# Improvement of Hexacopter UAVs Attitude Parameters Employing Control and Decision Support Systems
## 4 Laboratory and In Situ Measurement Results and Discussion
### Hover Flight Tests
#### 4.3.2 Hexacopter Flight Parameters Extracted from Sensors during Hover Flight
The operation of the motors is observed in the range 1000-1725 us, as the rotational speed range does not reach 100%, leaving room for additional manoeuvring if needed. It is noticed that there are small delays in response times between 100 and 200 us, representing less than 10%, possibly due to the natural frequencies of the motors or the structural elements on which they are mounted. High vibrations can cause wrong accelerometer altitude and horizontal position estimations, leading to problems in maintaining altitude. The hexacopter may start an uncontrolled lift manoeuvre without the operator being able to intervene. Position control problems in flight modes such as loiter, PosHold and auto may also occur. Vibrations are best visualized by plotting the VibeX, VibeY and VibeZ values in the VIBE menu. These represent the raw vibration values before being filtered by the accelerometers. Vibration levels below 30 m/s\({}^{2}\) are normally acceptable. Levels above 30 m/s\({}^{2}\) can cause problems, and levels above 60 m/s\({}^{2}\) nearly always cause position or altitude maintenance problems. The graph in Figure 29b shows acceptable vibration levels that are consistently below 30 m/s\({}^{2}\) and around 13 m/s\({}^{2}\). Figure 27: Tests performed on the motors without propellers. (**a**) Speed command given by the operator; (**b**) Engine response to the operator command. Figure 28: The altitude of the test site. (**a**) Operating altitude of the in situ location; (**b**) Test location. Similar results in terms of accelerometer vibrations (0) were found when performing another flight under similar conditions with the hexacopter, as shown in Figure 30a. The phenomenon of accelerometer clipping, which means that the accelerometers have been exposed to a level of vibration that exceeds their full measurement range, is illustrated in Figure 30b. These are feedback signals to the control loop; therefore, if they are not operating in optimal parameters, the attitude control cannot be maintained. This phenomenon usually occurs when the drone collides with a hard object, such as crashing or landing on a hard surface. If the value increases during the flight, it is recommended to rebuild the damping system by fitting double adhesive strips or soft rubber mounts to allow the three-axis movement and avoid inducing vibration in the flight controller housing, which is then transmitted to the on-board sensors. In the case of the studied hexacopter platform, it was observed to have a value of 0. To illustrate the operation of gyroscopes, in Figure 31a, the measured raw values of the rotational speeds in rad/s of the gyroscopes are represented. Very low values are recorded because the hexacopter does not pitch, roll or yaw during the ascent to the fixed-point hover altitude, but only compensates in very small increments the constant attitude. Because the controller has two IMUs, namely (0) and (1), both graphs are plotted comprising data from both subsystems. As expected, the measured values of both IMUs gyroscopes are identical, indicating the appropriate operation of these sensors. In the case of the GPS signal received by the GPS antenna, which has the Ublox M8N GPS built-in receiver, in Figure 31b, the accuracy of the received GPS signal is presented. HAcc indicates a horizontal positioning accuracy of 0.5-1.2 m, VAcc indicates a vertical positioning accuracy of 0.55-1.45 m and SAcc indicates a velocity measurement accuracy of up to 0.2-0.4 m/s. NSats indicates the number of satellites received, up to a maximum of 15. Figure 30: Vibration and clipping. (**a**) Accelerometer (0); (**b**) Clipping. Figure 29: Results processed with the online platforms. (**a**) Engines response to the lift command; (**b**) Accelerometer (0) vibration recordings. While operating in one of the autonomous modes (loiter, RTL, auto, etc.