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Research Papers

Nonlinear Passive Cam-Based Springs for Powered Ankle Prostheses

[+] Author and Article Information
Jonathan Realmuto

Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195
e-mail: realmuto@uw.edu

Glenn Klute

Department of Veterans Affairs,
Center of Excellence for Limb Loss,
Prevention and Prosthetic Engineering,
Seattle, WA 98195
Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195
e-mail: gklute@uw.edu

Santosh Devasia

Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195
e-mail: devasia@uw.edu

Manuscript received July 16, 2014; final manuscript received September 15, 2014; published online November 14, 2014. Assoc. Editor: Rita M. Patterson.

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Med. Devices 9(1), 011007 (Mar 01, 2015) (10 pages) Paper No: MED-14-1210; doi: 10.1115/1.4028653 History: Received July 16, 2014; Revised September 15, 2014; Online November 14, 2014

This article studies the design of passive elastic elements to reduce the actuator requirements for powered ankle prostheses. The challenge is to achieve most of the typically nonlinear ankle response with the passive element so that the active ankle-torque from the actuator can be small. The main contribution of this article is the design of a cam-based lower-limb prosthesis to achieve such a nonlinear ankle response. Results are presented to show that the addition of the cam-based passive element can reduce the peak actuator torque requirement substantially, by ∼74%. Moreover, experimental results are presented to demonstrate that the cam-based design can achieve a desired nonlinear response to within 10%.

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Figures

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Fig. 1

Biomechanics of level ground walking. The gait cycle can be divided into a stance phase and a swing phase.

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Fig. 2

Gait trajectories for nonamputee walking at self-selected walking speed [24]. The dotted lines represent the phase transitions from Fig. 1. Top: angular displacement θd during the gait cycle. Middle: ankle-torque Td normalized by body mass M. Bottom: ankle power Pd normalized by body mass M.

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Fig. 4

Schematic representations of 1DOF powered ankle configurations. (Top) The PEA configuration is characterized by an ideal torque source Ta in parallel with a rotational passive component. The parallel component generates a torque Tp as a function of the angular displacement θd. (Bottom) The SEA configuration consists of an ideal torque source Ta in series with a rotational passive component.

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Fig. 3

Ankle-joint stiffness characteristics (ankle-torque Td normalized by body mass M versus angular displacement θd) during stance phase for self-selected walking speed. The loading phase begins with HS and ends with MDF. Unloading begins with MDF and ends with TO.

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Fig. 5

Normalized component costs (Eq. (16)) versus polynomial degree n for optimal PEA. The optimization weights are w = [0 0 1] in Eq. (9).

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Fig. 6

Normalized component costs (Eq. (16)) versus polynomial degree n for optimal PEA. The optimization weights are w = [1 1 1] in Eq. (9).

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Fig. 7

Performance comparison of PEAs: positive energy Ea+ (left); peak positive peak power Pa,p+ (middle); and peak torque Ta,p (right)

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Fig. 8

The nonamputee ankle stiffness characteristics (normalized torque T/M versus desired angular displacement θd) for the entire gait cycle, superimposed with the linear PEA1 and third-degree nonlinear PEA3 stiffness profiles

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Fig. 9

Comparison of normalized actuator torque Ta/M and power Pa/M for optimal linear PEA1 with those for the optimal, third-degree nonlinear PEA3 with w = [1 1 1]

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Fig. 10

Design concept for cam-based parallel component. The cam is fixed to the foot segment and displaces the spring attached to the shank segment.

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Fig. 11

Schematic of the cam-follower mechanism adapted from Ref. [33]. The cam rotates about point O (the center of the prime radius Rp), which is d away from the line of action of the cam follower, whose displacement is s and velocity is vf. The pressure angle φ is the angle between the normal force N and the follower’s line of action. The instant center of rotation is A, which is b away from the center of rotation O.

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Fig. 12

Pressure angle φ for different follower stiffness kc and eccentricity d. Dotted line denotes maximum allowable pressure angle, max|φ| = 30 deg. The red dot denotes the prototype configuration.

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Fig. 13

Cam prototype for PEA3 with parameters in Table 3. The black dashed lines represent the boundaries of the functional area: −22 deg for plantarflexion and 20 deg for dorsiflexion.

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Fig. 14

Images of the prototype powered ankle prosthesis. (a) Cross section with major components labeled. (b) Picture of actual device.

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Fig. 15

Images detailing the static loading experiment. (a) Free body diagram of static loading experiment. (b) Schematic of static loading experiment illustrating the geometry and the angular displacement θ∧, caused by input displacement Δy. (c) Image of experimental setup.

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Fig. 16

Experimental validation of cam elastic response. The circles represent the experimental data points for the nominal case. The triangles are experimental data points with a 3 mm shim added to the device.

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