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Design Innovation Paper

Design of Lower Limb Prosthesis Transverse Plane Adaptor With Variable Stiffness

[+] Author and Article Information
Corey Pew

Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195
Department of Veterans Affairs Center of Excellence for
Limb Loss Prevention and Prosthetic Engineering,
1660 S. Columbian Way,
Seattle, WA 98108
e-mail: Corey.Pew@gmail.com

Glenn K. Klute

Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195
Department of Veterans Affairs Center of Excellence for
Limb Loss Prevention and Prosthetic Engineering,
1660 S. Columbian Way,
Seattle, WA 98108
e-mail: gklute@u.washington.edu

Manuscript received June 27, 2014; final manuscript received April 16, 2015; published online July 16, 2015. 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(3), 035001 (Sep 01, 2015) (6 pages) Paper No: MED-14-1199; doi: 10.1115/1.4030505 History: Received June 27, 2014; Revised April 16, 2015; Online July 16, 2015

A lower limb prosthesis is able to restore mobility to patients who have lost a limb; however, no current replacement is as moveable and adaptable as the limb that was lost. Therefore, all amputees suffer from a reduction in function at some level. Movement in the transverse plane of a lower limb prosthesis is often negated in a traditional prosthesis, and those devices that do allow for transverse plane motion are set to a single, fixed stiffness, and incapable of adapting to the varying activities of the user. A prototype device has been created that allows for varying stiffness in the transverse plane of a lower limb prosthesis. The variable stiffness torsion adapter (VSTA) functions by way of a movable pivot lever arm that can actively modify the mechanical advantage between the outer housing and the internal spring. The motion of the pivot is perpendicular to the external torque allowing for low power adjustments of the stiffness. Bench tests were performed and demonstrate the ability of the VSTA to vary torsional stiffness between 0.12 and 0.91 N m/deg over a ±30 deg rotational range of motion. The device also includes a mode for fully locked operation. The VSTA may improve the mobility of lower limb amputees by allowing for activity-dependent transverse plane stiffness.

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Figures

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Fig. 1

Basic VSTA concept. This shows the VSTA layout as an applied force activating a simple lever attached to a torsion spring. The lever is allowed to slide along the pivot (in the direction of the horizontal arrows), effectively changing the lever arm length and the force required to deflect the spring to a specified angle. In this representation, the spring is fixed, while in the actual VSTA the spring and pivot move, and the lever arm position is fixed, but allowed to pivot around the point of force application.

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Fig. 2

Representation of the VSTA installed in a standard prosthesis

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Fig. 3

VSTA. A—upper housing, B—lower housing, C—lever arm, D—carriage, E—spring carrier, F—electric motor, G—guide rail (2×), H—ACME lead screw, and I—main bearing.

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Fig. 4

The VSTA in the (a) low and (b) high stiffness settings

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Fig. 5

Geometric layout of VSTA. This top view shows how the geometric relations between the lever arm and the spring center offset change as the VSTA is deflected. In the depicted orientation, the spring offset is closer to the lever pivot than the device center. As the lever pivot sweeps along its diameter, the lever arm will effectively lengthen.

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Fig. 6

Model of VSTA behavior at various settings. These curves depict the modeled behavior of the VSTA at individual stiffness settings. The torque increases linearly as the VSTA is deflected between 0 and 30 deg.

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Fig. 7

Results from bench testing of VSTA on MTS. The physical VSTA can be seen to match well with the model predicted torque curves, with deviation at the highest torque setting due to internal deflection of the components.

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Fig. 8

Depiction of internal moment causing deflection of components at high stiffness settings. The long distance between the lever arm force application and the grounding of the torsion spring causes a moment that deflects the guide rails of the spring carrier. This results in a deviation of the VSTA from model predictions.

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