Energetic autonomy of a hydraulic-based mobile field robot requires a power source capable of both electrical and hydraulic power generation. While the hydraulic power is used for locomotion, the electric power is used for the computer, sensors and other peripherals. An internal combustion engine was used as the prime mover due to the high energy density of gasoline. The primary specification for this hybrid hydraulic-electric power unit (HEPU) is that it must output constant pressure hydraulic power and constant voltage electric power. An on-board computer uses a pressure sensor and a speed sensor to regulate the pressure and voltage by modulating a hydraulic solenoid valve and an engine throttle. The speed regulation also results in a system noise with predictable frequency band which allows for optimal muffler design. A novel characteristic of this power source is its cooling system in which hydraulic fluid is used to cool the engine cylinders. Several hydraulic-electric power units were built and successfully demonstrated on the Berkeley Lower Extremity Exoskeleton (BLEEX) shown on bleex.me.berkeley.edu/bleex.htm. A prototype power unit weighs 27 Kg, outputs 2.3 kW (3.0 hp) hydraulic power at 6.9 MPa (1000 psi), and 220 W of electric power at 15 VDC.

We have developed a novel device to supply hydraulic power for anaerobic mobile robotic systems. This power source demonstrates the possibility of operation in oxygen-free environments, such as underwater or in space. Unlike the internal combustion engine, the design produces power on demand, eliminating idling when there is no load on the system. This monopropellant-powered free piston hydraulic pump (FPHP) was designed as a human scale (1.0-3.0 kW) power source, and produces only steam and oxygen as its byproducts. The FPHP utilizes high concentration hydrogen peroxide as the monopropellant energy source, which decomposes into hot gas when exposed to a catalyst. Energy is extracted from the hydrogen peroxide and transferred directly to hydraulic fluid by expanding the hot decomposition gas in an integrated piston/cylinder arrangement. Based on a specific power and specific energy analysis using a Ragone plot, the performance of the FPHP potentially exceeds that of a battery-based hydraulic power source for short operation times. Note to Practitioners-This project was motivated by the need for a simple free piston engine that is capable of producing hydraulic power. A free piston engine has no crankshaft or rotating motion, only a piston assembly that is constrained to move in a linear fashion within a bore. The energy of the free piston can be harnessed by pressurizing a working fluid or generating electrical power. Instead of relying on the combustion of gasoline or diesel fuel to move the free piston, which involves problems with fuel/air mixing and ignition timing, we created a device that is powered by the catalytic decomposition of a monopropellant. This concept overcomes the problems with a combustion powered free piston engine and also affords operation in environments with no atmosphere, such as underwater or in space. The device we constructed is an integrated free piston engine and hydraulic pump with very few moving parts. This monopropellant free piston engine could be adapted for use with any source of pressurized gas (preferably one with a high specific energy) and could be scaled to varying power outputs. The largest hurdle to overcome before realizing reliable operation of the free piston hydraulic pump (FPHP) is ensuring that the catalyst effectively decomposes the monopropellant.

We have developed a novel device to supply hydraulic power for anaerobic mobile robotic systems. This power source is capable of operation in environments with no oxygen, such as underwater and space. The design, unlike the internal combustion engine, produces power on demand, eliminating idling when there is no load on the system. Steam and oxygen are the only byproducts of this power supply. This monopropellant-powered free piston hydraulic pump (FPHP) was designed as a human scale (1.0 to 3.0 kW) power supply. The FPHP utilizes high concentration hydrogen peroxide, which decomposes into hot gas when exposed to a catalyst, as the monopropellant energy source. Energy is extracted from the hydrogen peroxide and transferred directly to hydraulic fluid by expanding the hot decomposition gas in an integrated piston/cylinder arrangement. Based upon a specific power and specific energy analysis using a Ragone plot, the performance of the FPHP potentially exceeds that of a battery based hydraulic power supply for short operation times.