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).

Optimal path of a UAV engaged in wind-influenced circular towing

“…In [11] simulation results show that by tilting the orbit vertically it is possible to reduce the peak-topeak oscillation magnitude that the towed system experiences in 3 m/s steady wind from 26 meters to 6.5 meters. For object retrieval missions, reducing the vertical oscillations as much as possible is critical to success.…”

“…O is assumed to be placed at the target position on the ground where an object should be placed/picked-up. In the presence of a constant wind in the direction of the X-axis (East) previous studies [11], [12], [13] have shown that the desired UAV orbit center must be shifted a significant distance upwind and also slightly to the South for the towed endbody to remain at/near O. The slight shift to the South is due to the fact that the UAV spends much more time flying upwind (West) than downwind (East).…”

“…• The equations of motion defined in Section II are integrated forward in time, while the wind magnitude is slowly ramped up to the desired value, the UAV orbit is tilted in response to the wind as in [11] and the orbit center is adjusted to stabilize the endmass at the desired ground-fixed point (inertial origin O) • When the system has achieved steady-state, the cable shape is captured for use in the OCP.…”

“…The main contribution of this paper is the concept for UAV path adjustments in winds, particularly solving for the optimal vertical path using optimal control methods. A path control strategy based on offline optimal control will be shown to greatly reduce the motion of the towed endmass compared to the purely "tilted" path (constant positive/negative vertical velocity when flying with/against the direction of prevailing wind) suggested in [11]. Furthermore, the work presented here includes a concept for how to adjust the path of the UAV in the horizontal plane to remove the effects from constant winds, to allow the towed endmass to stabilize close to the desired ground-fixed target.…”

Control of an End Body Towed by a Circling Unmanned Aerial Vehicle

“…The strategy involves the development of a Sliding Mode Controller (SMC), as the robust properties are expected to help stabilize the towed cable-body system. Also, [15] presents an early version of the work contained in this Note. New content includes demonstration of the SMC, the path compensations to allow the towed endbody to stabilize around the ground-fixed target in winds and expanded simulator results that cover more wind cases and also demonstrate robustness to uncertainties.…”