A Novel Pneumatic Spherical Robot
While working at Disney Research, I developed a deformable pneumatic spherical rolling robot. The main structure of the robot is a pair of hemispherical acrylic components that have a large number of holes drilled into them. Attached to the outer surface of both of these hemispheres are a number of hexagonal and pentagonal spherical segments that are each an elastic rubber bladder. The bladders are custom molded to exactly form a sphere when assembled using the exact same geometry as a soccer ball (i.e. a truncated icosahedron).
The bladders each contain a rigid plastic element that allows the robot to hold a spherical shape when the bladders are deflated and also feature tapped holes that allow the bladders to act as their own sealing gasket when attached to the acrylic hemispheres.
Internal to the hemispheres are the valves that control airflow into and out of the bladders. The valves used are solenoid, 3/2 valves that normally connect the bladders to the environment. There is one valve for each bladder (i.e. there are 32 valves and bladders) and they are each mounted to be coaxial with the center of their bladder. When a valve is energized, pressurized air is allowed to flow into its bladder, inflating it. The inflation is constrained to be radially outward from the center of the sphere only by the rigid hemispherical structure.
When this deformation occurs in a bladder that is located near to the sphere's contact point with the ground, the inflating bladder will make its own contact with the ground and impart a moment to the spherical robot. This causes the robot to begin to roll. Repeatedly inflating a bladder that is near to the ground on the opposite side of the robot's direction of travel will result in a continuous rolling motion.
The action of actually determining which of the bladders are nearest to the ground and on the correct side of the robot is done here using an optical commutation approach. First, note that inflating only a single bladder would result in an indeterminate direction of travel. Instead, a swath of the bladders must be inflated so that the direction of travel matches the desired direction. In the optical commutation used here, each of the bladders' control valves is driven by a phototransistor and accompanying electrical circuitry so that when the phototransistor is illuminated, the valve is energized and air begins to flow into the accompanying bladder.
Within this scheme, the valves to inflate are selected by shining a light on them. To exploit this arrangement, a second, smaller acrylic sphere is placed concentric with the one that supports the rubber bladders. This sphere contains a disc that is fitted with three spherical casters so that is allowed to roll passively and freely. Gravity and these casters cause the disc to fall to the bottom of the sphere. By attaching a remotely directed spotlight to this disc, a user of the robot can select the direction of travel of the robot. Gravity and alignment of the spotlight will ensure that the correct bladders are inflated to achieve a continuously rolling motion.
The research culminated with a conference paper being written and published to the IEEE International Conference on Robotics and Automation, held in Anchorage, Alaska in May, 2010.
