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KUKA, Hereward Rise, Halesowen, West Midlands B62 8AN
Copyright © 2006 SAE International
ABSTRACT
This paper describes work being conducted by OC
Robotics and Airbus to develop snake-arm robot
technology suitable for conducting automated inspection
and assembly tasks within wing boxes.
The composite, single skin construction of aircraft
structures presents new challenges for robotic
assembly. During box close-out it is necessary for
aircraft fitters to climb into the wing box through a small
access panel and use manual or power tools to perform
a variety of tasks. These manual interventions give rise
to a number of health and safety concerns. Snake-arm
robots provide a means to replace manual procedures
by delivering the required tools to all areas of the wing
box. The advantages of automating in-wing processes
will be discussed. This paper presents early stage
results of the demonstration snake-arm robot and
outlines expectations for future development.
BACKGROUND
ROBOTICS IN THE AEROSPACE INDUSTRY
The automotive industry was very quick to embrace
automation as a means to achieve mass production,
while the aerospace industry has been much slower. In
recent years, however, there has been a general move
towards automation as a means to increase throughput
and standardise processes.
The reasons for the slow uptake of industrial robots into
the aerospace can be largely attributed to the need for
high accuracy over large structures. In particular, holes
must be drilled within large structures with both high
absolute and relative accuracy, relative to other holes
and features of the aircraft assembly.
Airbus has been researching low cost, highly flexible
robot automation for a number of years. A robot test
programme determined that standard industrial robots
cannot meet the high accuracy process requirements.
Closed loop control systems have been developed to
achieve adequate position accuracy with industrial
robots [1], thus enabling high precision drilling tasks to
be automated.
However, tasks within rib bays and other low access
areas found throughout aircraft structures have
remained inaccessible to automation. Manoeuvring an
industrial robot (Figure 1) through a small opening
becomes an ‘eye of the needle’ problem (Figure 2): it
becomes practically impossible to use a conventional
robot-arm to pass through an access panel, for example,
and conduct work within a wing box.
Figure 1 - 350kg payload Kuka
Figure 2 - "Through the eye of a needle"
Operating within a rib bay requires some of the
capabilities of industrial robots, e.g. the ability to place
tools precisely, but other capabilities must be added in
order to be able to operate within confined spaces. In
particular it is necessary to have a robot structure that
does not have prominent ‘elbow’ joints. A suitable
structure is one with low profile elbows or continuous
curvature that is able to snake into confined structures.
This type of robot, called a snake-arm robot is the
subject of research being conducted by OC Robotics [2]
and Airbus.
SNAKE-ARM ROBOTS
Robots with many independently controlled degrees of
freedom are referred to as ‘super-redundant’
mechanisms, where ‘redundant’ is a mathematical term
that means ‘excess’. In particular, a super-redundant
snake-arm robot has many joints, similarly to a natural
snake. This belongs to a body of work related to
continuum robots and this is being conducted around the
world e.g. Walker [3] This paper is a useful reference for
citations including work by Hirose, Chirikjian, Burdick
and Kobayashi By introducing more joints into the robot
structure it becomes physically possible to pass the arm
through a small hole. However, standard mathematical
techniques of motion control no longer apply since, for
any given tip position, there is now an infinite number of
joint solutions. One of the mathematical challenges
solved by OC Robotics is the ability to choose a series
of solutions in time so that the robot arm follows a path
and avoids collisions.
The OC Robotics patented design uses a number of
flexible ‘segments’ that can be independently controlled
by applying a moment at the end of each segment.
Consider a flexible rule: end loads cause the rule to
bend along its length rather than bending at a joint.
Whilst there are a number of ways to introduce moments
at the end of each segment, the OC Robotics design
uses three actuators that each pull an individual wire.
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