IEEE Robotics & Automation Magazine - December 2014 - 106

As expected from the swing controller characterization,
the maximum knee flexion angle in the swing phase remained unaltered at different walking speeds. This result is
particularly significant if we consider the high variability of
the swing starting conditions in terms of torque, angle, and
velocity. On the other hand, passive prosthetic knees were unable to entirely compensate for the variation of the inertial
torque following the change in walking speed, as shown by
the variation of knee maximum flexion angle. Unlike passive
prostheses, the proposed controller allowed a biomimetic
swing trajectory at any walking speed [22].
The analysis of the stride, stance, and swing phase durations shows that the proposed swing controller restored physiological gait symmetry independently of walking speed. This
result is significant because the effective swing phase duration
depends not only on swing movement duration, but also on
the volition of the patient. If patients had waited too long at
the end of the swing movement before loading their body
weight on the prosthesis, physiological gait symmetry would
have not been achieved. Our results show not only that swing
movement duration was properly adapted at each step but
also that patients trusted the variable cadence control and
moved in synchrony with the robotic prosthesis.
The time needed to tune a powered transfemoral prosthesis for each patient has been identified as a main obstacle to
their clinical viability [36]. Our control framework can provide physiological function without any need for tuning; we
only had to input patient's BM and height into the controller.
All patients could walk comfortably with the prosthesis at
varying speeds after minimal training-about 15 min of
walking practice-with no tuning. This result encourages us
to test the proposed controller on a larger population, focusing on clinical outcomes.
A limitation of this article is the lack of a direct comparison with previously proposed powered prostheses and control methods. Nonetheless, the goal of this article was to
thoroughly assess the control performance of the proposed
framework and to obtain a preliminary validation on a limited sample size that could justify a larger clinically oriented
study. Future work will aim to assess the clinical benefits of
the proposed approach in comparison to other powered
prostheses and control methods. Future studies will need to
include a larger sample size to properly verify the ability of
the proposed controller to generalize across different individuals. In addition, further insights could be gained through
additional outcome measurements, for example, by recording kinematics, kinetics, and electromyography signals from
the residual limb and contralateral leg. Moreover, heart rate
and metabolic consumption measurements would be desirable to verify possible systemic benefits.
Conclusions
Providing positive net energy over the gait cycle is one of the
biggest advantages of powered prostheses over passive devices [26], [37]. Positive net energy is needed to walk on level
ground at moderate to high speeds [11], [38], to properly
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IEEE ROBOTICS & AUTOMATION MAGAZINE

December 2014

propel and support the body [39], and to accelerate the leg
into swing [40], [41]. However, the amount of positive net
energy required depends on walking speed, which imposes a
complex, nonlinear modulation of torque [42]. For the first
time, the proposed stance phase controller enabled a transfemoral prosthesis to restore physiological gait energetics
over a wide range of walking speeds without any subject- or
speed-specific tuning by relying on able-bodied intact leg
quasistiffness profiles. In addition, the proposed swing controller enabled the robotic prosthesis to provide a smooth
swing movement that drove the prosthetic leg through a
physiologically appropriate trajectory independently of the
stance-phase controller. By timing the swing movement
based on the duration of the previous stance phase, the robotic prosthesis was able to restore physiological gait symmetry, which is fundamental to restore natural gait stability and
efficiency to persons with transfemoral amputations [43].
Future work will be dedicated to embedding the controller
onto the onboard electronics of the prosthesis and evaluating
its performance in a larger clinical population, to assess improvements in walking stability and metabolic efficiency
compared to passive prostheses and powered prostheses
using other control approaches.
Acknowledgment
We would like to thank Suzanne Finucane, Elizabeth Halsne,
and Rudhram Gajendran for their help with the experimental
setup and data acquisition.
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Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2014