# Hybrid Power System for the Range Extension of Security Robots: Prototyping Phase
## 4 Vehicle Testing
### Integration Test Results  
The integrated plug-in HPS prototype was bench-tested on the dynamometer during development (see Figure 4). Owing to the lack of capabilities, the integration tests were mostly carried out at a typical speed (5 kph). In accordance with the previous tests, the first test was aimed at verifying an increase in operating hours thanks to larger batteries and a wider SOC in the plug-in hybrid structure. In the simulation, the widest SOC window (90%) was assumed, but it is not feasible in reality. The upper limit of the SOC was fixed at 81% because of the limit imposed by the low quality of the charger in the plug-in HPS prototype. The lower limit of the SOC was first set at 15%, and it would be optimized in the following test. In consequence, the CD mode lasting for 1.27 h was added before the transition to the CS mode (see Figure 5a). Note that the battery bears the load alone during the CD mode that is followed by the CS mode in which the fuel cell shares the load with the battery. The added hours in the actual test is nearly 40% shorter than that in the simulation. The greater part of this shortage is attributable to the narrower SOC window (90% to 66%), but it cannot cover all. The remaining part is related to inaccurate and unstable load regulation in the dynamometer. While spinning the rollers at a constant speed (5 kph), the load demand abruptly dropped at 5.67 h. This was witnessed by a change in the power supply at low-side (see Figure 5b), which was reduced from 390 W to 240 W approximately. This can also be manifested by a  
Figure 4: Design drawing of the integrated plug-in HPS. (**a**) Side view and (**b**) Top view showing (1) fuel cell, (2) fuel storage, (3) battery, and (4) DC/DC converter. (**c**) Pre-prototype under integration tests. variation in the rate of change in the hydrogen pressure (see Figure 5c). By accident, the reduced load (240 W) is close to the estimated power (200 W) required to move the security robot at a typical speed. Details on the estimation process can be found in our original paper [22]. In this circumstance, we observed that the integrated plug-in HPS prototype continuously drove the security robot on the dynamometer at 5 kph over 24 h.  
The second test was aimed at determining the lower limit of the SOC. Note that by optimizing the usable SOC range, the operating hours of the security robot can be maximized while maintaining the operational stability of the plug-in HPS. The lower limits of the SOC were set at 15%, 46%, and 22% in sequence. Based on the characteristics of the one-stage series structure, these limits were realized by adjusting the output voltage of the converter to 23.8 V, 24.8 V, and 24.3 V, respectively. A lower final SOC makes the operating time of the security robot longer but also makes the plug-in HPS more vulnerable to sudden load changes. Hence, as performed in the first test, the final SOC was first set at 15%, and at the beginning, the vehicle speed was set to 5 kph, which is approximately equivalent to a motor speed of 1500 rpm. A sudden change in the load demand was imposed by stepping the motor speed up by 500 rpm. At 24.188 h, we observed that the load increased to nearly 860 W by revving up from 2500 rpm to 3000 rpm (see Figure 6a). At this moment, the fuel cell shut down with an OC alarm, and the battery bore the load alone. The fuel cell controller subsequently attempted to reconnect the load every ten  
Figure 5: Results of integration tests performed to verify an extended hours of operation based on the plug-in hybrid structure. (**a,b**) Power and (**c**) Pressure with Energy versus time. seconds but failed because of the same fault. About 4.2 min later, the fuel cell recovered when the motor speed was revved down to 1500 rpm (see Figure 6b). The operating points of the fuel cell when it stopped were measured by our data acquisition system to be 28.5 V and 30.1 A (see Figure 6c,d). Unlike the previous unit test, an OC (30.1 A > 30 A) alarm went off, as specified in the user's manual.