# Aerial Grasping of an Object in the Strong Wind: Robust Control of an Aerial Manipulator
## 4 Experiments and Results
### Aerial Grasping in the Wind  
When the aerial manipulator grasps a target object, the hex-rotor is hovering at one point to keep the object in the operational space, so that the manipulator can grasp the target safely. Thus, high precision hovering is essential for the aerial manipulator to grasp a target in the wind. If the hex-rotor cannot keep the target in the operational space all the time, the target will not be reachable for the robotic arm.  
The speed of wind disturbance in the experiments is shown in Figure 7a. The wind was sweeping periodically, as in Figure 7a, because the axial flow fan is switched between turn-on and turn-off to generate gusty wind disturbance. The mean and variance of the gusty wind speed were 6.54 m/s and 1.5265 m/s, respectively. The wind speed in the frequency domain is shown in Figure 7b. The main frequency of the wind disturbance was 0-0.9 Hz. Therefore, to eliminate the main part of the wind disturbance, 0.9 Hz was chosen as the cut-off frequency of the low pass filter, Q(S), in the acceleration feedback enhanced controller.  
In order to validate the wind disturbance rejection performance of the acceleration feedback method, in the experiments the aerial manipulator was controlled by the robust \(H\infty\) controller without and with the acceleration feedback enhanced term. The system output control results are shown in Figure 8. The mean and variance of absolute value of control errors are shown in Figure 9. Contrasting the position and yaw error in the two experiments, we can see that the position control accuracy of the hex-rotor was improved significantly with acceleration feedback enhanced term. It means that the acceleration feedback term can reject the wind disturbance effectively.  
Figure 6: Projection of the operational space \(\Omega\) in the \(X_{B}\)-O-\(Z_{B}\) (left) and \(X_{B}\)-O-\(Y_{B}\) (right) planes.  
Figure 7: The speed and frequency of the gusty wind disturbance in the experiments: (**a**) The speed of the gusty wind disturbance; (**b**) The frequency domain diagram of the of the gusty wind disturbance.  
The position of the target object in the operational space in the experiments is shown in Figure 10. It can be seen that without the acceleration feedback enhanced term, the aerial manipulator cannot keep the target object in the operational space all the time because of the weaker disturbance rejection ability. Therefore, the target becomes unreachable for the manipulator, as shown in Figure 11. As a result, the aerial grasping task fails. With the acceleration feedback enhanced term, the hex-rotor has sufficient disturbance rejection ability, and the position control accuracy of the hex-rotor is improved. Thus, the target object is kept within the operational space all the time, and the aerial grasping task is accomplished in the end, as shown in Figure 11. (The experimental video is available at the following website: [[https://v.youku.com/v_show/id_XNDA4](https://v.youku.com/v_show/id_XNDA4) NDEwODE4 NA==_html?spm=a2h3j.8428770.3416059.1]([https://v.youku.com/v_show/id_XNDA4](https://v.youku.com/v_show/id_XNDA4) NDEwODE4 NA==_html?spm=a2h3j.8428770.3416059.1))  
Figure 8: The position and yaw angle of the hex-rotor: (**a**) The hex-rotor is controlled without the acceleration feedback enhance term; (**b**) The hex-rotor is controlled with the acceleration feedback enhance term.  
Figure 9: The mean and variance of absolute errors of the position and yaw angle of the hex-rotor controlled with and without the acceleration feedback enhanced term.