# Energy Sources of Mobile Robot Power Systems: A Systematic Review and Comparison of Efficiency
## 2 Power Requirements for Mobile Robots
### Walking Robots and Wheeled Mobile Robots  
For each robot, designers need to select an energy source. An adult human weighing 80 kg needs around 270 to 560 W of metabolic energy to walk [17]. This could be used as the first approximation of energy demand for a human-size walking robot. The 80 kg DURUS robot uses an onboard 2.2 kWh lithium-polymer battery [18], and the BigDog robot uses a small internal combustion engine from a go-kart (11 kW) to drive a hydraulic pump and electric generator [19]. However, the BigDog robot's internal combustion drive was deemed too noisy for military applications, which redirected development toward an all-electric drive. The Lucy robot uses external compressed air tanks or compressors [20]. Thus, pneumatic actuation can be implemented either by integrating a compressor (powered by an electric or internal combustion engine) or by using a compressed air tank. The hydraulically powered robots (PETMAN [21], ATLAS, and ATLAS DRC [22]) used external sources of supply pressure. The ATLAS unplugged robot [23] used a supply pressure source located on its deck, which increased its total weight from 152 kg to 182 kg (i.e., by 19.74%). The current version of the ATLAS robot [18], based on an improved hydraulic system design, weighs 80 kg and measures 151 cm in height. Source [23] presents simulation results for a HOAP2 humanoid robot. The study suggests that a hybrid fuel cell power supply is superior to conventional batteries. There are emerging new possibilities for powering robots. The small 88 mm RoBeetle robot [24] is powered by catalytic combustion of methanol (with a specific energy of 5.6 kWh/kg). In comparison, a good-performance Li-ion battery provides just 0.5 kWh/kg, and the specific energy of animal fat is almost twice as high (10.6 kWh/kg). Scaling up this solution could be groundbreaking for mobile robotics. Scientific and industrial centers are conducting intensive research in the area of photovoltaics to power robots [25]. A small solar-powered walking robot has been presented [26]. A hybrid approach, where solar power is used to charge a rechargeable battery pack can be seen in humanoid robots such as ATLAS [25]. Interesting designs of a solar robot with wheels include Tertill [27] and Vitirover [28]. Wheeled robots are often used as Autonomous Mobile Robots (AMRs) [13]. These robots require a suitable onboard energy system that provides appropriate voltage, power, and capacity. This depends on the technology and function required. AMRs have been equipped with advanced sensors for the perception system (digital camera, Lidar system, etc.). The AMR picker has been equipped with robotic arms with a gripper. AMRs also require a motion drive power supply. For interested readers, a detailed explanation of the sensors and equipment used in AMRs can be found in [13]. Li-ion batteries have been used for most of these robots. Most AMRs are equipped with modern Li-ion batteries with a LiFePO\({}_{4}\) cathode and a graphite anode. As the voltage of a single LiFePO\({}_{4}\)/graphite cell is \(\sim\)3.25 V, the batteries are used in a series-parallel [13] connection to achieve the necessary voltage and capacity. The operating voltage for AMR batteries can range from 12 to 96 V and the capacity can vary from 10 to 200 Ah. AMRs have specialized functions affecting total mass, sensory equipment, and power supply parameters. Selected examples are shown in the Table 1 for LiFePO\({}_{4}\)/graphite and Li-ion cells. Boston Dynamics Handle robot is an AMR equipped with two legs with wheels [38]. This robot also uses two arms. Its kinematics combines the rough-terrain capability of legs with the efficiency of wheels. Using a manipulator arm, it is capable of picking up heavy boxes, weighing up to 15 kg. This is the reason why this robot is so heavy \(-\)150 kg and high \(-\)200 cm (maximum reach 280 cm), and can work with pellets that are 1.2 m deep and 1.7 m high. The robot uses an onboard battery typical of Boston Dynamics.