## 3 Design and Sizing of the Hydrogen-Battery Hybrid Power System  
The electrical power system was designed according to load requirements and primarily consists of six AC-powered wheels. DC/AC converters (Shenzhen Renhao Weiye Technology Electronics Co., Ltd., Donnguan, China) are used, operating at 36 VDC/wheel, with a current consumption ranging from 0 to 1.5 A/wheel (Figure 6).  
Figure 4: (a) CNC machine, the Lagun 1000. (b) Detail of the machining process of a mobile part.  
Figure 5: (a) Detail of the turning of an M16 stainless steel screw on a manual lathe. (b) The final result.  
The hybrid electrical power system comprises a hydrogen-powered PEM fuel cell and a battery pack (B1, B2, B3), which together regulate the DC bus voltage, supply peak power demand for the wheels, and provide sufficient energy to last over the required 8 h of autonomy.  
Both the PEM fuel cell and battery pack are connected directly to the DC bus with relays RLY-01 and RLY-02. This configuration avoids the use of heavy power electronics, mitigating excess weight on the mobile platform.  
The PEM fuel cell subsystem is built from the PEM fuel cell and all the accessories needed for its operation (balance of plant, or BoP). It is monitored with voltage and current transducers, labelled VT-01 and CT-01. The hydrogen supply is controlled with the solenoid valve SOV-01 and the pressure regulator PR-01, which is connected to the output of the hydrogen tank. The flow transductor FT-01 provides details of the PEM fuel cell's hydrogen consumption. A hydrogen purge is achieved with a solenoid valve, SOV-02. All the BoP's accessories have been selected using the basis of a common 12 VDC operating voltage. From the battery subsystem output, a DC/DC converter is then needed to convert the 36 V received from the DC bus to the 12 V required by the BoP. The connection between the battery subsystem and the 36-to-12 V DC/DC converter is established by the interrupter INT-01.  
The platform's driving system is based on a 6-channel radio-controlled (RC) module (described in Section 4) that allows the user to move the vehicle in different directions and speeds. The radio control module communicates with the power converters of the wheels through the signal modulation carried out by the microcontroller.  
Finally, a rheostatic braking system has been included on the platform, with the aim of dissipating the energy surplus that could eventually appear when the PEM fuel cell and battery subsystem are working together. The rheostatic brake is connected to the DC bus by means of a MOSFET transistor, controlled by the microcontroller, which can activate or deactivate the transistor, depending on the bus voltage.  
Regarding the power balance, which is based on the direct connection of the PEM fuel cell subsystem and the battery subsystem with the DC bus, a passive power management system is implemented. The nominal battery subsystem voltage and PEM fuel cell subsystem can be expressed as \(V_{bat}=V_{FC}=36\) V. The battery voltage is monitored. In this case, its value is \(V_{bat}\geq 38\) V; this means that the battery subsystem is overcharged. Then, the PEM fuel cell is disconnected, the rheostatic brakes are activated, and the battery  
Figure 6: Hydrogen-battery hybrid power system for the Hygrobot platform.  
subsystem provides the power demanded by Hygrobot. This will involve the battery subsystem discharges. In the case of \(V_{bat}\leq 33\) V, this means that the battery subsystem is over-discharged. Then, Hygrobot is stopped, and the PEM fuel cell recharges the battery subsystem. When \(33\) V \(<V_{bat}<38\) V, the PEM fuel cell and battery subsystems supply the power demanded by Hygrobot, Figure 7.  
Each subsystem is described in detail in the following sections.