The Rcsearch On Vision-based Small Unmanned Helicopter Autonomous Landing

Autonomous landing, as a key part of fully autonomous flight of unmanned helicopter, has become a focus in this field. Autonomous landing techniques has improved autonomous flight performance of unmanned helicopter to some extent, which also reduces the flight accidents, but there are still some unsolved problems in the actual landing task. Such as selection of a landing site, how to choose a suitable landing site to implement precise landing, and the ground effect interference problems when it lands close to the ground. These are the practical problems we faced. It is worthy of further exploration and research.In this dissertation, visual navigation was introduced into the autonomous landing based on the former autonomous landing research. A specific landmark was introduced to improve the reliability and accuracy of the autonomous landing. The ground effect problem during the landing process was also discussed. The most important work in this dissertation is as follows:(1) This dissertation discussed the ground effect problem from the most affected aspect, that the impact of ground effect on lift force, and put forwards a method to test the lift force generated by ground effect. Through the real flight hovering experiments, the numerical relationship between the lift force and collective close to the ground can be got, with contrast of it which is far to the ground, the magnitude of impact to lift force of ground effect can be worked out. And then the hovering experiment was conducted at different height near the ground, a relation model of ground effect on lift with the distance above ground could be got. And then the model was taken into account of the helicopter nonlinear modeling. Simulink simulation results verified the accuracy of the newly-added ground effect model.(2) Control strategy of the landing process was discussed in details. The helicopter always focuses on the ground reference(landmark) during the landing process, and adjusts the position of the horizontal direction timely, to ensure the accuracy of the landing position. The landing process is divided into two parts:high-altitude descending and close-to-ground landing, and they are respectively discussed and tested while the control strategies were different. The control strategy was verified valid through real flight experiments. It is noteworthy of the control strategy close to the ground, the usual "no control" method was abandoned, a certain stage of self-control was achieved through upper planning, and then was switched into open-loop control at last. The stability of the near-surface landing was improved, in order to achieve the soft landing result of the helicopter. Real flight experiments have verified the effectiveness and robustness of the control strategy during the entire landing process. Actual landing of the average location error was within30cm, which greatly improved the accuracy of landing.