## 5 Simulation Verification
### Drive Cycle  
The HPS of a security robot is similar to the powertrain in modern hybrid electric vehicles. For such vehicles, there is a drive cycle that is commonly used in the industry, and based on it, their powertrain can be designed and validated. However, this is not the case with our security robot, which is in the early specification development phase.  
Despite limited knowledge regarding the actual operating conditions of the security robot, its operation mode was defined by modifying one of the popular drive cycles, namely a charge-depleting cycle life testing profile for an electric vehicle battery [28]. The modified drive cycle lasts for a total of 12 min, with the security robot simultaneously surveilling and patrolling for the first half, and only surveilling for the second half (see Figure 12). Throughout this simulation, the electrical load required for surveillance was continuously applied and was uniform at 250 W. The mechanical load required for patrolling was often added to that and varied above 200 W. Overall, this simulation consists of three relatively light loads in a row followed by one heavy load (see Figure 12). The final simulation we conducted focused on the power flow in the load transitions from light to heavy and vice versa. The hydrogen flow rate was also used to double check the operating time estimated based on the energy.  
Figure 11: Simulated (**a**) power, (**b**) current, and (**c**) voltage of HPS under the short-circuit condition.  
Initially, the load increases to 250 W, and 16 s later, it steps up to 500 W (see Figure 13). After 28 s, the load steps up again to 750 W. The maximum load is held for 11 s, after which it steps down to 250 W. Similar to the first simulation, compared to the battery, the slower transient response of the fuel cell can be clearly observed. Under rising loads, the battery provides power to the load before the fuel cell, ensuring the responsiveness of the entire system. Under a falling load, the fuel cell provides the power remaining after supply to the load with the battery, which promptly recovers its SOC. Therefore, the behavior of the entire system and its components is consistent throughout the three consecutive light loads.  
Next, at 236 s, the load suddenly increases to 2.25 kW. After 8 s, it steps back down to 1.5 kW. In response to the heavy load, the fuel cell supplies power up to a maximum of 945 W, which corresponds to a current less than or equal to 30 A. As stated previously, this reflects the OC fault that protects the selected fuel cell. The battery power compensates for any shortcomings. At 360 s, the first half of the drive cycle ends, and the second half continues until 720 s.  
We observed that the hydrogen flow rate increases with an increasing load (see Figure 12). The amount of hydrogen consumed per cycle is estimated to be approximately 90 L, which is equivalent to 7.2 g based on the weight of 1 L of hydrogen (0.08 g). As mentioned previously, if compressed at 310 bar, the selected cylinders can store up to 0.71 kg of hydrogen, allowing them to complete 95 cycles on a single charge. Considering that the duration of each cycle is 12 min, it was predicted that 19 h of continuous operation can be achieved. Based on our calculations, in electrical terms, the operating time was estimated previously. Through computer simulations, the results were cross-checked by evaluating hydrogen consumption to verify the consistency of the information provided. The result of falling short of the goal time of 24 h is understandable given that the load applied during the drive cycle, particularly the mechanical power required for patrolling, is greater than the average power.  
Figure 12: Simulated (**a**) power and (**b**) hydrogen flow rate with accumulated mass of HPS throughout a full cycle (until 720 s).