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linear movement of the base of the snake-arm robot with
the motion of the snake-arm robot axes. Figure 6 shows
the industrial robot providing the linear movement
required for path-following with the snake-arm robot
attached as a forearm at the industrial robot’s wrist. It is
the coordinated motion of the industrial robot and snakearm
which gives the system its flexibility.
Since the industrial robot and the snake-arm have
separate control systems, and different mathematical
architectures it is necessary for the industrial robot to act
as the slave of the snake-arm control system. This
allows for the overall system to be decoupled into two
kinematic chains.
The snake-arm is also equipped with a wrist and
interface to attach different tools. As a focus for the
work, three tasks are being considered. The tasks are
swaging a standard aerospace fastener, applying
sealant and conducting inspections.
The swaging and sealing tasks are to be carried out at
the front spar and at the rib feet. Inspection is to be
carried out around the whole rib bay.
THE DEMONSTRATOR
A prototype has been designed which will be used to
demonstrate all the required tasks inside a mock-up of a
rib bay.
The complete snake-arm robot demonstrator system
(Figure 7) is comprised of a number of sub-systems. The
design of the snake-arm itself required careful
consideration of size, flexibility and payload. Establishing
these parameters determined the design of the links that
make up the arm. The arm is driven by an actuator pack,
which includes linear actuators that pull the wires, plus
the necessary control circuits. The actuator pack is
mounted on a mechanism to allow it to be introduced
into the working environment in the required orientation.
The arm has a single degree of freedom (“pitch”) wrist at
its tip to allow a tool to be placed in the required
orientation for a task. The entire snake-arm robot is
rotated about its axis to enable orientation of the wrist in
the “roll” axis.
The arm was designed using proprietary software tools
that allow the kinematics of the arm to be optimised for a
given environment. This software is also used to drive
the actual hardware and can therefore be used for
training.
The demonstrator snake-arm is 1.2m in length and
100mm in diameter. The hollow bore is 25mm. The
complete system has a mass of 50kg and a centre of
gravity 250mm from the attachment to the industrial
robot wrist axis. The arm has 10 segments, with each
segment be able to rotate in two dimensions. This gives
the arm the flexibility to nose-follow into the rib bay. The
complete system has 27 degrees of freedom.
Figure 7 - Demonstrator and rib-bay mock-up
The snake-arm robot follows a path into the wing-box,
either by joystick control or from a pre-determined ‘path
library’. The arm then moves in ‘cartesian mode’ either
by joystick control or automatically using visual servoing
to ensure it is correctly aligned before beginning each
task. When applying sealant, a camera on the toolpiece
tracks the line of the seam to ensure accurate and even
application.
END EFFECTORS
In order to maximise the benefit of a snake-arm’s pathfollowing
capability, the diameter of the end effector’s
envelope must be equal to or less than the diameter of
the snake-arm. The length of the end effector must be
minimised, ideally to the diameter of the snake-arm or at
least to less than 1.5 the diameter.
In addition to these considerations, further restrictions
were placed on the design by the snake-arm robot
specification and the rib bay geometry.
Three interchangeable end effectors have been
designed by OC Robotics:
AN INSPECTION TOOL
This tool will contain several cameras with various
functions to ensure adequate inspection of all areas of
the wing box. Although it is probably the easiest of the
three tasks, the inspection of the wing box is
complicated by several factors: low lighting in the wing
box and the uniform colour of the interior compounded
the differentiation and identification of defects. The
approach being taken is to use a range of wide-angle,
spot and close-up lights around the cameras.
Figure 8 - Inspection tool
A SWAGE TOOL
Swaging involves high forces which can be reacted
through the structure. Automated tools must be
electrically actuated whilst remaining small enough not
to restrict motion within the rib bay. This tool will swage
a rivet and direct the removed section into a collection
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