# Drone-Based Vibration Monitoring and Assessment of Structures
## 3 System Development
### UAS Platform  
The standalone sensor package does not require connection to the power grid, so it can be deployed to any structure. Traditional (human operator) deployment and retrieval though may be challenging in remote locations. For example, in remote bridges, a lift from  
Figure 4: The docking and retrieval system of the sensor package featuring two electropermanent magnets: #1 on the top of the package, used to mount it to the underside of structures; and #2 on the drone docking platform, used to secure, transfer, deliver, and retrieve the package.  
Figure 3: Constituent components of the sensor package.  
the ground is required to reach the underside (which may be impractical if the bridge crosses a body of water or a busy highway) or a crane on the topside of the bridge, which can position the operator underneath the structure. Both methods are risky and require costly equipment and roads to be shut down to make the underside of the structure accessible. A drone platform has been developed to overcome these limitations and securely deploy and retrieve sensor packages in difficult-to-access areas.  
The drone platform was based on the DJI F550, a hexoopter with six standard 2312E motors, shown in Figure 5[26]. Pixhawk 4 was used as the flight controller, and the system was operated via the ArduPilot Mission Planner. Onboard sensors included an IMU, accelerometers, gyroscope, magnetometer, and barometer to provide data for a variety of parameters including drone orientation and altitude. There was also an external GPS used for global positioning when the signal was available. The F5 Kky X8R radio was used as the onboard receiver to communicate with the F5 Kky 2.4 GHz Taranis X9D radio controller for manual flight commands. The sensor package, as previously described, was attached to the UAS via a second electropermanent magnet (#2 in Figure 4), which was controlled using one of the RC servo ports available on the power management board of the Pixhawk 4.  
The system was designed to work as follows: On the ArduPilot Mission Planner, two channels from the FrSky Taranis were set to send high, low, and neutral commands to the two magnets. During the high command, the magnet begins the charging sequence, which can only be completed once it is in contact with a ferromagnetic surface. Once the magnet is mounted on the metallic surface, this command can be shifted to neutral as the electropermanent magnet will remain magnetized until the low command is sent. When the low command is sent, it will disengage the magnet.  
During operations, the sensor package is delivered onboard the drone (Magnet #2 engaged) to the desired point on the structure where Magnet #1 arms and connects the sensor package and drone to the structure. Once the package is secured in place, Magnet #2 disengages, and the drone detaches and flies away. The sensor package then periodically collects and stores acceleration data for DSR processing. When this operation is complete, the drone conducts a retrieval sequence by reversing the docking procedure. A summary of the status of both magnets in each phase of operations is given in Table 2.  
Figure 5: Overview of the custom Unmanned Aircraft System (UAS or drone) developed for sensor deployment and retrieval.  
To assist with the retrieval of the sensor package, the UAS was outfitted with a 3D printed alignment tool (docking platform in Figure 4). The alignment tool features angled walls, which help to center the sensor package over the electropermanent magnet and prevent the sensor package from coming into contact with the propellers.