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Research Papers

Sliding-Bar MACCEPA for a Powered Ankle Prosthesis

[+] Author and Article Information
R. Jimenez-Fabian

Department of Mechanical Engineering,
Vrije Universiteit Brussel,
Pleinlaan 2,
Brussels 1050, Belgium
e-mail: rjimenez@vub.ac.be

L. Flynn

Department of Mechanical Engineering,
Vrije Universiteit Brussel,
Pleinlaan 2,
Brussels 1050, Belgium
e-mail: flynniv@gmail.com

J. Geeroms

Department of Mechanical Engineering,
Vrije Universiteit Brussel,
Pleinlaan 2,
Brussels 1050, Belgium
e-mail: jgeeroms@vub.ac.be

N. Vitiello

Assistant Professor
The BioRobotics Institute,
Scuola Superiore Sant’Anna,
Viale Rinaldo Piaggio, 34,
Pontedera 56025, Italy;
Fondazione Don Carlo Gnocchi,
Center of Florence,
Via di Scandicci 256,
Firenze 50143, Italy
e-mail: n.vitiello@sssup.it

B. Vanderborght

Professor
Department of Mechanical Engineering,
Vrije Universiteit Brussel,
Pleinlaan 2,
Brussels 1050, Belgium
e-mail: bram.vanderborght@vub.ac.be

D. Lefeber

Professor
Department of Mechanical Engineering,
Vrije Universiteit Brussel,
Pleinlaan 2,
Brussels 1050, Belgium
e-mail: dlefeber@vub.ac.be

1Corresponding author.

Contributed by the Mechanisms and Robotics Committee of ASME for publication in the JOURNAL OF MECHANISMS AND ROBOTICS. Manuscript received March 2, 2014; final manuscript received December 16, 2014; published online April 6, 2015. Assoc. Editor: Robert J. Wood.

J. Mechanisms Robotics 7(4), 041011 (Apr 06, 2015) (11 pages) Paper No: JMR-14-1045; doi: 10.1115/1.4029439 History: Received March 02, 2014

This paper describes a new design that improves several aspects of the mechanically adjustable compliance and controllable equilibrium position actuator (MACCEPA). The proposed design avoids premature wear and attachment issues found in the cable transmission used in previous MACCEPA designs and allows the use of high-performance compact compression springs. The mechanical configuration of the actuator provides an adjustable stiffness with a nonlinear stiffening output torque. The output position of the actuator and its global stiffness are independent from each other. In this work, we provide a mathematical description of the actuation principle along with an experimental verification of its performance in a powered ankle–foot prosthesis. This work is part of the CYBERLEGs project funded by the European Commissions 7th Framework Programme.

Copyright © 2015 by ASME
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References

Figures

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Fig. 1

Diagram of the conventional MACCEPA. The lever arm sets the rest position of the output link. Any deviation of the output link position with respect to this rest position is counteracted by the restoring force of the spring.

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Fig. 2

Schematic drawing of an early prototype of a conventional MACCEPA. The lever arm motor sets the rest position of the actuator. The pretension motor changes the output stiffness by changing the effective length of the cable between the attachment point in the spring and the pivot. This configuration have shown a very short life cycle due to mechanical wear on the cable and fast degradation of the cable at the attachment points.

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Fig. 3

Diagram of the proposed actuator mechanism (with the compression mechanism on the output link). The angular position of the lever arm (segment ac¯) determines the rest position of the actuator. The compression mechanism m is fixed on the output link (segment ab¯) and pushes against the spring to change its initial deformation.

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Fig. 4

Actuator with compression mechanism on the output link and inverted lever arm

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Fig. 5

Actuator with compression mechanism on the secondary link

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Fig. 6

Constrains for the links’ lengths, according to the available space in the foot. The length B of the moment arm is limited by the available space between the axis of the ankle joint and the bottom of the foot. Length C might also be constrained by the distance between the ankle joint and the front of the foot.

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Fig. 7

Torque developed by the first configuration

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Fig. 8

Stiffness developed by the first configuration

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Fig. 9

Comparison of the MACCEPA behavior in terms of the power, torque, and position characteristics ideally developed by the human ankle. The graph in the top shows the reduction of the instantaneous ankle power during one gait cycle of an 80-kg person: Healthy ankle (reference, solid line) required motor power (dashed line). This calculation was made assuming that the torque provided by the actuator perfectly matches that of the healthy ankle, as show in the graph in the middle. The graph in the bottom shows the required moment arm trajectory to achieve this torque match.

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Fig. 10

Velocity and power changes due to the selected preload. The initial deformation of the spring can be varied from 0 to 10 mm with an improvement in the required velocity of the lever arm but a general rise in the peak power of the system.

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Fig. 11

Design drawings of the prosthesis prototype. (a) Main components. Note that the spring system in this illustration corresponds to an elastomeric spring that was later changed to a compression spring in steel to minimize damping effects. (b) Lateral section view showing the main dimensions (in mm). The total weight of the system, including the motors is around 1.85 kg.

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Fig. 12

Physical implementation of the VSA in the prosthesis’ foot

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Fig. 13

Verification of the output torque using three different methods: internal load cell measurements (TLC), external torque sensor measurements, and estimation based on the preload and the driving angle (TαP = T(α, P)). The difference between the average torque provided by the torque sensor at a given angle and the two proposed torque approximations is lower than 2.0% of the average value.

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Fig. 14

Experimental (solid line) and theoretical (discontinuous line) output torque for three different preloads. The actuator torque well matches the theoretical prediction and shows a low level of hysteresis.

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Fig. 15

Theoretical stiffness (discontinuous line) and experimental stiffness (solid line) estimated using a polynomial approximation of the measured torque

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Fig. 16

Free ankle angular position response of the spring-slider-foot system for different values of preload

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Fig. 17

Torque frequency response for three different values of preload using the closed-loop system. The bandwidth of the system is severely limited by the main motor characteristics and the slow compression mechanism.

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Fig. 18

Tracking of the ankle torque profile scaled to 75% of the actual requirements for an 80-kg person at ground-level walking

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Fig. 19

Transfemoral amputee wearing both the ankle prosthesis and a knee module tethered to the supervisory control system in the CYBERLEGs platform

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Fig. 20

Ankle torque-angle characteristics developed by the prosthesis in a test with a transfemoral amputee. The torque-angle trajectory of the prosthesis differs from that used as a reference due to the bandwidth limitations of the system and the inherent walking dynamics of the user. Nevertheless, the system is able to produce positive work ant the ankle joint.

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Fig. 21

Comparison between the output power of the prosthesis and the power requested from the lever arm motor in a test with a transfemoral amputee. The peak output power corresponds to the power that should be provided by the system if it were to be actuated by a stiff motor to obtain a similar torque-angle trajectory. Using the proposed actuator, a reduction of 16.6% of this required power can be achieved thanks to its energy store–release features.

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