# Aerial Robotic Solution for Detailed Inspection of Viaducts
## 1 Introduction  
The inspection of viaducts and bridges is a very time-consuming and resource-intensive activity. It requires heavy involvement from highly qualified and specifically trained personnel. Additionally, these inspections pose health and safety risks that are mainly derived from working at height and the difficulty of the operation. Current inspection methodologies involve the use of climbing operators, who, by means of ropes, hang from the structure and perform the measurements required by the inspectors to evaluate its current state (see Figure 0(a)). These works present many potential accident risks due to the difficulty and technical level required to access certain complicated areas at heights, the possible physical fatigue of the workers, human errors in the safety of the operation, or even problems with the use of specific measuring tools.  
An alternative method is the use of heavy machinery, like cherry pickers, truck-mounted lifts, and cranes (see Figure 0(b)). This machinery requires new specialized personnel to operate it and perform inspections and does not eliminate the problem of having to expose people to work at heights. The surfaces to be inspected are usually located at a high altitude, and under it, several types of obstacles could be found, such as traffic of vehicles or trains, water flows, or rough terrain. This means that the use of machinery has to be done from the top surface of the structure, interrupting its service and increasing the operational costs of the inspection.  
Currently, the highly qualified staff required for inspections is normally composed of civil engineers working for engineering firms specialized in structures. Every little deformation, crack, or defect can be the cause of a potentially bigger critical problem, so they must be identified as soon as possible. An example that requires high accuracy isthe measurement of cracks width with an error smaller than 0.1 mm. Moreover, crack depth can only be measured using contact sensors. However, visual inspection is the most extended way of assessing the preliminary status of the viaduct before deciding if specialized equipment is needed. This means that, in order to find the smallest defects, the inspector must be very close to them. Furthermore, human subjectivity or the lack of experience could lead to an undervaluation of the severity of a defect.  
This whole process presents many inefficiencies that can be minimized using aerial robotic technologies combined with computer vision algorithms (e.g., artificial intelligence) and other computerized technologies supporting the post-processing of the acquired data. This article proposes an aerial robotic solution for gathering all the needed data to analyze the status of a viaduct. Our proposed solution drastically improves the safety of the inspections, as it does not involve the work at height of any human being or the use of heavy machinery. It reduces inspection times and costs by reducing the number of specialized people required to perform the inspection and avoiding the need to interrupt the use of the structure. On the other hand, it improves the quality of the data obtained, since it is the inspectors themselves who indicate the points to be inspected by the aerial robotic platforms, which can always be quickly sent back to obtain more information if necessary. In addition, the system is flexible to use the different sensors needed: either cameras or sensors that require physical contact with the structure.  
Figure 1: Examples of current inspection methodologies. (**a**) Current method for the viaduct inspection with rope access, from Ayres Associates, Inc. [1]. (**b**) Current method for the viaduct inspection using specialized machinery, from Forsgren Associates, Inc. [2]. (**c**) Proposed new methodology using the flight platform AERO-CAM in Alora, Malaga (Spain) performing an inspection under a viaduct deck.