## 2 Materials and Methods
### Construction of the Measurement Device  
A few steps were taken to make sure that all sensors measured the same distance from the object under study. (1) Sensor base: A PVC plate (Figure 5a) provided a solid, uniform, and smooth base for the sensors. (2) Sensor connection to the PVC plate: Three-dimensional printed clamps were designed to connect the sensors to the plate (Figure 5b). These 3D-printed clamps were developed to hold the HC-SR04, VL53L0X, and VL53L0X at a known height. These clamps were glued to the PVC plate (Figure 5a) and then the sensors were glued to the 3D-printed clamps (Figure 5c). At this position, the measuring part of the ultrasonic sensors (the top of the HC-SR04 sensors) was located at a 25 mm height from the PVC plate. The thickness of the ToF sensors was fixed to 2 mm. These heights were taken into account by implementing these values to the Arduino code. (3) Wiring: The sensors were wired as shown in Figure 5d. The 25 ultrasonic sensors were connected to the digital ports of the microcontrollers, and the 50 sets of ToF sensors were connected to I2C ports of the microcontrollers using multiplexors (Figure 6). (4) The Raspberry Pi was connected to the Arduino Uno for wireless control and saving measurements. Then, the Arduino Uno was connected to the Arduino Mega using two jumper wires (Figure 6). Finally, the Arduino Mega and Raspberry Pi were connected to an external power source. Figure 4(e) shows the developed distance measuring device in the laboratory. As seen, the device was located on a fixed platform experiment testbed. The mobile part of the actuator is the upper jaw of the jack (Figure 7). In addition, the flowchart of the construction of measurement device is presented in Figure 4(f).  
In Figure 6 shows all of the assets required to make the multisensor distance measuring device. Figure 6 also displays the serial connection of Arduino Uno and Mega, as drawn by Fritzing software [57]. Pins 0 (RX) and 1 (TX) on the Arduino Uno are connected to pins 1 (TX) and 0 (RX) on the Arduino Mega, respectively. RX and TX pins stand for receiving and transmitting pins of the Arduino serial communication.  
Figure 5: _Cont_.  
Figure 5: The components of the distance measuring device: (**a**) PVC sheet for attaching the sensors and the data acquisition equipment, (**b**) designed 3D-printed base for holding the various sensors together at a known height, (**c**) sensor allocation, (**d**) wiring the system together, (**e**) the experiment platform for validating the accuracy of several different low-cost sensors and (**f**) flowchart of the construction of the proposed measurement device.  
Since this paper targets static sensors, the sensors were not synchronized with microsecond resolution. Even though the ToF sensors have a very high sampling frequency, the ultrasonic sensors need some time to measure data. For that, some delay functions were used in the Arduino code of the tailored device. This system measured distance once every 10 s. In the absence of ultrasonic sensors, ToF sensors could have been synchronized with microsecond resolutions, as in the recently published article by the same authors [50].  
The current system has already been used with accelerometers, as well. Recently, Komarizadehas et al. [50] published a paper discussing noise reduction of dynamic sensors  
Figure 6: Scheme of the connections between the microcontrollers and the Raspberry Pi.  
Figure 7: The laboratory experiment equipment.  
using several synchronized low-cost accelerometers. Thus, a system made from dynamic sensors can be used for tests with dynamic loadings.