Microprocessor Technology in

Ankle Prosthetics

Arizona State University

Dr. Thomas Sugar

Former Students

LTC Joseph Hitt, PhD

Dr. Kevin Hollander

Dr. Matthew Holgate

Dr. Jeffrey Ward

Mr. Alex Boehler

Mr. Ryan Bellman

Human Machine Integration Laboratory

• Design Unique Compliant Actuators

• Developing Powered Prosthetic Ankles

• Developing Exoskeletons for Running

Robotic Tendon drives a powered AFO

Translating a spring back

and forth to achieve the

desired position and forces

• The Robotic Tendon (RT) – Designing SPARKy (Spring Ankle with

Regenerative Kinetics)

Agenda

SPARKy (Spring Ankle with

Regenerative Kinetics)

• Our goal:

Develop a new generation of powered

prosthetic devices based on lightweight,

energy storing springs that will allow for

more functional gait.

Microprocessor Controlled Prostheses

The Endolite Adaptive Knee and the Otto Bock C-Leg

Proprio Foot by Ossur PowerFoot by MIT and iWalk

Robotic knee/ankle Goldfarb

Human Centric Approach

to Wearable Robotics Human Centric

Compliant

Actuators

Continuous

Control

System Efficiency

Robotic Tendon Based Ankle

• Powered Ankle Prosthetic – Walking, Walk on inclines/declines, Walk backwards

– Ascend/Descend stairs, Jumping, Running

Robotic Tendon Based Ankle

• Powered Ankle Prosthetic – Walking, Walk on inclines/declines, Walk backwards

– Ascend/Descend stairs, Jumping, Running

Studying Human Gait

A single human walking gait cycle.

Ankle angle and normalized moment data. The highlighted region is the push off phase of gait.

Passive Systems

No push-off at

the ankle.

No rotation at the

ankle.

• Passive and untunable.

• Provides minimal

power generation (25%

of AB) and ankle

motion 15% of AB).

Sagittal plane ankle angle, moment, and power for a male below the

knee amputee using a SACH foot walking at 1.13 sec/step, solid line,

versus that of an average able-bodied subject, dashed line.

Ankle Gait Analysis

Plantarflexion (Toes Down)

Dorsiflexion (Toes Up)

Ankle Gait Power

Assumptions: 80 kg person, walking at 0.8 Hz (1.25 sec/cycle)

Robotic Tendon Concept

Robot Tendon

Concept

m g o

Fx x a

K

gait powerspring power

m g

F FP F x

K

Motor Power:

Robotic Tendon Actuator

• Robotic Tendon

m

Why use springs?

• Springs are Powerful

• Springs are Efficient

• Springs are Lightweight

• Springs are Economical

• Springs are Compliant

(308,000 W/kg)

(0.999 for spring steel)

(easily mass produced)

( 0.05 kg)

(safety built-in )

The spring and motor power add to provide the desired output power

required for gait. Notice that at 40% of gait, the spring and motor work in

opposite direction to store elastic energy and at 50% gait, the spring

provides majority of the output power.

Power Decomposition

The subject walks on a treadmill at 2.2 mph. The ankle has 9 degrees of

dorsiflexion and more importantly 23 degrees of plantarflexion. The user

has complete control of the ankle motion because the output side of the

spring is not controlled. The ankle motion fits the model extremely well.

Ankle Motion

The ankle moment matches the model very well.

Ankle Moment

The subject walks at 2.2 mph. Measured power out, Po, and power at the nut, Pm,

for the test series with a 36KN/m spring and a 9 cm lever at 1 m/s (2.2 mph). The

device achieves a very high level of power amplification of 3.7. This is the unique

advantage of a Robotic Tendon.

Ankle Power

Key Accomplishments

• User has full range of sagittal ankle motion comparable to able-bodied gait. (23 degrees of plantar-flexion, 7 degrees of dorsiflexion.)

• User has 100% of the required power for gait delivered at the correct time and magnitude.

• The peak output power is 3-4 times larger than the peak motor power allowing a reduction in motor size and weight.

• Allows a highly active amputee to regain high functionality and gait symmetry.

Design and Build SPARKy 2

• Actuation: A Maxon RE40, 150 Watt motor, roller screw and helical spring assembly.

• Sensors: motor encoder, and ankle encoder, rate gyro

• FS 3000 Keel from Freedom Innovations.

Lever arm

Spring

FS 3000 Keel

RE 40 Motor

Roller screw

Robotic

Tendon

Ankle

Joint

Electronic System

• Control Platform: Matlab, Simulink, Real Time Workshop Toolbox.

Electronic System

• Design Code using a Graphical Interface in Simulink/Matlab

• Use Specific Toolboxes for Device Hardware – We use the Kerheul Toolbox for Microchip dsPIC processors

• Matlab Real Time Workshop generates C-Code automatically

• Download code using MPLAB

Solution

A Robotic Tendon stores and releases

energy during the gait cycle

A tuned spring for a given individual

reduces peak motor power and energy as

compared to a traditional motor/gearbox

system

The proximal side of the spring uses robust

position control

Vision

Compliant Actuators:

• Study the kinematics and kinetics to use springs that are

tuned to the body’s movement

• Energy storage

• Reduce power/energy requirements

Microprocessors:

• Easier to program and develop high-level control

• Very cost-effective

• Future – BeagleBoards, Rasberry Pi ….