## 6 Research Areas
### Sensing  
Robots can be equipped with various types of sensors to perform different types of inspections. They can be equipped with high-precision servo-accelerometers to measure story drift in earthquake-affected structures without the need for a human inspector [234]. Many other sensors have been used with robots for non-destructive testing and structural health monitoring, such as eddy current, RADAR, infrared (IR), and ultrasound sensors [95; 170]. [PERSON] et al. [100] proposed using gas detectors with a pipe-climbing robot for detecting gas leakage in high-rise towers. [PERSON] and [PERSON] [235] developed a robotic arm to automatically collect water samples from a solar pond. The arm was equipped with multiple sensors for in situ measurement of density, salinity, pH, and turbidity. [PERSON] et al. [236] embedded sensors, such as strain gauges, in the building structure. They used a UGV to power the sensors and collected data wirelessly through electromagnetic induction. This avoided the need for embedding power cables for powering the sensors. [PERSON] and [PERSON] [92] installed RADAR on a robotic cart to create a digital reconstruction of the interior of a bridge deck through diffraction tomography. [PERSON] et al. [129] used audio sensors to perform a hammering test on steel structures using a hexapod robot. The hammering test detects and analyses the vibration of a structural element after striking it with a hammer. Anomalies in the resulting vibrations indicate loose bolts or other structural defects [129]. An emissometer connected to robotic arms has been used for autonomous mapping of the thermal emissivity of the building envelope for energy studies [161]. Robots equipped with IR sensors are used to monitor the building envelope for any thermal anomalies [158; 237; 238; 239]. When using IR, the angle of tilt of the sensors affects the accuracy of measurement [240]. Therefore, UAVs can be advantageous in maintaining the right angle while collecting data for high-rise buildings. Experts may choose to combine multiple sensors into a single robotic platform for faster inspection and more detailed inspection. These sensors can be grouped together into six categories--thermal/IR sensors, ground-penetrating radars (GPR), accelerometers, laser-based sensors (LIDAR or laser scanners), strain gauges, microphones, and electrical resistivity sensors. The frequency of works reporting the use of these sensors found in the review is presented in Figure 5. The numbers in the figure represent the number of papers reviewed reporting the use of such type of sensors.  
When using multiple sensors together in a single robotic platform, fusing multiple sensor data together to present more meaningful data is important and was the focus of research in [94] and [86]. [PERSON] et al. [95] combined data from a ground-penetrating RADAR, thermal IR camera, LIDAR, GPS, and accelerometer into a long short-term memory neural network for detecting road defects. Combining and processing the data from different types of sensors, such as positioning sensors (GPS, IMU), visual sensors (camera), and non-visual sensors (RADAR, thermal camera) can provide more in-depth inspection in a shorter time [86; 94]. However, more sensors will increase the weight the robot needs to carry, and its power requirements [241]. The more sensors a robot carries, the larger the capacity required, and the larger it has to be in size, which in turn increases the power requirements even further.