## 3 Development of Retaining Wall Displacement Measurement System
### System Configuration and Design  
The retaining wall displacement measurement system comprises: (1) retaining wall displacement measurement hardware mounted under the corner strut, (2) software for controlling the hardware, and (3) a displacement analysis device that stores and analyzes the data collected by the hardware. A 2D LiDAR sensor was used for displacement  
Figure 2: Measurement method of the 2D LiDAR sensor-based retaining wall displacement measurement system.  
analysis [28; 29], and SICK's LMS511-10100 PRO product, which is more precise than other 2D LiDAR sensors, were used. Since it is a 2D line scanner, it performed only line scanning, as shown in Figure 3a and a driving device for the 360\({}^{\circ}\) rotation of the 2D LiDAR sensor at a constant speed (Figure 3b) was used to collect the 3D point cloud data about the retaining wall. Consequently, a servomotor (SANKYO of Japan: MM101A2 LA18) and servo driver for controlling the servomotor were applied to the driving device of the hardware. Additionally, a slip ring device (COVIS of Korea: CSH025-15-1006-01E), which prevents cable twisting and allows the 2D LiDAR sensor to rotate infinitely, was applied to the hardware.  
Figure 4 shows the configuration of the 2D LiDAR sensor-based retaining wall displacement measurement system. The displacement measurement hardware receives power from an external power supply (220 V) and the power supply to the hardware (2D LiDAR sensor, servomotor, and control computer) can be confirmed through an external light-emitting diode (LED). The control computer (LATTEPANDA in China: Latte Panda Alpha 864s) controls the servomotor (RS-485 communication) and 2D LiDAR sensor (TCP/IP communication) using the software for hardware control, which stores the data collected using the 2D LiDAR sensor. The displacement analysis device (used by the construction site manager) controls the hardware by accessing the control computer remotely using WiFi and receives the point cloud data stored in the control computer.  
Based on Figures 3 and 4, the displacement measurement hardware was designed, and it is depicted in Figure 5. The hardware was mounted under the corner strut using a fixing bracket, and the outer case was designed so that only the top case could be separately dismantled to allow the maintenance of the components inside the hardware. The servomotor was mounted on the right-angled reducer and set to prevent offsets from occurring when collecting the point cloud data. This is achieved by aligning the rotation axis of the reducer with the centerline of the 2D LiDAR sensor during the 360\({}^{\circ}\) rotation of the sensor (Figure 5d).  
Figure 3: 3D point cloud data collection using the 2D LiDAR sensor. (**a**) Scan range of the 2D LiDAR sensor; (**b**) 360\({}^{\circ}\) rotation of the 2D LiDAR sensor.  
Figure 4: Configuration of 2D LiDAR sensor-based retaining wall displacement measurement system.  
Figure 5: Design of the retaining wall displacement measurement hardware. (**a**) Perspective view; (**b**) perspective view inside the hardware; (**c**) layout of components inside the hardware; (**d**) alignment between the rotation axis of the reducer and centerline of the 2D LiDAR sensor.