of this paper to develop robust UAV path control that is able to stabilize the towed endmass in the presence of both moderate disturbances and modelling uncertainties. An optimized disturbance-free planned path that considers the UAV performance constraints is computed offline for the desired UAV towed system. A nonlinear sliding mode controller is developed to provide robust path control. To compensate for persistent winds or disturbances, the optimized disturbance-free orbit is inclined vertically to achieve an even tug on the towcable (i.e stabilize the measured cable tension force).