Mini-Whegs™ are small mobile robots, designed according to abstracted cockroach locomotion principles, that can run and can climb obstacles taller than themselves. Mini-Whegs are derived from larger mobile robots denoted Whegs™, with modifications to reduce size, reduce the number of mechanisms, and increase relative mobility. The name "Whegs" originated as a contraction of "wheel-legs," referring to three-spoke appendages by means of which these robots move, as described below.

Figure 1. Several Stages in the Motion of a pair of front wheel-legs approaching and climbing an obstacle illustrate important principles of design and operation. A far (black) wheel-leg makes contact first and then, by virtue of passive torsional compliance in the drive train, a near (grey) wheel-leg rotates into phase with the far wheel-leg. Once in phase, the two wheel-legs can propel the front of the robot over the obstacle. Atop the obstacle, the two wheel-legs spring back to their nominal 60° rotational separation.

Whegs combine the speed and simplicity of wheels with the climbing mobility of legs, and offer a compromise between climbing ability and smooth motion. Figure 1 depicts several aspects of the Whegs concept. The spokes of each wheel are spaced apart at 120° angular intervals. Two wheel-legs are mounted on each axle on opposite sides of the robot. The two wheel-legs on each axle are nominally oriented at 60° out of phase with each other.

Each axle contains a passive, compliant mechanism that enables the two attached wheel-legs to rotate by as much as 60° from their nominal phase difference of 60°. This feature is a key element of both mobility and relative simplicity of design in that it enables a robot to adapt its motion to changing terrain in a manner similar to that of a cockroach, without need for an active adaptation control subsystem. In particular, as illustrated by the example of Figure 1, this feature can cause the wheel-legs in each pair to become aligned as needed for climbing an obstacle and then spring back to the normal 60° phase difference once the obstacle has been surmounted.

A Whegs has six wheel-legs and, hence, contains three axles: one for each pair of wheel-legs. A Mini-Whegs contains four wheel-legs and, hence, contains two axles: one for the front pair of wheel-legs and one for the rear pair of wheel-legs. One motor drives all three axles in a Whegs or both axles in a Mini-Whegs via chains and sprockets, causing the robot to nominally walk in a cockroach-like alternating tripod gait. Steering is accomplished by means of an electrically actuated servomechanism connected to a front-wheel steering linkage similar to that of an automobile.

Figure 2. One of the Mini-Whegs Robots is shown with its front wheel-legs climbing an obstacle.

To date, five Mini-Whegs have been built, each incorporating a different advancement. All are battery powered and most are operated by remote (radio) control. The first one, shown in Figure 2, has a body about 3.25" ~(8.3 cm) long, 2.5" ~(6.4 cm) wide and 0.8" ~(2.0 cm) high; wheel-legs having a radius of 1.3" ~(3.3 cm); and a total mass of 125 g. Most of the Mini-Whegs can move at a sustained speed of more than 10 body lengths per second and can run over obstacles taller than their body lengths. They can run forward and backward and on either side. They are sufficiently robust that they can fall down stairs without sustaining noticeable damage. Each can carry a payload equal to twice its weight. One of the Mini-Whegs contains a jumping mechanism that enables it to surmount an obstacle is high as 9" ~(23 cm) — more than the height of a standard stair step.

This work was done by Roger D. Quinn, Roy E. Ritzmann, Jeremy Morrey, and Andrew Horchler of Case Western Reserve University for the Defense Advanced Research Projects Agency.

This Brief includes a Technical Support Package (TSP).
Miniature Wheel-Leg Mobile Robots

(reference DARPA-0004) is currently available for download from the TSP library.

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