## 4 Platform and Sensors Implementation and Evaluation
### Sensors
#### 4.2.3 Distance Sensors  
In order to measure the distance between slab tracks it is necessary to allocate one or two distance sensors in the adjacent borders of two slab tracks. On the market there are two well-known kinds of distance sensors: infrared and ultrasonic. The former are recommended for use principally in indoor environments since their measurements are highly affected by sunlight. That is, since light does not  
\begin{table}
\begin{tabular}{c c c c} \hline  & **SQ-SI-360 DA** & **SCA100T-D2** & **ADXL345** \\ \hline
**Manufacturer** & SignalQuest & DFRobot & Analog devices \\
**Axis** & 2 & 2 & 3 \\
**Consumption** & 4.6 mA & 4 mA & 40 \\
**Price** & 363 \(\upepsilon\) & 63.14 \(\upepsilon\) & 16 \(\upepsilon\) \\
**Interface** & analog, UART & SPI & SPI \\
**Precision** & Analog: 3.49\% (2 ) & 0.01\% (0.0035 ) & 0.17\% (0.1 ) \\
**Image** & & & \\ \hline \end{tabular}
\end{table}
Table 3: Technical characteristics of the evaluated inclinometers.  
reflect the same way off every surface, the infrared sensor reading will be different for different surfaces, different colors, and different shades even if the range is the same. However, ultrasonic sensors not only can be used outside in bright sunlight but also they are able to carry out accurate measurements in these conditions. Due to the fact that the designed monitoring device is going to be installed outdoors, three ultrasonic distance sensors have been analyzed in order to choose the most suitable one: MB1043, SRF08 and PING ultrasonic sensor. Table 4 shows their main features.  
After testing in our lab (see Figure 5) the performance of the sensors presented in the Table 4, we realized that sensors with a dual-element ultrasonic module are able to read much closer distances than those with a single-element. Namely, the MB1043 sensor was able to read distances of between 30 cm and 5 m, the SRF08 sensor distances between 3 cm and 6 m, and the PING sensor distances between 2.41 cm and 3 m. Over short distances, the PING model was the sensor able to provide better and more accurate measurements. However, SRF08 was the model able to measure longer distances (up to 11 m) although when we tried to measure distances of 2.5 m, again, the PING sensor provided more accurate results. It is worth noting that both sensors were able to detect changes with a precision of 1-2 mm. In conclusion, we find that the SRF08 model is a highly configurable sensor. For instance, it can be tuned to performance faster reads over short distances, its gain can be modified and it can be also put into Artificial Neural Network mode to detect several obstacles located at different distances at the same time. But, from the project requirements point of view the most important aspects to be taken into account are energy consumption and accuracy measurements over short distances. Thus, despite the fact that the SRF08 sensor consumes much less energy than the PING sensor, we consider the latter to be more suitable for this project as it is not only able to provide accurate measurements when the obstacle is located very close but it is also half the price.  
We are aware that the requirements state that the minimum measurable distance in this sensor is approximately 1 cm while the distance between slab tracks could be lower, but this issue can be solved by installing the sensor and setting it back from the slab track edge.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **MB1043** & **SRF08** & **PING** \\ \hline
**Manufacturer** & MaxBotix & Devantech & Parallax \\
**Consumption** & 3.1 mA & 3 mA & 35 mA \\
**Price** & 28 \(\upepsilon\) & 42 \(\upepsilon\) & 24 \(\upepsilon\) \\
**Interface** & Analog, UART & Digital & Analog \\
**Image** & & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Technical features of the evaluated distance sensors.