## 4 Types of Robots
### Unmanned Aerial Vehicle (UAV)  
UAVs are by far the most commonly used type of robot for inspection and monitoring of the built environment, as found in the literature. According to Association for Unmanned Vehicle Systems International (AUVSI), the market for UAVs is estimated to be $11.3 billion in the U.S. alone, which will grow to $140 billion in the next 10 years [38]. Examples of commercial UAVs used in research are DJI Phantom 3 [39], DJI Phantom 4 [40], Parrot AR.Drone 2.0 [41], DJI M600 Pro [42], DJI Matrice 100 [43], FlyTop FlyNovex [44], DJI Mavic Mini [45], DJI Mavic Pro [46], and Tarot FY680 [47]. The use of UAVs, also known as drones, started in military operations; however, they are now increasingly being used in the construction and maintenance of civil infrastructures for inspection and monitoring [30; 48; 49]. Their versatility and low operating and maintenance costs make them appealing to a wide range of sectors, including construction [50; 51]. UAVs are a preferred tool for data collection because of their maneuverability and higher angles of measurement [52; 53]. They are also very lightweight and take little time to set up. They can reach places where it is difficult to reach for humans [51; 54; 55]. UAVs can also undertake at-height inspections isolating humans from fall hazards [56; 57]. UAVs are also faster than human inspectors [58]. High-rise towers with glass facades often get damaged after extreme weather events. These facades need to be inspected before re-occupancy of the building is allowed [59]. Due to their speed, UAVs can provide accurate information much faster and more frequently than humans [58; 60]. UAVs can thereby reduce the cost and risk of inspection of the built environment [61]. Figure 3: Types of robots used in literature. UAVs are classified into two categories--fixed-wing and rotary-wing UAVs. While fixed-wing UAVs are faster, they cannot hover or take-off vertically. Rotary-wing UAVs can take off vertically from any location, eliminating the need for a horizontal plane on which to gain speed [62]. The most popular rotary-wing UAV is a quadrotor with four rotors [63]. They are agile and can hover in one place. They are, however, slower and have a shorter range [62]. As a result, rotary-wing UAVs are better suited for building applications. However, fixed-wing UAVs can be useful in long, linear infrastructure projects such as highways and railways. A relatively rare UAV type is a lighter-than-air-platform; examples are balloons and kites [64; 65]. These systems are substantially slower and have very little wind resistance. Typically, UAVs carry digital cameras as payloads to capture visual data from the site [66; 67]. They can also carry other equipment, such as thermal cameras [68; 69]. However, images or videos captured from UAVs can be noisy and require post-processing [66]. The design and payload of the UAV may vary based on the desired application [70]. The common use of UAVs in building inspections is to collect visual data by utilizing their on-board cameras [15; 71]. UAVs are also commonly used for bridge and powerline inspections, because of the high risk of fall accidents in the manual inspection of bridges [72; 73]. [PERSON] et al. [74] identified the following requirements for a UAV system for building inspection:  
* UAVs can hover anywhere to take photos. * UAVs can zoom in and focus on a small region of interest. * UAV operation should be simple, and the operator can get started without professional training. * UAVs should have a long enough flight time to improve operational efficiency. * UAVs should be capable of autonomous flight. * UAVs should be small enough for transportation and maintenance. A UAV can be operated either remotely or autonomously and also fly over difficult-to-access areas [75]. Researchers [72] have found that for large linear infrastructure projects, and with manual control of the UAV, frequent turning and climbing maneuvers can be exhausting for the UAV pilot.