## 3 Component Specifications
### Energy Specification  
The energy specification can be set based on the main development goal of our security robot, namely 24 h continuous operation on a single charge. As stated previously, the operation modes of the security robot can be divided into surveillance and surveillance during patrolling. The average power required for each mode was estimated to be approximately 250 and 450 W, respectively. The mechanical power required for patrolling is thus 200 W, which is mostly consumed by electric motors. The electrical power required for surveillance is primarily consumed by sensors and computers in the security robot. Figure 3: Variations in the power flow within the selected structure. (**a**) Incoming to battery and (**b**) outgoing from battery. Assuming that the vehicle is moving on paved, rough, and flat ground at a constant velocity, the required power, \(P\) (200 W), is estimated as follows:  
\[P=\frac{F\cdot v}{\eta}, \tag{1}\]  
where \(F\) is the total tractive effort (92.75 N), \(v\) is the average velocity (5 km/h = 1.39 m/s), and \(\eta\) is the lumped efficiency (0.657). The total tractive effort, \(F\), which is the force propelling the vehicle forward, is calculated as:  
\[F=F_{rr}+F_{ad}+F_{hc}+F_{la}+F_{aa}, \tag{2}\]  
where \(F_{rr}\) is the rolling resistance force, \(F_{ad}\) is the aerodynamic drag, \(F_{hc}\) is the hill climbing force, \(F_{la}\) is the force required to provide linear acceleration to the vehicle, and \(F_{aa}\) is the force required to provide angular acceleration to the electric motors. Under the assumptions above, only \(F_{rr}\) and \(F_{ad}\) are accounted for. The rolling resistance force, \(F_{rr}\) (92.75 N), is calculated as:  
\[F_{rr}=\mu_{rr}mg, \tag{3}\]  
where \(\mu_{rr}\) is the coefficient of rolling resistance (0.03) and \(m\) is the vehicle weight (270 kg). Although negligible compared to \(F_{rr}\), the aerodynamic drag, \(F_{ad}\) (0.53 N), is evaluated as:  
\[F_{ad}=\frac{1}{2}\rho AC_{d}v^{2}, \tag{4}\]  
where \(\rho\) is the air density (1.25 kgm\({}^{-3}\)), \(A\) is the frontal area (1.1 m\({}^{2}\)), \(C_{d}\) is the drag coefficient (0.4), and \(v\) is the average velocity (1.39 m/s). Different power conversion efficiencies are combined into the overall efficiency, \(\eta\) (0.657), which includes the electric motors with their reduction gear (0.75) and the DC/DC converter (0.9). Therefore, the mechanical power required for patrolling was estimated to be 196 W. For simplicity, we consider a value of 200 W. The electrical power required for surveillance was assumed to be 250 W. If the security robot surveils for half of 24 h (250 W \(\times\) 12 h = 3 kWh) and surveils while patrolling for the other half (450 W \(\times\) 12 h = 5.4 kWh), the total energy required adds up to 8.4 kWh. As sources of the parameter values used to evaluate the mechanical power required for patrolling, \(\eta\), \(\mu_{rr}\), and \(\rho\) were as referred to in [19; 20], \(m\) and \(A\) were measured, and \(C_{d}\) was estimated. Assuming that the gravimetric energy density of the battery used in the HPS is approximately 100 Wh/kg and the required energy is supplied by batteries alone, batteries weighing over 80 kg would be required. Although the weight of batteries varies in direct proportion to the energy stored inside them, this is not the case for fuel cells. Fuel cells can \begin{table}
\begin{tabular}{c c c} \hline Speed (km/h) & Avg. & 5 \\ Max. & 13 \\ \hline Climbable angle (\({}^{\circ}\)) & 18 \\ \hline Climbable step-obstacle height (cm) & 20 \\ \hline Charging time (h) & 1 & Compressed hydrogen used as a fuel \\ \hline Operating time (h) & 24 \\ \hline Range (km) & 70 \\ \hline Operating temperature (\({}^{\circ}\)C) & \(-\)20–45 \\ \hline Terrain capacity (mm) & 40 & Based on the size of pebble used as a road surface material \\ \hline \end{tabular}
\end{table} 
Table 2: Security robot’s requirements pertaining to HPS. supply as much energy as necessary without increasing their weight.