# Towards Autonomous Bridge Inspection: Sensor Mounting Using Aerial Manipulators
## 1 Introduction  
The world of unmanned aerial vehicles (UAVs) has been rapidly growing in recent years. As their design and control are perfected, these aerial vehicles have become more and more available. Nowadays, off-the-shelf ready-to-fly UAVs can be found and bought in shops, which makes them available to virtually anybody. This, in turn, has sparked a great deal of public interest in UAVs since their potential can be found in applications such as agriculture, various inspections (bridges, buildings, wind turbines), geodetic terrain mapping, the film industry, and even for hobby enthusiasts to fly and record videos from a first-person perspective. The vast majority of commercially available UAVs are equipped with a camera, while more specialized vehicles for terrain mapping or crop spraying offer a more diverse sensor suite.  
All of the aforementioned systems primarily observe and gather data about the environment, while having little to no ability to interact with and change the environment. One way to augment these vehicles for physical interaction is to attach a lightweight manipulator to their body, which is the main interest of the aerial manipulation field. Although such vehicles are more complex for both modeling and control, their benefit lies in performing versatile tasks that require interaction with the environment.  
In general, there are three types of bridge inspections: periodic, special and damage inspections. Periodic bridge inspections differ from country to country according to national standards, and are usually performed at least once every two to three years. Special inspections are typically used to monitor the condition of deficient elements at specific locations based on predefined requirements. Damage inspections are usually performed after events that have occurred due to environmental impacts or human actions. The aim of a bridge inspection is to evaluate and assess structural safety and reliability. Current techniques are based on traditional visual inspection with a combination of non-destructive methods (NDTs). Traditional visual inspection is performed by experienced (trained) engineers and using specialized trucks equipped with the cranes and basket, that allow inspectors to review the conditions underneath the bridge. During the inspection, the engineers are equipped with various NDT [1] tools to detect construction faults and defects such as corrosion, cracks, voids, weakening connections, and concrete delamination. Some of these NDTs require mounting small sensors to collect data, such as accelerometers, strain gauges, tilt meters and various transducers for acoustic or pressure measurements. Afterwards, the bridge is excited with vibrations, sound waves, tapping, etc., and mounted sensors record responses to these specific excitations. Furthermore, there are usually requirements for performing measurements during the bridge inspection, such as the short- and long-term monitoring of vibrations, strains, displacements, etc. Mainly, these inspections offer valuable information about the current bridge conditions, but there are a number of disadvantages. The use of trucks during inspections requires total or temporary road closures, which at the same time require safety measures to keep traffic flowing as freely as possible. In addition, inspectors often encounter challenges in reaching all portions or elements in narrow areas, such as tight spaces between girders, beams and vaults. The aforementioned significantly increases the time and overall cost of the inspection. An aerial robot, with the potential to reach these challenging locations on the bridge, could significantly reduce the time and cost of these inspections and improve worker safety. Moreover, we note that the aforementioned sensors are relatively lightweight, which makes them suitable for transportation and mounting with an aerial robot.