This paper describes the results of a six-year project aiming at designing and constructing a flapping twin-wing robot of the size of hummingbird (Colibri in French) capable of hovering. Our prototype has a total mass of 22 g, a wing span of 21 cm and a flapping frequency of 22 Hz; it is actively stabilized in pitch and roll by changing the wing camber with a mechanism known as wing twist modulation. The proposed design of wing twist modulation effectively alters the mean lift vector with respect to the center of gravity by reorganization of the airflow. This mechanism is modulated by an onboard control board which calculates the corrective feedback control signals through a closed-loop PD controller in order to stabilize the robot. Currently, there is no control on the yaw axis which is passively stable, and the vertical position is controlled manually by tuning the flapping frequency. The paper describes the recent evolution of the various subsystems: the wings, the flapping mechanism, the generation of control torques, the avionics and the PD control. The robot has demonstrated successful hovering flights with an on-board battery for the flight autonomy of 15-20 s.
In this paper, vehicle stability enhancement, based on the integrated vehicle control notion, is presented. A new method for adaptive optimal distribution of braking and lateral tyre forces is employed. The control inputs considered are the individual wheel steering and braking for each wheel. Since a unique set of tyre forces satisfying control objectives cannot be easily determined, an adaptive optimization problem subjected to two equality and four inequality constraints has been solved to achieve an optimal solution. A proper adaptation mechanism is suggested to minimize the negative effects of direct yaw moment control, such as the undesirable decrease in the total speed of the vehicle. The effectiveness of the proposed vehicle stability enhancement system, especially online balancing of tyre forces in an optimal form with and without an adaptation mechanism, is demonstrated through digital simulations. A comprehensive non-linear vehicle dynamics model is utilized for simulation purposes. The results indicate that the proposed control system can effectively utilize the tyres' frictional forces and significantly improve the vehicle stability and handling performances.
Hovering flapping wing flight is intrinsically unstable in most cases and requires active flight stabilization mechanisms. This paper explores the passive stability enhancement with the addition of top and bottom sails, and the capability to predict the stability from a very simple model decoupling the roll and pitch axes. The various parameters involved in the dynamical model are evaluated from experiments. One of the findings is that the damping coefficient of a bottom sail (located in the flow induced by the flapping wings) is significantly larger than that of a top sail. Flight experiments have been conducted on a flapping wing robot of the size of a hummingbird with sails of various sizes and the observations regarding the flight stability correlate quite well with the predictions of the dynamical model. Twelve out of 13 flight experiments are in agreement with stability predictions.
This study describes the design, development, and flight tests of a novel control mechanism to generate yaw control torque of a hovering robotic hummingbird (known as Colibri). The proposed method generates yaw torque by modifying the wing kinematics while minimizing its influence on roll and pitch torques. To achieve this, two different architectures of series and parallel mechanisms are investigated; they are mathematically analyzed to investigate their behavior with respect to cross-coupling effects. The analysis is verified by measuring the control torque characteristics. The efficacy of the proposed method is also explored by flight experiments.
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