# Hybrid Power System for the Range Extension of Security Robots: Prototyping Phase
## 2 Unit Testing
### Unit Test Bench  
Rather than using large-scale and high-cost equipment specialized for batteries or fuel cells, our unit test bench consists mostly of general-purpose equipment. Static and dynamic loads were simulated using a DC electronic load (Kikusui, PLZ1004W) coupled with a DC power supply (Kikusui, PWR800L). The electronic load discharges a battery like in a situation where the security robot speeds up. The power supply charges a battery as in a situation where the security robot slows down, although regenerative braking might be insignificant in such a low-speed vehicle. Using the electronic load and power supply, a load transient from charge to discharge and vice versa is possible, but a significant latency period should be provided. To make the transition seamless, the electronic load and power supply were integrated with a charge-discharge system controller (Kikusui, PFX2512) that can guarantee a latency times less than 50 ms, which is required to emulate drive cycles for electric vehicles.  
Our unit test bench also comprises multi-purpose and modular data acquisition systems (National Instruments, cRIO-9035), to monitor, save, and post-process test data. The charge-discharge system controller is also capable of measuring the voltage, current, and temperature over a few channels. The fuel cell can take such measurements internally,and the battery can do so. However, such individual measurements were not sufficient because system-level, rather than component-level, data acquisition was required. The use of the data acquisition systems allowed us to meet our requirements, which include multiple point measurements, exact time synchronization between multiple measurements, and the provision of an interface to external equipment such as a chassis dynamometer.  
In the power network of the HPS, the voltages were measured at the output of the fuel cell (high-side) and converter (low-side). Because of the characteristics of the one-stage series structure, the low-side voltage is equipotential with the DC bus voltage and battery voltage. As the high and low-side voltages span from 21 V to 45 V, voltage input module with a 60 V input range (National Instruments, 9229) were used for both the high and low-sides. The currents were also measured at both the high and low-sides. At the low-side, the current was additionally measured at the output of the battery to trace the power flow. The high and low-side currents range up to 50 A, and thus the hall-effect current transducers (LEM, DHAB S/145) were used for both sides. The voltage produced by the current transducer was acquired using another voltage input module with a 5 V input range (National Instruments, 9239). This was also used to measure the voltage produced by a hydrogen pressure transducer ([PERSON], PA-23). The hydrogen pressure was used to estimate the hydrogen consumption and fuel cell efficiency. The temperatures were also measured. Because the temperature within the fuel cell was already given by its controller, the temperatures of air flowing in and out of the fuel cell were measured using a temperature input module (National Instruments, 9212). The apparatus in our unit test bench are listed in Table 2.