Design and Evaluation of a Reaction-Force Series Elastic Actuator Configurable as Biomimetic Powered Ankle and Knee Prostheses


John Soares

John Soares

Carney, Matthew E. “Design and Evaluation of a Reaction-Force Series Elastic Actuator Configurable as Biomimetic Powered Ankle and Knee Prostheses By.” Massachusetts Institute of Technology, 2020.


All commercial leg powered prostheses have been, up to this point, a one-size fits-all
design, and of those existing systems, none has yet managed to fully achieve biological
walking range of motion, torque and power. Yet, no human body is the same as the
next. A configurable prosthesis potentially offers improvements in battery run-time,
prosthesis mass, acoustic noise, user comfort, and even enables sport and economy
modes within the same fundamental hardware. In this thesis, a reaction-force, serieselastic
actuator (RFSEA) is presented that is capable of achieving biomimetic ankle
and knee kinetics and kinematics during level-ground walking across a range of body
masses, heights and walking styles. The platform is configurable to inertial load by
swapping a simple-to-manufacture flat-plate composite spring that allows tuning the
actuator dynamics to match different user requirements. The RFSEA also comprises
a high torque and pole-count drone motor that directly drives a ball screw with a tunable,
low-gear ratio lead. The design enables high dynamic range providing a closedloop,
torque-controlled joint that can demonstrate arbitrary levels of impedance. This
control fidelity is important to support smooth control in free-space and high-inertial
output conditions, such as the swing and late-stance phases of walking, respectively.
A simulation framework is presented that defines mechatronic design specifications
for the motor, spring, and gear-reduction components. The optimization procedure
clamps output joint dynamics to subject-specific biological gait data, and searches
for minimum electric energy solutions across the motor, gear-reduction and spring
component space. A second optimization procedure then searches for optimal linkage
and spring geometry to best approach the design targets as constrained by the availability
of discrete drivetrain components. In this thesis, ankle and knee designs are
presented with optimized components using biological joint data from a non-amputee
subject walking at 2.0m/sec with a body mass equal to 90Kg. For these designed
biomimetic joints, system specifications are verified using bench test evaluations, and
preliminary human gait studies. With a minimum viable actuator mass of 1.4Kg, the
platform has a nominal torque control bandwidth of 6Hz at 82Nm, a repeated peak
torque capacity of 175Nm, peak demonstrated power over 400W (with theoretical
limits over 1kW), a 110 degree range of motion, as well as torque and power densities
of 125Nm/kg and 286W/kg, respectively. Configured as an ankle-foot prosthesis,
there are 35 degrees of dorsiflexion and 75 degrees of plantar flexion, and as a knee the
full 110 degrees of flexion are available to enable activities on varied terrain such as
stairs and inclines. Walking dynamics are evaluated with a finite state-machine ankle
controller piloted by N=3 subjects with below-knee amputation walking at 1.5m/sec
on an instrumented treadmill and one subject walking on stairs. In preliminary experiments,
net positive work of 0.2J/Kg, peak joint torque of 1.5Nm/Kg, and peak
mechanical power of 4.3W/Kg all fall within one standard deviation of the intact-limb
biological mean. Configured as an ankle-foot prosthesis, the system mass is 2.2Kg
including battery and electronics, and as a knee the system mass is 1.6Kg, making
the RFSEA platform the lightest, most adaptable, and most biomimetic leg system
yet published.

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