Experimental Evaluation of Payload Induced Oscillations of an Unmanned Rotorcraft System

Abstract

Among the wide-ranging fields of application for Unmanned Aerial Vehicles (UAV), Aerial delivery has demonstrated a high potential to become a mainstream delivery service in e-commerce industries. Compared to other methods of payload carriage with a UAV, a tethered carriage can be a flexible and heavy load capacity system. However, the change in dynamics of a rotorcraft due to a tethered payload leads to instability and reduced efficiency on a mission of the Unmanned Rotorcraft System (URS). This study analyzes the performance of a quadrotor during different segments of its flight, including takeoff, cruising, and landing to gain a more detailed understanding of its behavior and capabilities. For this study, Pixhawk 4 controller-based quadrotor was assembled on an S500 frame due to its ubiquitous use in academia. The payloadinduced oscillation data in non-dimensionalized tether length and payload weight were studied by flying the URS in two distinct autonomous missions. The autonomous mission was set and controlled using qgroundcontrol, an open-source ground station system for Pixhawk controller. Furthermore, the inevitable vibrations due to extraneous factors were evaluated by conducting the autonomous mission with and without load conditions separately. Thus, obtained data were assessed case-specifically for different values of cable length, payload mass, speed, and altitude. In order to harmonize the time duration of the total flight and flight time of each segment, the time factor was normalized to define all possible cases. These data were analyzed and plotted in MATLAB to study the effect of change in variables on the preset autonomous missions. This study found that destabilizing effect of an underactuated cable-suspended payload caused secondary oscillations. Also, with increasing cable length and increasing payload mass, the pitch, roll, and altitude fluctuations were found to increase significantly in all cases with the tethered payload attached to the URS. However, the time period of pitch angle oscillation decreases by 28.3% when speed is increased from 5m/s to 7m/s on a segment of quadrilateral path. Each segment of the flight path is studied in details. So, in order to address the fluctuation and deviation characteristics of the quadrotor, further consideration in the control scheme for the quadrotor control system.