# Mechanical Design and Testing of Mobile Monitoring Units for Security Systems
## 2 Materials and Methods
### Simulation Model of the Mobile Monitoring Units  
A finite element method (FEM) model of the mobile monitoring unit was created to determine the stability of the construction and to analyze if there is a possibility that the mobile monitoring unit can be turned up due to a dynamic impact caused, for example, by wind gusts. A modal analysis was also performed on the FEM model to determine the natural frequency of the mobile monitoring unit, which is the input parameter for the consequent dynamic frequency analysis, the goal of which is to evaluate the system's periodic response to cyclical loads of frequency-dependent amplitudes. The load input takes the form of a driving frequency with the corresponding amplitudes. In our case, the amplitude and phase of the displacements and related velocities in response to a load oscillating at different frequencies were calculated.  
Numerical analyses of an idealized model consisting of shell and beam elements using the finite element method were performed. For the applied elements, the material and mechanical properties were defined. In Figure 5, the computational model of the mobile monitoring unit is shown. The 3D model of the mobile monitoring unit was created in the Creo-Parametric software. Consequently, an idealized calculation model was created, and the FEM simulations were performed in the Creo-Simulate software. A detailed view of the base of the mobile monitoring unit together with the defined beam element properties can be seen in Figure 6.  
The other problem resulting from the mobile monitoring unit's utilization in exterior conditions is temperature dependence. To increase the accuracy of the security sensors, temperature sensors for the calibration of temperature dependence must be included in the mobile monitoring units. However, for accurate measurements, it is necessary to also know the temperature dependence of the materials used for the mobile monitoring unit's construction. Therefore, the temperature dependence of the dynamic characteristics of the whole construction was analyzed using simulations performed in the Creo-Simulate software.  
Figure 4: Basis of the mobile monitoring unit with the adjustable legs: (**a**) CAD (computer aided design) model; (**b**) prototype.  
Figure 5: Simulation model of the mobile monitoring unit.  
Figure 6: Simulation model of the mobile monitoring unit: (**a**) detailed view of the basis; (**b**) defined properties of the beam elements.  
To confirm the correctness of the computational FEM model, experiments with the mobile monitoring unit prototypes were performed. During the experiments, the dynamic characteristics of the mobile monitoring unit prototypes were measured using an Adash 4400 VA4 Pro device manufactured by Adash company in Ostrava, Czech Republic. The measurements were performed so that the mobile monitoring unit was deflected by a transverse force of 50 N at one point, at a distance of 38 cm from the solar panel holder in the top part of the mobile monitoring unit, where several types of sensors are attached. The overall height of the mobile monitoring unit is up to 2 m because of the limitations resulting from transportation requirements. The overall weight of the mobile monitoring unit with the accumulators is 44.7 kg. The solar panel can therefore be easily removed and attached again to the construction. The sensor positioning fixtures were designed specially for this purpose and are manufactured using 3D printing [63]. For comparison, a whole set of measurements with two prototypes was performed, involving mobile monitoring unit designs without and with the tensioning steel wire ropes, with different lengths of legs, and of course, in different ambient temperatures.