Flight Testing of a 1-DOF Variable Drag Autonomous Descent Vehicle

This paper details the hardware development and testing of an autonomous descent vehicle. The developed vehicle utilizes a circular parachute and wind data to control the landing location. The benefit of this system over alternative precision aerial delivery techniques is the envisioned ability to use the large inventory of circular parachutes already in use for uncontrolled cargo and personnel deliveries with minimal training or system modifications. Parachute control is obtained via reversibly reefing of the canopy, thereby modifying the descent speed. For this study, the parachute size is assumed to be constant for the remainder of the descent; however, the desired parachute size computation is periodically updated to assist in reducing landing errors due to inaccurate wind data. A small mechanical reeling system has been developed, comprising a microcomputer, RF modem, electronic speed controller, and an electric motor. The hardware is coupled with a quarter-spherical canopy with four suspension lines, similar to those used in automotive drag racing. The combined weight of the parachute-payload system is 53.5N (12.0lb). Flight testing was conducted using a small single engine aircraft (Cessna 172), with preliminary flight testing conducted using an Arcturus T-20 UAV. Release ceilings were approximately 3050m (10,000ft) MSL. Typically, dropsondes were used to collect predicted wind for the descent vehicle. The time needed to collect the wind data, upload it into the descent vehicle software, takeoff, and reach the desired deployment location was approximately two hours. Preliminary testing of the parachute-payload system was performed to determine the appropriate control gains for the motor angle control routine to achieve the desired descent rate. Release altitudes were between 450m (1,500ft) and 610m (2,000ft) AGL. Using Zeigler-Nichols gain tuning rules and an experimental step response, gains were determined for both a Proportional-Integral (PI) controller and a Proportional-Integral-Derivative (PID) controller. Additional testing was conducted to verify the ability of these control gains to achieve a desired descent speed prior to flight testing the full path planning system and control algorithm. Flight testing results demonstrate the ability for the autonomous descent vehicle (ADV) to successfully navigate towards a target line segment when using accurate wind prediction data. As previously published results have noted, when the predicted wind data is inaccurate, the vehicle is not always capable of improving the landing location accuracy compared to an uncontrolled parachute. Additional considerations in developing a descent rate control system for use in circular parachutes are also presented.