# Aerial Robotic Solution for Detailed Inspection of Viaducts
## 1 Introduction
### Aerial Robots for Inspection  
The use of autonomous unmanned aerial vehicles (UAVs) to capture images and their subsequent use in infrastructure analysis is currently on the rise. After an exhaustive search of the projects developed in the research world, relevant applications have been found that make use of autonomous UAVs for image capture in the inspection of railway tracks [3], where autonomous flights are carried out to follow the tracks, capturing images for subsequent analysis. Other applications, like mining inspection obtained 3D maps [4], make it possible to evaluate the earthworks carried out; wind turbines inspection for evaluation of deformations or damage [5]; inspection of civil infrastructures capturing images and their subsequent analysis through the use of neural networks to identify possible cracks or landslides [6]. Finally, in [7], bridge inspection is studied in a similar way as proposed in this article, where images are captured autonomously using a UAV platform and then analyzed using photogrammetry software. The main limitation raised is that they use Global Navigation Satellite System (GNSS) positioning to automate flights, while in the system proposed in this article, the aircraft that captures images does not navigate with GNSS but with onboard sensors such as LIDAR. The physical characteristics of the viaducs cause the GNSS signal to be partially or totally degraded when flying near or under them. These degradation problems with global positioning signals are discussed in [8], where the most critical ones affecting this article are signal masking and multipath. Therefore, the degradation of this signal leads to localization problems in the UAV in which it can drift its flight or even make sudden changes in its positioning, seriously compromising the safety and integrity of the operation. In addition, GNSS poses problems of repeatability of inspection operations, since the number and position of available satellites varies over time. There are currently some commercial systems for infrastructure inspection such as the one offered by Skydio [9], which makes use of several onboard cameras to navigate and perform the inspection autonomously. Unlike Skydio's system, our system is able to reference the inspection to a global coordinate system that can be used from the construction of the viaduct until the end of its life. This feature makes our methodology better suited to the current workflow of inspectors, who already use total stations to check the displacements of structures against global references defined during construction. Additionally, thanks to the global coordinate system, the solution offers the possibility for more than one aircraft to navigate and perform an inspection while maintaining the same references. By using a generic gimbal, our system is much more flexible than Skydio's system in the choice and configuration of the required camera sensor, as it is not limited to the built-in camera. In addition, our system uses an aerial robot that performs inspection by making physical contact with the structure. This provides information that cannot be acquired by pictures exclusively, such as precise measurements of crack depth and width, material hardness, concrete humidity, etc. In relation to this inspection by contact with an aerial robot, there are different lines of research with a number of projects that focus on maintaining stability during physical contact [10; 11]. In [12] the predecessor aerial manipulator of the one used in this work is presented, which was patented [13] and awarded with the EU Radar Innovation Award 2017 [14]. In [15], an aerial vehicle that operates overhead using a rigid arm, and that is even capable of keeping in contact with the surface [16], is presented. In [17] an aerial vehicle that operates at the bottom as well as at the front, is presented. Aerial manipulators capable of operating in either direction are presented in [18]. Stable contact operations have been achieved using a pusher trirotor in [19] or, a quadrotor in [20; 21].