Development of Hydraulic-electric Power Units for Mobile Robots

Energetic autonomy of a hydraulic-based mobile robot requires a power source capable of both hydraulic and electrical power generation. The hydraulic power is used for locomotion and the electric power is used for the control computer, sensors and other peripherals. In addition, the power source must be lightweight and quiet. This study presents several designs of internal combustion engine-based power units. Each power unit is evaluated with a Ragone plot which shows its performance over a wide range of operation times. The best-performing design of the hydraulic-electric power units (HEPU), based upon the Ragone plot analysis, is described in detail. This HEPU produces constant pressure hydraulic power and constant voltage electric power. The pressure and voltage are controlled on board the power unit by a computer. A novel characteristic of this power unit is its cooling system in which hydraulic fluid is used to cool the engine cylinders. The prototype power unit weighs 27 kg and produces 2.3 kW (3.0 hp) hydraulic power at 6.9 MPa (1000 psi) and 220 W of electric power at 15 VDC. INTRODUCTION Most human scale and field robotic systems are currently powered by tethers or heavy battery systems. In order for a robotic device to obtain energetic autonomy free from tethers and heavy batteries, a compact, portable power unit providing both mechanical power for actuation and electrical power for computation and control is essential. The NiMH battery pack in ASIMO, Honda’s humanoid walking robot, is one example of a battery-based power unit [1]. However, batteries have a low specific energy (energy per unit mass): 150 W-hr/kg for a high performance lithium ion battery [2]. Due to this low specific energy, batteries become large and heavy unless the operation time is short or the robotic system requires little power. A fuel with a higher specific energy than batteries is desirable in a mobile robotic system. Internal combustion (IC) engines utilize the high specific energy of gasoline: 12000 Whr/kg. The power units described here utilize IC engines to produce compact, lightweight power sources. This is primarily motivated by the fact that IC engines have been the primary source of power for automobiles, earthmoving machinery, motorcycles, and other wheeled vehicles. We envision mobile field robots as another class of these field vehicles that operate outdoors for periods of hours. In fact several field and service robotic systems have already experimented with IC engines as their prime mover [3]-[4]. This paper describes the basic design challenges of a generic hydraulic-electric power unit (HEPU) for robotic applications. First, an analysis of the HEPU systems based on Ragone plots is presented. Then the specifications of several versions of the HEPU are described. Finally, the final architecture of the HEPU, including its hydraulic and electric power generation, cooling system, and control, is described in detail. RAGONE PLOT ANALYSIS Ragone plots are useful for evaluating the performance of a power unit for a wide range of operation times [5]. They plot the power unit’s specific power (power divided by total mass) versus the specific energy (energy divided by total mass). The specific energy can be expressed by Ê