Characterisation and image-based flight control of an autonomous free fall skydiving robot

Abstract

The Freefall Camera (FFC) is a robot designed to film skydivers in free fall and is, to the author's knowledge, the first robot to control its position and terminal velocity relative to another falling object. The robot is equipped with a vision sensor to track objects of a chosen colour. The control surfaces for free fall motion are four vertical ailerons that control planar position and orientation and two retractable flaps that control terminal velocity. To develop a robust controller, equations of motion were created and validated experimentally for movement in the vertical and planar rotational directions. The static and dynamic response of the system were then tested in a vertical wind tunnel to validate the models and use them to design and tune a PID + Filter controller for the free fall flight of the robot. Planar rotation tests showed a decrease in steady state angular velocity and dynamic response gain compared to the model and a slight increase in dynamic response phase shift. This was attributed to the effect of the turbulent air near the shell of the robot. Vertical translation tests showed a decrease in maximum terminal velocity compared to the model, likely due to the effect of air viscosity at higher Reynolds numbers. The models were used to tune a PID + Filter controller to achieve stable target tracking in the rotational and vertical degrees of freedom using image based visual servoing. Controlled flight of the FFC was then tested in a vertical wind tunnel, where the robot was able to successfully track a coloured target within 15° of the centre of the field of view.