Tiny Hydraulics for Human-Assist Machines

Achievement date: 

Researchers affiliated with the Center for Compact and Efficient Fluid Power (CCEFP), an NSF-funded Engineering Research Center (ERC) headquartered at the University of Minnesota, have demonstrated the capabilities and advantages of tiny hydraulics for untethered, powered human-assist machines. The key concepts of such machines have been realized in the Hydraulic Ankle Foot Orthosis (HAFO) (see figure).


HAFO-type devices have the obvious benefit of creating novel powered exoskeletons to assist persons with disabilities, such as mobility impairments, to walk with greater stability, confidence, and independence, including persons coping with the aftermath of stroke or acute trauma or cerebral palsy. In addition, addressing the complex modeling and engineering issues associated with these devices facilitates development of a broad range of miniature fluid-power systems by pushing the practical limits of weight, power, and duration for wearable human-assist machines, likely leading to new market opportunities for the fluid-power industry in areas including miniature and integrated power supplies, novel actuators, valves, transmission lines, and housings.


The HAFO work required advances in system and component-level modeling to predict the efficiency and weight of small hydraulic systems. The models, including simple models of O-ring seals, were validated by experimental data and enabled system-level thinking about tiny hydraulic designs where every component in the power transmission path is a transformer whose transmission ratio influences system efficiency and component weight.

These concepts have been realized in the HAFO, where the power supply is worn at the waist, leaving just small hydraulic actuators at the ankle to minimize weight carried on the foot. The HAFO runs at 2,000 psi, provides 90 Nm at 100 deg/s, weighs less than 1 kg at the ankle, and fits under a pair of loose-fitting pants.


Experimental data showed that (1)hydraulic exoskeletons must operate at high pressure (>1,000 psi) to take advantage of their inherent power and force density and (2) actuator bore sizes of less than about 4 mm cause a rapid drop in system efficiency. System-level thinking focused especially on power-transmission-path components including batteries, electric motors, hydraulic pumps, conduits, cylinders, and linear-to-rotary transmission elements.