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UWE Bristol Engineering showcase 2015

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Ben Watson<br />

BEng (Hons) Robotics<br />

Project Supervisor<br />

Sanja Dogramadzi<br />

Civilian Assistive Powered Exoskeleton – Conceptual Design<br />

Introduction<br />

A powered exoskeleton is an electromechanical<br />

system akin to a wearable robot, but differs greatly<br />

from purely robotic systems. It is a device which is<br />

worn by or mounted to a user who controls it with<br />

their own body to aid them in a manual task. The<br />

exoskeleton becomes a secondary system to the<br />

user that acts in tandem with them to support<br />

their own physique with modularity to allow for<br />

specific sections of the body to be assisted. The<br />

apparatus itself would essentially be a string of<br />

motors and sensor arrays with a control system<br />

linking these.<br />

The apparatus can be used to increase the<br />

strength and endurance of the user when coupled<br />

with their own musculoskeletal system. Other<br />

applications include improving dextrous precision,<br />

increasing an individual’s load bearing limit or<br />

giving mobility to those with limb disabilities.<br />

Kinematics and Force Analysis<br />

To properly analyse the range of motion for the<br />

exoskeleton and the forces it can exert, I first<br />

analysed my own arm.<br />

The human arm is a redundant system with more<br />

controllable DOFs than the total DOFs. Therefore<br />

constructing a system intended to replicate the<br />

motion of the whole human arm is a complex task<br />

.<br />

To reduce this complexity, the exoskeleton in this<br />

project will work with only one of the seven DOFs<br />

of the human arm, the elbow.<br />

The kinematics of my own arm were analysed and<br />

when coupled with average weights (which I fall<br />

below) for each segment of the arm (Clauser, C. E.,<br />

et al, 1969) the minimum torque exerted to move<br />

my arm at the elbow can be found (14.715N X<br />

0.5m = 7Nm). This is the absolute minimum torque<br />

that the exoskeleton should exert. I aim to double<br />

the force exerted by the arm, so the exoskeleton<br />

system should exert 14Nm of torque at minimum.<br />

After rendering the final frame through CAD<br />

software the mass of the forearm part could be<br />

simulated and the torque necessary to perform<br />

flexion on it could be calculated. As it is the<br />

forearm half that moves relative to the upper arm<br />

half of the exoskeleton , I focused solely on the<br />

forces involved in moving that part. Through<br />

simulation of the part in ABS plastic, the most<br />

likely material to be used in fabrication the mass of<br />

this frame was given as 0.39Kg. With its length<br />

being 25cm, I calculated the torque needed to<br />

move this piece as 0.96Nm which I rounded up to<br />

1Nm. When added to the torque to move my<br />

forearm and doubled to account for the<br />

exoskeleton’s expected strength level, 16Nm was<br />

found to be the necessary minimum torque for a<br />

linear actuator to produce in the system.<br />

Materials<br />

Metals like titanium are attractive for use in such<br />

an application, but for the more generally<br />

appealing approach I am taking with my design,<br />

and the fact such a level of durability is perhaps<br />

dismissible here, I am electing to avoid the use of<br />

metals and alloys in the base frame.<br />

Military grade ruggedness is not a necessity in a<br />

system intended for in home or hospital use and<br />

also these metals are expensive and definitely<br />

outside the remit of this particular project.<br />

Therefore in this system the materials used will be<br />

strong but lightweight polymers. These are much<br />

cheaper to manufacture with but also allow for<br />

the implementation of rapid prototyping via 3D<br />

printing; a technology quickly emerging, like that<br />

of the exoskeleton.<br />

Project summary<br />

An exploration of the best design<br />

considerations for a common-use strength<br />

enhancing exoskeleton and the conceptual<br />

design made from these considerations.<br />

Project Objectives<br />

•The exploration of current exoskeleton<br />

technologies.<br />

•The selection of optimal materials, actuators,<br />

sensors, power source etc for the application.<br />

•The completion of a conceptual design.<br />

•The construction of part of the design.<br />

Project Conclusion<br />

After beginning the project with slightly<br />

overambitious zeal, I had to dial back the<br />

extent to which I would implement my<br />

design. Factors such as budget, my own<br />

ability, but most importantly time, combined<br />

to leave me with less than I had hoped for at<br />

the start of the project.<br />

However, my proof of concept involving 3D<br />

printing was mostly successful, producing a<br />

sturdy and lightweight frame for the system.<br />

Unfortunately, I was unable to progress with<br />

actuation of the frame or implementing<br />

sensing.<br />

As for the overall conceptual design, I feel I<br />

was successful in considering each of the<br />

myriad of facets (of which only a fraction are<br />

touched upon here) that make up the design<br />

of a powered exoskeleton in order to provide<br />

the basis for an exemplar civilian system.

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