# Mixed-Integer-Based Path and Morphing Planning for a Tensegrity Drone
## 7 Experimental Study
### Tensegrity Drone with Strut Contact  
In order to improve the performance of the drone and to make the structure more compact, an additional horizontal strut was added, making the drone frame a seven-bar tensegrity. The levels of pre-stress were increased, leading to struts touching each other, making the structure transcend the definition of pure tensegrity, as the struts in this case can experience forces other than pure compression. The design of the drone and an in-flight photo are shown in Figure 8.  
Figure 8: Second generation tensegrity drone. (**a**) CAD model. (**b**) In-flight photo.  
Figure 7: Pure tensegrity drone; ESC stands for Electronic Speed Controller; the flight controller used is PX4.  
The following series of experiments was conducted. The second generation tensegrity drone and a rigid-frame drone QAV-R 5\" Racer were outfitted with visual markers designed to work with the motion capture system OptiTrack. Both drones were required to fly over a pre-determined 8-shaped trajectory and their flight parameters were logged using PX4 measurement and logging solution, processed via Flight Review software. Figure 9 shows the trajectories of the rigid-frame QAV-R 5\" Racer and tensegrity drones; the accuracy of the motion capture system measurements is 0.3 mm. Analyzing the graphs, one can note the less consistent behavior of the tensegrity drone as compared with the rigid-body drone; however, it needs to be taken into account that the flight controller used in both experiments was designed for rigid-frame drones. This includes the way the control actions are computed and the way the state of the drone is estimated. The video recordings of the experiments are publicly available: flight tests for tensegrity and rigid-frame drones: [[https://youtu.be/X8](https://youtu.be/X8) SFrV3l4A8]([https://youtu.be/X8](https://youtu.be/X8) SFrV3l4A8) (accessed on 30 April 2022), [[https://youtu.be/LxREx53](https://youtu.be/LxREx53) Sna8]([https://youtu.be/LxREx53](https://youtu.be/LxREx53) Sna8) (accessed on 30 April 2022).  
The same experiments were used to produce spectra of acceleration power spectral density for both the rigid-frame drone QAV-R 5\" Racer (see Figure 10) and the tensegrity drone (see Figure 11). The measurements were performed using CUAV PX4 V5 nano board with IMUs ICM-20689, ICM-20602, and BMI055; their readings were combined in an extended Kalman filter, implemented in PX4 software. As can be seen in the spectra, the rigid drone exhibits a band of higher accelerations at higher frequencies compared to the tensegrity drone. The tensegrity drone, on the other hand, exhibits a clear dominant frequency that remains stable throughout the whole experiment. The amount of lower frequency accelerations for the tensegrity drone is higher than for a rigid-frame drone, as could be expected based on the presence of the elastic elements.  
Figure 10: Spectrum for a rigid-frame drone QAV-R 5” Racer collected via PX4 log collector and plotted using PX4 Flight Review.  
Figure 9: Drone trajectory captured by the visual motion capture system OptiTrack in the horizontal plane. (**a**) Rigid-frame drone QAV-R 5” Racer. (**b**) Second-generation tensegrity drone.