Rotational Legged Locomotion

The Rotopod


What is it?



o     The Rotopod is a novel robot mechanism which combines the features of wheeled and legged locomotion in an unusual way.

o     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.

o     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




o     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).

o     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.

o     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?


o     The Rotopod moves by rotating around each of its legs in turn.

o     A rotation of q radians around a leg is call a step, and it results in the center of the robot moving a distance

o     A regular sequence of steps is called a gait.

o     The robot can produce a broad set of gaits: stepping for various values of q on one leg, or any sequence of legs

o     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.)



o     Sharp turns are not necessarily a problem for a rotopod, since its continual rotation allows it to change direction dramatically under certain conditions.


o     The diagram to the right shows a rotopod traversing a square traveling clockwise and then counterclockwise.



Some example videos


ODE simulation video 1 – continuously moving


ODE Simulation video 2 – large but discontinuous steps



How does Stepping Happen?





o     The rotopod steps by shortening the length of one leg.

o     When the reaction mass traverses that leg, it lifts the other two legs off the ground momentarily.

o     Without a reaction torque, the whole body rotates.

o     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?


o     The only purpose of the leg joints on the rotopod are to raise and lower the body to cause timed unbalance.

o     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.

o     In its most efficient form, the rotopod is essentially a “virtual” wheel, the legs acting as spokes.




[1]. Rotational Legged Locomotion. Lyons, D., and Pamnany, K., IEEE Int. Conf. on Advanced Robotics, Seattle WA, July 2005.


[2]. Analysis of Gaits for a Rotating Tripedal Robot. Lyons, D., and Pamnany, K.,  SPIE Intelligent Robots & Computer Vision, Boston MA, Oct. 2005.


[3]. Energy Efficient Searching using  Rotational Legged Locomotion. K. Pamnany, MS Thesis. May 2005.