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These three wires are equally spaced at 120° intervals
around the circumference of the segment and terminate
at the end of each segment, Figure 3. By controlling the
length of these wires, the segment can be made to bend
to form an arc. This plane of bending can also be rotated
by appropriate actuator wire length changes. Each
segment therefore has two degrees of freedom –
curvature and orientation of the plane of curvature.
Figure 3 - Principles of wire drive
An OC Robotics snake-arm robot can have a large
number of segments, whilst maintaining the same cross
section along the length of the arm. An arm is therefore
a series of serially connected flexible tubular segments.
The stiffness (compliance) of the tubular segments can
be chosen according to the task.
The design of the segments is critical to the performance
of the arm. OC Robotics’ design is similar to a
backbone, with alternating rigid sections (vertebrae) and
thin elastomer layers (discs). These sub components of
a segment are called links. The stiffness of the link must
also be balanced by the ability of the actuators to apply
loads in order to bend the segments. This bending
moment is a function of the diameter of the arm and the
tension in the actuator wire. The tension in each wire
and the length of each wire are controlled by an
individual motor. These motors are grouped together at
the base of the arm, in the actuator pack.
Having the actuators at the base of the arm greatly
simplifies wiring and sealing considerations, as well as
offering a suitable location for a single quick-release
interface. This enables very quick arm replacement,
while retaining the high-cost actuators and controllers.
An important design feature of the patented arm design
is the constant arm diameter. This leads to a simple arm
design, reducing both complexity and cost. It also means
that the curved external surface of the arm is reasonably
smooth and can be easily sealed using a thin elastomer
or bellows tube.
SNAKE-ARM ROBOTS IN INDUSTRY
In April 2003, OC Robotics signed a contract with the UK
MoD to build a 2.5m long snake-arm robot able to carry
a payload of 25kg (Figure 4).
Figure 4 - 2.5m snake-arm robot for UK MoD
In early 2005 OC Robotics successfully demonstrated
the capability of the arm to reach into a car through an
open window. The arm reached through the driver’s
window to the back seat of the car where an object was
viewed through the tip-mounted camera. The contract
also stated that the arm must be strong enough to tow a
car. Snake-arm robots are axially strong because of the
high loading ability of the wires running along their
length so the 95mm diameter arm was able to tow a car
with ease.
OC Robotics completed its first commercial nuclear
contract in the summer of 2004. Two types of robot (five
in total) were supplied to Ringhals AB to complete an
urgent pipe replacement in an extremely awkward area
below one of their reactors [4]. Before the operation
could be performed on the real environment, there was
extensive training on a custom-built mock-up. After
thoroughly practising the whole procedure, the team, led
by Uddcomb Engineering AB, conducted a trial
installation and operation within reactor containment as
part of the acceptance tests. Replacing the leaking
section of pipe involved more than 30 distinct
procedures with the majority being conducted by the
robots working cooperatively. The more flexible snakearm
was used to get the ideal camera location to monitor
the process (Figure 5) whilst the other snake-arm was
used to deliver the processing tools and fixtures, remove
the old pipe, introduce the new pipe and conduct tasks
such as welding and inspection.
In August 2004 a single pipe repair was successfully
completed and, a month later the generic solution based
on the robots, which is able to repair any of the other
156 similar pipes, successfully completed the Factory
Acceptance Tests.
Figure 5 - The Inspection Arm carried cameras to the work site
AEROSPACE APPLICATIONS
Figure 6 - Snake-arm robot reaching into the wing box
Airbus UK are actively developing aerospace robots [1]
working in close collaboration with KUKA. These large
industrial robots will deliver end effector packages
capable of drilling and sealing.
The approach being taken is to consider a snake-arm
robot as an additional tool that the larger industrial robot
can deliver. The basic concept is shown in Figure 6.
The nose following motion is achieved by coordinating
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