Rotational
Legged Locomotion
The Rotopod
What is it?
· The Rotopod is a novel robot mechanism which combines the features of wheeled and legged locomotion in an unusual way.
· This robot has the advantage of legged locomotion in stepping its 1-DOF
legs over objects, but its drive mechanism is a rotating reaction mass that
rotates the robot, in a controllable fashion, around each of its legs, similar
to a rotating wheel.
· The mechanism has the potential to transfer the energy from the
rotating reaction mass in an efficient manner to the legs, effecting a spinning
forward motion
One of the inspirations for
the Rotopod Design was the tripedal “spider” creature described in
Arthur Clark’s novel Rendezvous
with Rama
(p.185 of the 1974 Pan books edition).
The
picture to the left shows our first prototype Rotopod. It uses 1DOF rotational “knee”
joins on each leg in contrast to the 1DOF translational joints shown in the
figure above. This was cheaper and easier to build but makes stepping more
difficult to control.
Control
is offboard in this prototype. An onboard controller
would be carried on a triangular platform between the legs just below where
they join.
How does a
Rotopod Move?
· The Rotopod moves by rotating around each of its legs in turn.
· A
rotation of q radians around a leg is
call a step, and it results in the
center of the robot moving a distance 
· A regular sequence of steps is called a gait.
· The robot can produce a broad set of gaits: stepping for various values
of q on one leg, or any
sequence of legs
· One of the most interesting gaits is spiral walking. This is a very natural behavior for the rotopod
mechanism. The final result looks a little like a prolate cycloid. It leads to paths that have a width element as well as a length
(i.e. a fractal dimension) and hence may be very efficient for covering space
(e.g., searching, surveillance, exploring, etc.)
· 
Sharp turns are not necessarily a problem for a
rotopod, since its continual rotation allows it to change direction
dramatically under certain conditions.
· The diagram to the right shows a rotopod traversing a square traveling
clockwise and then counterclockwise.
· Coming Soon: Images and
Videos of 3D rotopod model in action.
How does
Stepping Happen?
· The rotopod steps by shortening the length of one leg.
· When the reaction mass traverses that leg, it lifts the other two legs
off the ground momentarily.
· Without a reaction torque, the whole body rotates.
· When the reaction mass sufficiently clears the leg,(assumes
friction in the reaction joint), the leg is raised again, the suspended legs
drop back to the ground and reestablish a reaction torque
Why is it Efficient?
· The only purpose of the leg joints on the rotopod are to raise and
lower the body to cause timed unbalance.
· The
movement of the body itself is powered by the rotating reaction mass,
essentially a flywheel, storing energy that is converted to motion by the
sequenced raising and lowering of legs.
· In its
most efficient form, the rotopod is essentially a “virtual” wheel,
the legs acting as spokes.
References
[1]. Rotational Legged Locomotion. Lyons, D., and
Pamnany, K., IEEE Int. Conf. on Advanced
Robotics,
[2]. Analysis of Gaits for a Rotating Tripedal Robot. Lyons, D., and Pamnany, K., to appear: SPIE Intelligent Robots & Computer Vision, Boston MA, Oct. 2005.
[3]. Energy Efficient Searching using Rotational Legged Locomotion. K.
Pamnany, MS Thesis. May 2005.