UWE Bristol Engineering showcase 2015

Ben Watson
BEng (Hons) Robotics
Project Supervisor
Sanja Dogramadzi
Civilian Assistive Powered Exoskeleton – Conceptual Design
Introduction
A powered exoskeleton is an electromechanical
system akin to a wearable robot, but differs greatly
from purely robotic systems. It is a device which is
worn by or mounted to a user who controls it with
their own body to aid them in a manual task. The
exoskeleton becomes a secondary system to the
user that acts in tandem with them to support
their own physique with modularity to allow for
specific sections of the body to be assisted. The
apparatus itself would essentially be a string of
motors and sensor arrays with a control system
linking these.
The apparatus can be used to increase the
strength and endurance of the user when coupled
with their own musculoskeletal system. Other
applications include improving dextrous precision,
increasing an individual’s load bearing limit or
giving mobility to those with limb disabilities.
Kinematics and Force Analysis
To properly analyse the range of motion for the
exoskeleton and the forces it can exert, I first
analysed my own arm.
The human arm is a redundant system with more
controllable DOFs than the total DOFs. Therefore
constructing a system intended to replicate the
motion of the whole human arm is a complex task
.
To reduce this complexity, the exoskeleton in this
project will work with only one of the seven DOFs
of the human arm, the elbow.
The kinematics of my own arm were analysed and
when coupled with average weights (which I fall
below) for each segment of the arm (Clauser, C. E.,
et al, 1969) the minimum torque exerted to move
my arm at the elbow can be found (14.715N X
0.5m = 7Nm). This is the absolute minimum torque
that the exoskeleton should exert. I aim to double
the force exerted by the arm, so the exoskeleton
system should exert 14Nm of torque at minimum.
After rendering the final frame through CAD
software the mass of the forearm part could be
simulated and the torque necessary to perform
flexion on it could be calculated. As it is the
forearm half that moves relative to the upper arm
half of the exoskeleton , I focused solely on the
forces involved in moving that part. Through
simulation of the part in ABS plastic, the most
likely material to be used in fabrication the mass of
this frame was given as 0.39Kg. With its length
being 25cm, I calculated the torque needed to
move this piece as 0.96Nm which I rounded up to
1Nm. When added to the torque to move my
forearm and doubled to account for the
exoskeleton’s expected strength level, 16Nm was
found to be the necessary minimum torque for a
linear actuator to produce in the system.
Materials
Metals like titanium are attractive for use in such
an application, but for the more generally
appealing approach I am taking with my design,
and the fact such a level of durability is perhaps
dismissible here, I am electing to avoid the use of
metals and alloys in the base frame.
Military grade ruggedness is not a necessity in a
system intended for in home or hospital use and
also these metals are expensive and definitely
outside the remit of this particular project.
Therefore in this system the materials used will be
strong but lightweight polymers. These are much
cheaper to manufacture with but also allow for
the implementation of rapid prototyping via 3D
printing; a technology quickly emerging, like that
of the exoskeleton.
Project summary
An exploration of the best design
considerations for a common-use strength
enhancing exoskeleton and the conceptual
design made from these considerations.
Project Objectives
•The exploration of current exoskeleton
technologies.
•The selection of optimal materials, actuators,
sensors, power source etc for the application.
•The completion of a conceptual design.
•The construction of part of the design.
Project Conclusion
After beginning the project with slightly
overambitious zeal, I had to dial back the
extent to which I would implement my
design. Factors such as budget, my own
ability, but most importantly time, combined
to leave me with less than I had hoped for at
the start of the project.
However, my proof of concept involving 3D
printing was mostly successful, producing a
sturdy and lightweight frame for the system.
Unfortunately, I was unable to progress with
actuation of the frame or implementing
sensing.
As for the overall conceptual design, I feel I
was successful in considering each of the
myriad of facets (of which only a fraction are
touched upon here) that make up the design
of a powered exoskeleton in order to provide
the basis for an exemplar civilian system.