# Mechanical Design and Testing of Mobile Monitoring Units for Security Systems
## 3 Results  
As the mobile monitoring unit was designed to withstand demanding environmental conditions, the main goal of the stability analysis was to determine the force necessary to turn the mobile monitoring unit over. This situation was analyzed given that a solar panel is on the top of the mobile monitoring unit, and severe windy conditions can possibly cause problems with its stability, even turning it over, which can lead to damage to its integral parts, such as solar panels or sensors mounted on the external side of the construction. The initial step was to calculate the force \(F\) required to tip the mobile monitoring unit. Considering the weight \(m\) of the mobile monitoring unit being equal to 44.7 kg, the reaction \(R_{\text{A}}\) in every leg is 149 N--see Figure 7. The force \(F\) can be calculated from the equilibrium of the moments of the forces using the relationship  
\[F.h=R_{A}.r, \tag{1}\]  
where \(h\) is the overall height of the mobile monitoring unit and \(r\) is the distance of the reaction of the mobile monitoring unit's axis. For our case, considering the height of the mobile monitoring unit \(h=2000\) mm and the distance of the reaction \(r=800\) mm, the resulting force \(F\) was 59.6 N.  
Consequently, regarding the solar panel's dimensions of 425 \(\times\) 668 mm, the resulting pressure \(p\) from the force \(F\) acting on a solar panel with the area \(S\) of 0.2839 m\({}^{2}\) is 210 Pa. The calculated pressure represents the worst-case situation considering the impact of wind on the solar panel in a perpendicular direction. In this case, according to the Beaufort scale, the calculated pressure corresponds to grade 8, which represents a gale or fresh gale, where the wind speed is 17.2-20.7 m\(\cdot\)s\({}^{-1}\) (62-74 km\(\cdot\)h\({}^{-1}\)) and the wind pressure is in the range from 18.4 to 26.8 kg\(\cdot\)m\({}^{-2}\). Under gale conditions, whole trees are in motion, people feel inconvenience when walking against the wind and upright walking is not possible. It must also be mentioned that in this worst case, the mobile monitoring unit only starts to turn, and additional force is necessary to turn over the whole mobile monitoring unit. Furthermore, in real operation, wind force does not generally act on the solar panel perpendicularly; therefore, even higher forces will be necessary to turn the mobile monitoring unit over. A modal analysis was performed to find the natural frequency of the mobile monitoring unit. It is necessary to know this frequency to ensure correct function, as every sensor has specified ranges, after the exceeding of which an alarm situation occurs. Signal pre-processing by applying a band block filter can contribute to higher reliability and robustness of the whole mobile monitoring security system. The results of the modal analysis that we performed can be seen in Figure 8. The natural frequency of the mobile monitoring unit without tensioning steel wire ropes in the first mode was 3.9 Hz. Similarly, the natural frequency was also calculated for the version with tensioning steel wire cables, where the frequency was 6.23 Hz. Figure 8: Modal analysis results of the mobile monitoring unit without tensioning steel wire ropes. Figure 7: Simulation model of the mobile monitoring unit with calculated reactions in the legs. The modal analysis created the basis for dynamic frequency analysis. The dynamic analysis results for the design without tensioning steel wire cables and with legs with a length of 665 mm from the attachment, with the force of 50 N applied at a distance of 380 mm from the attachment of the solar panel, can be seen in Figure 5. Also, the results derived when some of the security sensors are attached can be seen in Figure 9. From the analysis results, it can be seen that, with a frequency of 3.9 Hz, the velocity at the analyzed point reached 2297 mm\(\cdot\)s\({}^{-1}\).