Research Papers

Preliminary Design and Engineering Evaluation of a Hydraulic Ankle–Foot Orthosis

[+] Author and Article Information
Brett Neubauer

Department of Mechanical Engineering,
University of Minnesota,
111 Church Street SE,
Minneapolis, MN 55455
e-mail: neub0070@umn.edu

William Durfee

Department of Mechanical Engineering,
University of Minnesota,
111 Church Street SE,
Minneapolis, MN 55455
e-mail: wkdurfee@umn.edu

Manuscript received June 23, 2015; final manuscript received March 22, 2016; published online August 5, 2016. Assoc. Editor: Rita M. Patterson.

J. Med. Devices 10(4), 041002 (Aug 05, 2016) (9 pages) Paper No: MED-15-1208; doi: 10.1115/1.4033327 History: Received June 23, 2015; Revised March 22, 2016

Ankle foot orthoses (AFOs) are used to correct motor impairments of the ankle. While current AFOs are passive, advances in technology and wearable robotics have opened the opportunity for a powered AFO. The hydraulic ankle foot orthosis (HAFO) is a device that takes advantage of the exceptional power-to-weight and force-to-weight of hydraulic fluid power. The device is untethered, and the power transmission chain is battery–electric motor–hydraulic pump–hose–cylinder, with the power supply worn at the waist and the cylinder actuators at the ankle. The fluid power circuit is configured as an electrohydraulic actuator (EHA) that is controlled by controlling the electric motor. The first prototype weighs 3.3 kg of which 0.97 kg is worn at the ankle. Steady-state torque–velocity performance showed that the prototype can provide 65 N·m of assistance torque and a no-load velocity of 105 deg/s. Closed-loop position control showed low steady-state error but a slow response. The current prototype demonstrates the potential of hydraulics for providing large torques in a compact, lightweight device. The speed performance of the prototype is inadequate for normal walking but can be improved by switching to servo valve control or by developing a custom hydraulic pump.

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Fig. 4

Cross section of piston showing the cable rod

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Fig. 2

Hydraulic power supply

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Fig. 1

Fluid power circuit

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Fig. 5

Power supply prototype

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Fig. 6

Ankle actuator (left) and power supply positioning on a user (right)

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Fig. 7

Weight distribution for system components

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Fig. 8

System and motor torque–velocity properties

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Fig. 10

Open-loop isometric torque frequency response

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Fig. 11

Open-loop isometric torque step response for system (solid) and motor (dashed) for torque steps of 20, 30, 40, and 50 N·m

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Fig. 12

Closed-loop P control unloaded position step response

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Fig. 13

Power supply mechanical efficiency excluding electric motor

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Fig. 14

System mechanical (left) and volumetric (right) efficiencies excluding electric motor

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Fig. 15

System total efficiency

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Fig. 16

Torque–velocity of HAFO compared to torque–velocity of the ankle during one step

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Fig. 9

Open-loop unloaded velocity frequency response



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