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Research Papers

Development of a Mechatronic Platform and Validation of Methods for Estimating Ankle Stiffness During the Stance Phase of Walking

[+] Author and Article Information
Elliott J. Rouse

Department of Biomedical Engineering,
Northwestern University,
2145 Sheridan Road,
Room E310,
Evanston, IL 60208;
Center for Bionic Medicine,
Rehabilitation Institute of Chicago,
345 East Superior Street,
Room 1309,
Chicago, IL 60611
e-mail: erouse@media.mit.edu

Levi J. Hargrove

Center for Bionic Medicine,
Rehabilitation Institute of Chicago,
345 East Superior Street,
Room 1309,
Chicago, IL 60611;
Department of Physical Medicine and Rehabilitation,
Northwestern University,
710 North Lake Shore Drive,
Chicago, IL 60611
e-mail: l-hargrove@northwestern.edu

Eric J. Perreault

Department of Biomedical Engineering,
Northwestern University,
2145 Sheridan Road,
Room E310,
Evanston, IL 60208;
Sensory Motor Performance Program,
Rehabilitation Institute of Chicago,
345 East Superior Street,
Room 1396,
Chicago, IL 60611
e-mail: e-perreault@northwestern.edu

Michael A. Peshkin

Department of Mechanical Engineering,
Northwestern University,
2145 Sheridan Road,
Room B224, Evanston, IL 60208
e-mail: peshkin@northwestern.edu

Todd A. Kuiken

Department of Biomedical Engineering,
Northwestern University,
2145 Sheridan Road,
Room E310,
Evanston, IL 60208;
Center for Bionic Medicine,
Rehabilitation Institute of Chicago,
345 East Superior Street,
Room 1309,
Chicago, IL 60611; and
Department of Physical Medicine and Rehabilitation,
Northwestern University,
710 North Lake Shore Drive,
Chicago, IL 60611
e-mail: tkuiken@northwestern.edu

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received November 28, 2012; final manuscript received April 10, 2013; accepted manuscript posted April 22, 2013; published online June 12, 2013. Assoc. Editor: Kenneth Fischer.

J Biomech Eng 135(8), 081009 (Jun 12, 2013) (8 pages) Paper No: BIO-12-1586; doi: 10.1115/1.4024286 History: Received November 28, 2012; Revised April 10, 2013; Accepted April 22, 2013

The mechanical properties of human joints (i.e., impedance) are constantly modulated to precisely govern human interaction with the environment. The estimation of these properties requires the displacement of the joint from its intended motion and a subsequent analysis to determine the relationship between the imposed perturbation and the resultant joint torque. There has been much investigation into the estimation of upper-extremity joint impedance during dynamic activities, yet the estimation of ankle impedance during walking has remained a challenge. This estimation is important for understanding how the mechanical properties of the human ankle are modulated during locomotion, and how those properties can be replicated in artificial prostheses designed to restore natural movement control. Here, we introduce a mechatronic platform designed to address the challenge of estimating the stiffness component of ankle impedance during walking, where stiffness denotes the static component of impedance. The system consists of a single degree of freedom mechatronic platform that is capable of perturbing the ankle during the stance phase of walking and measuring the response torque. Additionally, we estimate the platform's intrinsic inertial impedance using parallel linear filters and present a set of methods for estimating the impedance of the ankle from walking data. The methods were validated by comparing the experimentally determined estimates for the stiffness of a prosthetic foot to those measured from an independent testing machine. The parallel filters accurately estimated the mechatronic platform's inertial impedance, accounting for 96% of the variance, when averaged across channels and trials. Furthermore, our measurement system was found to yield reliable estimates of stiffness, which had an average error of only 5.4% (standard deviation: 0.7%) when measured at three time points within the stance phase of locomotion, and compared to the independently determined stiffness values of the prosthetic foot. The mechatronic system and methods proposed in this study are capable of accurately estimating ankle stiffness during the foot-flat region of stance phase. Future work will focus on the implementation of this validated system in estimating human ankle impedance during the stance phase of walking.

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Figures

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Fig. 1

Model of the Perturberator Robot with prominent design features highlighted

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Fig. 2

Servodrive controller closed loop bode plot denoting the position bandwidth of the Perturberator Robot. Note the natural frequency at approximately 200 Hz.

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Fig. 3

A series of parallel linear filters are shown mapping acceleration of the Perturberator Robot's motor angle to the forces from the force platform. Note, z-axis (vertical) force channels shown, but analysis used for all channels.

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Fig. 4

Diagram showing ground reaction forces acting on the foot (solid). The resultant (dashed) ankle torque, Ta, is computed by multiplying the ground reaction force components by their respective perpendicular distances.

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Fig. 5

Prosthetic ankle angle (a) and ankle torque (b) shown during stance phase, beginning with heel contact. The nonperturbed trials are shown in black and the perturbed trials are shown in blue with the average shown in bold and the standard deviation in translucent. The circle denotes the onset of the perturbation.

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Fig. 6

Estimated filters shown for vertical (z) axis channels. Note, the initial impulse, denoting the dominant feature of the dynamics is the robot's inertia.

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Fig. 7

Means are shown in bold with standard deviations in translucent. (a) Perturberator Robot's motor angle as a function of time, showing the 0.035 rad ramp perturbation with 75 ms ramp length and 25 ms hold. (b) resultant ankle angle and torque as a function of time, during the 100 ms analysis window. The model estimated torque profiles are also shown. Signals shown have been filtered by a bi-directional low-pass filter, with a cutoff frequency of 20 Hz.

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Fig. 8

Experimentally determined prosthetic foot stiffness plotted as a function of actuator distance (black) and the stiffness values estimated during walking (gray). Error bars denote standard deviation and perturbations were in the dorsiflexion direction.

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